FOXBAT WAS DESIGNED to be a
modern Category II and III Open 1⁄2A Gas
model. To accomplish this goal, a Cyclon
.049 engine (KCs, ADs, and Foras are also
suitable) is combined with a Variable
Incidence Tailplane (VIT) system and an
electronic timer in a model size that allows a
good balance between climb and glide
performance.
My secondary goal was to achieve this at
a reasonable level of building effort and
produce a design that the average FF
modeler could build successfully. Foxbat
incorporates the following features:
• A flat-bottomed airfoil, to avoid the need
for a contoured building bed while allowing
the use of a full-depth spar.
• A modest wing aspect ratio (approximately
10:1), to allow a vertical climb and smooth
transition using a VIT without bunt.
Foxbat has served me well. In addition
to many wins in local contests, the
model placed first in 1⁄2A Open Gas at
the 2003 and 2004 Southwest Regionals.
CONSTRUCTION
Build and cover the wing, stabilizer,
and rudder first, so that they are available
when you are building and rigging the
fuselage.
Wing: The heart of the wing is the spar,
which is full depth and capped with .007-
inch-thick unidirectional carbon fiber (CF).
Attach a piece of waxed paper to a flat 3⁄4-
or 1-inch-thick building board using 3M
Super 77 spray cement. Cover two pieces of
1-inch aluminum angle with waxed paper on
their outer surfaces using 3M Super 77. Affix
one piece 3 inches from the edge of the
board with 3M Super 77.
Cut a 3⁄16 x 1 x 29-inch spar blank and a
3⁄16-inch-wide strip of .007-inch CF. Spread a
thin layer of good-quality epoxy, such as
NHP 12-minute, to one edge of the spar
blank and add the CF strip.
Lay the assembly flat on the building
board with the CF against the face of the
aluminum angle. Press the vertical face of
the second angle against the back edge of
the spar and clamp with six or seven large
spring clamps.
Remove the spar subassembly when
cured, and cut one piece to 171⁄2 inches for
the main spar. The leftover 111⁄2-inch piece
will be the tip spar. Carefully cut the main
spar to the exact height of the main spar
shown on the rib-section drawing of the
plans, minus the thickness of the
top .007 CF strip, and add the
second CF strip.
To complete the tip spar,
make a 1⁄32 plywood template
from the wingtip spar drawing.
Transfer the shape to the 111⁄2-
inch piece and cut to shape and
length. There is no CF on the top
surface.
Because the airfoil is a log
spiral, you only need three
templates per wing. Make
the three wing-rib templates
using the rib drawings for
the main, front diagonal,
and rear diagonal ribs.
Make the templates as
shown in the “typical rib
template” drawing.
Place the rib template—
upside down—on the rib sheet stock
with the hardwood strip butted against the
lower edge. Cut the rib profile and then the
vertical LE and TE cuts. The forward
vertical edge of the ribs must be square with
the bottom of the rib.
The tip TEs are first assembled as blanks
with sandable aliphatic resin glue, using 3⁄16
x 1-inch strips. Then they are cut as one
piece using the wingtip template provided
on the plans.
To assemble the wing, cover the plans
with waxed paper and pin down the main
panel TE, spar, and two-piece LE. Use
Rocket City pin clamps for all parts pinning
to avoid putting pins through the balsa. Use
aliphatic resin for assembly of all laminated
LEs.
Carefully cut the forward portion of one
rib to fit between the LE and the spar. It is
essential that all cuts are perpendicular to
the bottom of the rib to assure a good glue
joint with the spar. To make sure of this, I
use Micro-Mark’s heavy-duty Chopper II
and True Sander tools.
When you are satisfied with the fit of the
first rib, place it on the Chopper with the
spar end against the blade, and set the
adjustable stop against its LE. Cut all of the
main-panel front ribs to this length. Cut a
1⁄16-inch strip from the bottom of each front
rib using a Master Airscrew balsa stripper.
Install the 3⁄32-inch dihedral ribs, at the
appropriate angles, using the dihedral-rib
fixtures provided on the plans. Put in the
remainder of the ribs, ensuring that they are
properly aligned and perpendicular to your
building board.
Set the Chopper’s stop to the
thickness of the spar, cut from the
front edge of each main-rib leftover,
and install. Install the front diagonal ribs
after you cut them to length (slice from the
rear face), and sand the front and rear faces
to an angle for a flush fit to the LE and the
spar. Repeat for the rear diagonals.
To build the tip panel, cut each rib to
length. Scribe a mark 3⁄16 inch from one edge
of a C-Thru plastic scale, place the scale
over the rib so that the edge is on the front
bottom corner and the mark is on the rear
top corner, and cut away the bottom. Square
both ends with the True Sander.
Pin down the TE and put a series of Tpins
along the inside of the LE, between the
rib locations. Cut four strips of 1⁄16 x .21-
inch balsa, and glue them together. Quickly
clean up the excess glue, and pin along the
LE against the T-pins before the glue sets.
After the LE dries, pin down the spar and glue its end to the LE with cyanoacrylate. Install the dihedral ribs
and straight ribs using the same procedure you used for the main
panel.
