18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada
Edition: Model Aviation - 2008/07
Page Numbers: 18,19,20,21,22,23,24,25,26,28
18 MODEL AVIATION
The Spectra II is fast even with the gear. In the water or out, it
flies like a clean Pattern model. Smooth maneuvers are its forte.
Laddie clears the model from the dock for taxi. The water rudder
is optional and adds authority to water steering, but it requires
careful management on takeoff.
The designer with his Spectra II for size comparison. The
functional elements are integrated in a way that makes this design
elegant as well as efficient.
The Spectra II can be slowed to a crawl for landing in tight spaces. The position of the
elevator relative to the propeller aids in performance.
As shown on the plans, removable fixed wire landing gear can be
attached to the model for grass or paved flying days.
An elegant
sport-scale
project that
leaps from
land or sea
July 2008 19
by Laddie Mikulasko
The Spectra II leaps onto the step with full throttle application, as
opposed to thrusting the nose into the water. Accurate lateral
balance and aileron control keep the wingtips out of the water.
The Spectra II has generous horizontal and vertical area. The
designer did a great job of maintaining the scale outline of this
intermediate-skill building project.
Photos by the author
YEARS AGO I saw an article in the July
1971 Radio Control Modeler magazine that
highlighted what I believed to be the nicest
scale seaplane: the Spectra II. I really liked
the design, but because of other priorities I did
not build it then. I recently stumbled across
the original RCM with the Spectra II on its
cover again, and this time I decided to build it.
The original floatplane had a 72-inch
wingspan and was powered by a .60 engine. I
was going to use the original plans as a base
for building this aircraft; however, I wanted to
have a 66-inch airplane that would be
powered by a .46 glow engine or a midsize
motor such as an AXi 4120/18. At the same
time I felt a need to simplify the construction.
From the original drawing I used only the
model’s outline, the location of the ribs, and
the shape of the formers. Before working on
this design I felt that I needed more
information than the 1971 construction article
offered. I learned from reading the original
introduction article by Don Dowey (RCM’s
publisher and editor at that time) that the
model was used in the engineering tests for
the full-scale prototype.
The original article and plans were by Don
Hains and Paul Rhen. I did not know who
designed the full-scale Spectra. When I asked
this question on the Internet, a modeler
20 MODEL AVIATION
The Spectra II is convertible from electric to glow power. An AXi
4120/18 with a Jeti 70 opto ESC does the job of a .46-size engine
when powered with a 6S Li-Poly battery system.
The balsa-and-plywood cowling can be built with or without the
option to explore power sources. The power pod is mounted high
enough to suit a 13-inch propeller.
The forward hatch secludes the two 3S batteries wired in series.
The servos shown operate the rudder and elevator control. Note
the magnetic closure.
Make the wing joiner by gluing three carbon-fiber tubes together.
Thick cyanoacrylate is recommended for construction because it
bonds extremely well with the composite material.
The center-section is built in halves, each of which is assembled
during the construction of its respective panel. As shown, the left
center-section is separated from the wing.
The LE and TE sheeting is glued on. The builder uses firm strips of
material pinned against the sheeting to keep the joints tight.
July 2008 21
The wingtips are made, showing the use of a balsa wedge to get
them at the right angle. The builder has the option of omitting
this wedge and angling ribs W11 and W12 instead.
The stabilizer sheeting joins the high edge of the full-length balsa
spar. Build the stabilizer in right and left sections, and join them
after the sheeting is complete.
The triangular stock is glued to the insides of the nacelle to add
body to the corners that will be rounded to shape later. The
laminated firewall is used as a sanding guide.
Above: The nacelle is built with two sides that are stacked while
shaped to ensure that they stay identical. Double-check to ensure
that the centers are aligned at all times.
Below: The nacelle and tail surfaces are finish-sanded and ready
for final assembly. Now is a good time to get the hinging finished.
The bottom formers are glued to the fuselage sides. The drawing
shows one-piece formers that the builder should use rather than
the heavier stick formers shown.
22 MODEL AVIATION
Former F1 has 5/32-inch-inside-diameter brass tube attached to it
for holding the nose-gear leg. The Kevlar thread is saturated with
epoxy resin after alignment is confirmed.
All bottom formers can be glued on, and the keel can be set in
position when the fuselage “box” is framed. No fiberglass is
needed after the bottom is sheeted.
The fuselage is built upside down. The balsa floor is glued in place
at this stage. It will be used later as a place to locate the powersystem
batteries.
Type: RC sport-scale amphibian
Skill level: Intermediate
Wingspan: 66 inches
Flying weight: 7.5-8.5 pounds
Wing area: 668 square inches
Length: 50.25 inches
Power: .46-.63 glow engine or 900-watt motor (AXi 4120/18 used)
Construction: Balsa with plywood and carbon-fiber
reinforcement
Covering/finish: Builder’s choice (UltraCote heat-shrink film
used)
Other: 8- to 10-ounce fuel tank, four to six servos, 6S Li-Poly
battery, 70-amp ESC, 2-inch spinner, landing gear, wheels
(optional)
July 2008 23
replied and enclosed part of the magazine
article. Roy LoPresti was mentioned as the
designer.
On the Internet I found LoPresti’s
company, Speed Merchants, which specializes
in improving the performance of generalaviation
aircraft by refining their
aerodynamics. I contacted the company and
received a reply from David LoPresti, who is
the son of the company’s founder.
The previous article mentioned the waterhandling
tests but included nothing about the
flight tests. David wrote that his father
terminated work on the Spectra for several
reasons, one of which was that after the waterhandling
tests he concluded that he required a
larger shop to continue.
David also mentioned that his father was
planning to build a factory in Florida to
produce this aircraft. However, for financial
reasons that never happened. At that time
David’s father, who was employed by the
Grumman company, was working on the
moon program.
Because of the program’s cancellation,
David’s father was offered a chief engineer
position at Grumman American, located in
Ohio. The Spectra, an advanced design for its
time, was never completed.
From the start I tried to design the model
so that it would be easy to build. As I
mentioned, I wanted an aircraft that could be
flown either as glow or electric. Therefore, the
nacelle can accommodate either power plant;
it can house the fuel tank or a speed controller.
The elevator and the throttle servos can also
be mounted inside the nacelle.
I made a plug-in wing to prevent the water
from seeping into the fuselage. The Spectra II
can easily be flown from both land and the
water.
To help less-experienced builders, I gave
each part a number. The numbers are used to
identify pieces in the construction section.
Also use the included materials list as a crossreference.
Please don’t make modifications. I didn’t
use fiberglass on the outside of the model.
Only the center-section of the wing, which sits
inside the fuselage, has fiberglass on both the
top and bottom, to strengthen the wing joint
point.
If you are planning to paint the fuselage,
cover the entire thing with 3/4-ounce
fiberglass. I recommend following the
building sequence as written.
As some photos show, parts of the Spectra
II were built in different ways. I strongly
suggest that you follow the written
instructions.
CONSTRUCTION
Wing: Make the two main joiners 24 by
adhering three 1/4-inch-diameter carbon-fiber
tubes with thick cyanoacrylate, as shown on
the plans and in the photo.
Cut all the ribs. Cut and drill holes in the
ribs as shown on the plans. Pin bottom main
spar 1 to the building board over the plans. All
wing spars are in three sections. The first
section is between ribs W1 and W2, the
second is between W3 and W11, and the third
is on the wingtip between W12 and W13.
Position and glue ribs W1 - W11 onto the
bottom main spar. Those ribs have to be
angled as shown on the drawing. Slide the
shim under the ribs’ TEs. Glue top main spar
2 to all the ribs. Glue in top rear spar 7 to the
1. 1/4 x 1/8 spruce (wing)
2. 1/4 x 1/8 spruce (wing)
3. 1/4 x 1/8 spruce (wing)
4. 1/4 x 1/8 spruce (wing)
5. 1/4 x 3/32 balsa (wing)
6. 1/4 x 1/8 spruce (wing)
7. 1/4 x 1/8 spruce (wing)
8. 1/4 balsa sheet (wing)
9. 1/8 balsa sheet (wing)
10. 3/32 balsa sheet (wing)
11. 3/32 balsa sheet (wing)
12. 3/32 balsa sheet (wing)
13. 1/4 balsa sheet (wing)
14. 1/4 balsa sheet (wing)
15. 1/4 x 1/4 spruce (wing)
16. 1/8 plywood (wing)
17. 3/4 x 1/2 hardwood (wing)
18. 1/2 balsa triangular stock (wing)
19. 1/16 plywood (wing)
20. 1/16 plywood (wing)
21. 1/16 plywood (wing)
22. 1/16 balsa sheet (wing)
23. 1/16 balsa sheet (wing)
24. 1/4-inch-diameter carbon-fiber tube (wing)
25. 1/8-inch-diameter carbon-fiber tube (wing)
26. 1/8 balsa sheet (wing)
27. 1/4 balsa sheet (stabilizer)
28. 1/8 balsa sheet (stabilizer)
29. 1/8 balsa sheet (stabilizer)
30. 1/16 balsa sheet (stabilizer)
31. 1/4 x 1/8 spruce (fuselage)
32. 1/4 x 1/8 spruce (fuselage)
33. 1/4 x 1/8 spruce (fuselage)
34. 1/4 x 1/8 spruce (fuselage)
35. 1/8 balsa sheet (fuselage)
36. 1/8 balsa sheet (fuselage)
37. 1/8 balsa sheet (fuselage)
Plans Identification Chart and Materials List
38. 1/8 balsa sheet (fuselage)
39. 1/8 balsa sheet (fuselage)
40. 1/8 balsa sheet (fuselage)
41. 1/4 x 1/8 balsa (fuselage)
42. 1/4 x 1/8 balsa (fuselage)
43. 3/32 balsa sheet (fuselage)
44. 3/32 balsa sheet (fuselage)
45. 1/4 balsa sheet (fuselage)
46. balsa block (canopy)
47. balsa block (fuselage)
48. 5/32-inch-diameter brass tubing (fuselage)
49. 5/32-inch-diameter piano wire (fuselage)
50. 1/4 x 1/8 balsa (canopy)
51. 1/4-inch-diameter carbon-fiber tube (fin)
52. 1/8-inch-diameter carbon-fiber tube (fin)
53. 1/4 balsa sheet (fin)
54. 1/8 balsa sheet (fin)
55. 3/8 balsa sheet (fin)
56. 3/8 balsa sheet (rudder)
57. 3/32 balsa sheet (rudder)
58. 1/8 balsa sheet (rudder)
59. 1/4 balsa sheet (nacelle)
60. 1/4 balsa sheet (nacelle)
61. 1/4 balsa sheet (nacelle)
62. 1/4 balsa sheet (nacelle)
63. 1/4 balsa sheet (nacelle)
64. 1/8 plywood (nacelle)
65. 1/4 balsa triangular stock (nacelle)
66. balsa block (nacelle)
67. balsa block (nacelle)
68. 5/32-inch-diameter piano wire (main gear leg)
69. 1/8-inch-diameter dowel (canopy)
70. 1/4-inch magnet (canopy)
71. 1/2-inch self-tapping screw
72. 1/8 plywood (rudder)
73. 1/8 plywood (rudder)
74. 1/32-inch aluminum (rudder)
ribs. Glue on LE spar 8 to the ribs. Sand this
spar so it follows the contours of the ribs.
Glue on top TE sheeting 11. Insert and
glue joiner 24 to ribs W3, W4, and W5.
Remove the wing and pin it upside down to
the building board. Place the shim under the
TE. Glue bottom rear spar 6 to all the ribs.
Glue on bottom TE sheeting 10.
Glue plywood shear webbing 19 between
ribs W1 and W2 and webbing 20 between ribs
W3-W6. Glue these webbings only to the rear
side of the top and bottom main spars. Glue in
fillers 3, 4, and 5 between the joiner and the
top and bottom main spars. Make sure glue
does not get onto the joiner between ribs W1
and W2.
Glue shear webbing 19 and 20 to the other
side of the main spars. Do not glue shear
webbing to joiner 24 between ribs W1 and
W2. Between ribs W5 and W6, glue plywood
shear webbing 20 to the rear of the main spars
and balsa shear webbing 21 on the other side.
Glue in 1/2-inch triangular stock 18 between
ribs W1, W2, W3, and plywood webbing 19
and 20.
Glue on bottom LE sheeting 11 to the ribs
and to the main spar. Glue in the aileron
servo-mounting tray between ribs W9 and
W10. Glue on the bottom sheeting between
the LE and TE over ribs W1, W2, W3, and
W4. Glue in landing-gear hardwood blocks
17. Remove the wing from the building
board.
Cut the aileron from the wing. Then glue
hinge spar 13 to the ribs and to rear spars 6
and 7. Glue the aileron’s LE 14 to the
aileron. Cap the ends of the aileron. Glue on
the bottom sheeting between ribs W9 and
W10. Glue on all the capstrips. Inside the
aileron, glue the 1/8 plywood plate to the
sheeting to support the aileron control horn.
Wingtips: Pin ribs W12 and W13 to the
building board. Glue the main and rear and
LE spar to them. Make sure rib W12 is
angled as shown on the drawing.
Sheet both sides. The top sheeting has a
different shape from the bottom one. Refer
to the drawing. Cap the end with balsa sheet
26. Glue the wingtips to the wing. Glue on
LE capstrips 9. Sand the wing to its final
shape.
Build the other wing half the same way.
Plug each half into its center-section. Smear
glue onto ribs W1 of each center-section.
Join the halves and place the dihedral shim
under each wingtip.
Once the halves are adhered, unplug the
wing panels from the center-section. Cover
both sides of the center-section with one
layer of fiberglass. Pull in the extension
wires for the aileron servos. Install the Y
harness in the wing center-section.
Tail Surfaces: The fin and rudder are made
in left and right halves. Pin fin ribs FN1,
FN2, and FN3 to the building board. Pin and
glue fin LE 53 and hinge spar 55 to the ribs.
The hinge spar extends up into the nacelle
by 1/4 inch and all the way down into the
fuselage. Glue on 1/8-inch carbon-fiber tube
52 to the ribs and to the hinge spar. Notice
that this tube extends up by 1/4 inch and
down 21/4 inches.
Glue on balsa sheeting 58 to the ribs, the
LE, and the TE. Build the other half of the fin
the same way.
If you are planning to have the servos in
front of the fuselage, install the plastic
pushrods into the fin. If you are going to have
the servos inside the nacelle, install the
extension cables. If your Spectra II is going to
be powered by a motor, install the battery
extension and ESC extension wire leads.
Insert and glue 1/4-inch carbon-fiber tube
51 to the ribs on one half of the fin, and then
glue the halves together. Notice that this tube
extends up by 1/4 inch and down by 3 inches,
as did tube 52. Glue LE capstrip 54 to the fin
and then sand the fin to its final shape.
Build the rudder in a similar fashion. Glue
plywood plate 72 to the sheeting on the inside
to support the rudder control horn.
Glue plywood plate 73 to the bottom of
the rudder. This will be needed to hold water
rudder 74. The water rudder is made from
thin sheet aluminum. It must be removed
when you are not flying from the water.
The stabilizer and elevator are built in left
and right halves. Cut out all the ribs for those
flying surfaces. Cut hinge spar 27 from 1/4
balsa sheet. Pin the hinge-spar shim to the
building board. Pin hinge spar 27 to the shim.
Pin the LE shim to the building board and pin
LE spar 28 to the shim.
Glue stabilizer ribs S1-S6 to the LE spar
and the hinge spar. Glue top sheeting 30 to
the ribs and the LE and TE. Flip the stabilizerupside down and pin to the shims. Glue on
bottom sheeting 30. Glue on LE capstrip 29.
Build the elevator the same way, using the
same shims. Do not forget to glue the
plywood plate for the elevator control horn.
Glue the stabilizer and elevator halves
together. Sand the stabilizer and the elevator
to their final shape.
Nacelle: Cut plywood formers N1, N2, and
N3. Cut out balsa sides 59 and 62. Glue the 1/2
triangular stock to the nacelle sides. Glue
firewall N3 to the sides. Keep everything
square.
Glue on top sheeting 60 and bottom
sheeting 61. Draw the centerline of the nacelle
onto the top and bottom sheeting. In the top
sheeting, cut the opening for the access hatch.
On the bottom, cut the openings for the LE
and the hinge spar of the fin to fit in. Make
openings for the wires or plastic tubes to enter
the nacelle from the fin.
Make the engine cowl by gluing
formers N1 and N2 to sides 62. Keep it
square. Glue on top and bottom sheeting
63. Glue pre-shaped nose block 67 to the
front of the cowl. Sand the nacelle and the
cowl to their final shape.
Fuselage: Cut all formers and fuselage sides
35. Notice that most of the formers are in top
and bottom halves. Glue longerons 31, 32, 33,
and 34 to the fuselage sides. Former F1 has a
5/32-inch brass tube attached to it with thread.
Start building the fuselage upside-down.
Glue bottom formers F5-F8 between the two
fuselage sides 35.
Glue in formers F1-F4. Glue the rest of the
formers in the back of the fuselage. Glue in
keels 37 and 39 to the formers. Glue on
bottom sheeting 36 to formers F1-F8 and
sheeting 38 to the formers behind the step.
Turn the fuselage right-side up and glue in
battery floor 40.
Position the fin onto the fuselage. If you
are using flexible control rods for controls,
feed them through the holes in the formers
and into the radio compartment. If you are
using extension wires, pull them into the radio
compartment.
Glue the fin to the fuselage. Make sure it is
square with the sides.
Glue on wing saddles 45 to each side of
the fuselage. Smear epoxy onto the wing
saddle. Place the wing on the wing saddle.
Ensure that the wing panels are plugged into
the center-section. Square the wing with the
fuselage and let the epoxy harden.
Unplug the wing panels from the fuselage.
Glue the top half of formers F7, F9, and F10
to the fuselage and the top of former F8 to the
wing center-section. Insert and glue longeron
42 into the slots in these formers. Glue on top
sheeting 45 between formers F7 and F10.
Glue on the top half of formers F3, F4,
and F5A. Make sure former F5A is glued on
the angle as shown on the drawing. Glue in
top longeron 41. Glue on top sheeting 43 over
formers F1-F5A. Glue balsa block 47 to the
nose.
To make the canopy, place clear plastic on
longerons inside the radio compartment so the
glue does not stick to the fuselage when you
are gluing the canopy pieces. Pin bottom
frame pieces 50 to the fuselage. Glue formers
F5B, F5C, F6, and F7A to frame piece 50.
Glue top longeron 42 to F5C, F6, and F7A.
Glue on canopy sheeting 44. Between former
F5B and sheeting 44, glue in either a balsa
block or a plank using balsa scraps.
The canopy is held in place with one
dowel in the back and two 1/4-inch magnets
embedded inside former F5A and F5B or one
1/2-inch magnet in former F5A and the other
in F5B.
Sand the fuselage to its final shape. Fix the
imperfections with filler. I used lightweight
water-based filler.
I covered the model with iron-on material.
If you are going to use a glow engine, cover
the entire fuselage with fiberglass that is no
heavier than 3/4 ounce per square foot, in
preparation to finish with your favorite
fuelproof paint.
Insert and glue in the stabilizer. Make sure
it is square with the wing and the fuselage.
Insert all the controls. Install the landing gear.
The nose-wheel leg slides into the tubing.
You can secure the steerable arm to it with the
long Allen wrench. Mount the glow engine or
motor.
With the glow engine, lead ballast has to
be secured in the nose so the model can be
balanced. If you are going to switch between
glow and electric, do not glue the ballast into
the nose. The electric-powered version does
not need lead since the motor battery will be
used to balance the model.
To secure the wing, plug the wing panels
into the fuselage. Drill a 3/32-inch hole 1/2-inch
into the main spar of the center-section. The
tip of the drill bit should penetrate the top
carbon-fiber tube of the joiner but go no
deeper. You must drill the hole approximately
1 inch in from W2 ribs. Drive a self-tapping
screw in place. Test the security by trying to
pull out the wing panels; they must not move.
Cover the model with your favorite
material. Install the hardware and test
everything before the first flight. Check the
CG; it should be located as shown, with the
fuel tank empty.
Flying: When flying from the solid surface,
taxi into the wind and apply full power. The
Spectra II tracks straight and will rotate
quickly. If balanced correctly, it will have a
solid feel.
Despite the model’s being short-coupled,
it is not sensitive in the pitch. It can perform
all basic maneuvers. Before going for
landing, fly high and then try to slow the
airplane until it stalls. This way you will
have an idea about its low-speed behavior.
Remove the landing gear (and seal the
gear sockets with matching covering or
tape) when flying from water. In light wind
there is no problem with steering. In a
stronger crosswind, the airplane requires
that the water-rudder extension be installed.
You may choose to make a longer water
rudder or add temporary extensions as
needed.
Taxi the model into the wind. Gradually
apply full power. Lift the wingtips out of the
water with the ailerons. The model will lift
off quickly.
The Spectra II is a delight in the air,
especially without the wire gear hanging from
it. The elevator’s location means that it stays
effective at all speeds. The large amount of
side area promotes good rudder authority for
point rolls. Inverted flight is well within this
model’s capabilities.
I hope you enjoy building and flying the
Spectra II as much as I did. Good luck. MA
Laddie Mikulasko
7 Giffen Rd.
Dundas, Ontario
L9H 6S1
Canada