To install the front diagonals, cut to length and mark the rear
edge to height using the wingtip-spar template as a guide. Cut along
the bottom from the forward bottom corner to the rear mark, and
square both ends. Angle-sand the edges, and put in the diagonals.
Prepare the rear diagonals by cutting to length and marking the
front-end height, using the rib it will butt up against as a guide. Cut
the bottom using the C-Thru scale, square the ends, angle-sand, and
install the rear diagonals. Add the 1⁄16-inch wing gussets.
Lift the wing panels from the plans, and lightly reglue all joints
with thin cyanoacrylate. Sand the LE tangent to the top surface of
the ribs, as shown in the wing-section drawing. On the tip panel,
place a piece of masking tape across the LE, aligned with the rear of
the spar. Sand the LE in this area to follow the shape of the tip spar,
and leave a 1⁄16-inch step across the LE, in line with the rear of the
spar. The sheeting will fit flush against the step in this area.
Cut the main-panel top sheeting 1⁄4-inch oversize in width. Put a
bead of medium cyanoacrylate along the top edge of the spar, and
seat the sheeting on the spar and against the rear ribs.
After the cyanoacrylate sets, turn the panel upside down, place it
on the building board, and slowly roll it toward the LE while
keeping the sheet in contact with the ribs. When it is pressed flush
on the LE and the ribs are still in contact, run cyanoacrylate along
the joint between the sheet and the front of the LE. Lift the panel,
and adhere each rib to the sheeting with cyanoacrylate.
Finally, cyanoacrylate-glue the sheeting to the inside edge of the
LE. Repeat the procedure for the tip panel, but cyanoacrylate-glue
the LE to the sheet, one rib bay at a time.
I use a simple contour sander for the upper surfaces. Use a 3 x 3
x 8-inch balsa block.
Attach the wing-section drawing along the bottom edge of one
side and the stabilizer-section drawing along the top edge of the
same side. Mark a line at the front and rear that smoothly extends
the top-surface curve to the edge of the block.
Using a band saw, cut along the upper surface of the wing
section. Repeat for the stabilizer section. Mount a piece of 150-grit
sandpaper to the cut surface with 3M Super 77. As you progress
along the tapered sections of the wing and stabilizer, adjust the fore
and aft position of the sander to match the shape of the airfoil at
each rib.
Cyclon .049 in the “go” end of the Foxbat. Plumbing for flood-off
is on the starboard side of this powerful engine.
Use rib templates to cut the ribs accurately. The flat-bottom,
spiral-log airfoil makes this assembly process easy.
Cut rib blanks using the rib template, and then use the
Chopper to accurately cut ribs to length as described in text.
Use True Sander to square ribs for tip panels. Front and rear faces
of ribs are slightly angled because bottom of ribs was cut to fit TE
and then to fit spar height.
This completes the structure for half of the wing. Repeat the
procedures to build the other half.
To join the wing panels, sand the end of each panel flush with
the angled dihedral ribs. A dihedral sanding fixture makes this job
easy and provides precise results.
Make a 10 x 10-inch piece of 3⁄4- or 1-inch finish-grade plywood.
Using a table saw or a band saw, tilt the blade and cut one edge to
the 31⁄2° center dihedral angle and the opposite edge to 11°, which is
half the tip dihedral angle. Mount the fixture slightly overhanging
the edge of your workbench with 3M Super 77 spray cement. The
31⁄2° edge should be facing off the bench.
Lightly spray the top of the fixture with 3M Super 77, and mount
the main panel with the bottom of the center dihedral rib flush with
the top edge of the angled face. Block-sand the end of the panel until
the sanding block is flush with the fixture’s angled face. Repeat the
process on the other edge of the fixture for the four tip dihedral ribs.
Pin the main panel to the building board, and block-sand the tip
panel to the proper angle. Apply aliphatic resin to the mating
surfaces, and pin the tip panel in place. You can use spring-type
clothespins to hold the ribs together until the glue dries. Repeat for
the main panels.
Using a hacksaw blade, cut a slot along the front face of the spar
into the center ribs and the next rib on each side. The slot should go
from the bottom surface of the wing to the depth of the dihedral
brace. Widen the slot for a slip fit onto the dihedral brace. Epoxy the
dihedral brace in place and clamp it until it’s cured.
Install the center bay sheeting inserts, install the fiberglass
reinforcement strips with thin cyanoacrylate, and the wing is ready
for covering after a final cleanup.
Stabilizer and Rudder: Build the stabilizer using the same
procedure you used for the wing. When the stabilizer is complete,
cut away the center gap in the LE, slot the bottom of the LE for the
stabilizer mounting wire, and epoxy it into place.
Build the rudder over the plans, using 3⁄16-inch strip stock.
Temporarily attach the movable portion of the rudder using 3M
Super 77. When the basic structure is complete, sharpen a piece of
.040-inch CF rod to a point and, gently twisting, insert it from the
bottom and through each rib like a drill until the tip is embedded 1⁄8
inch into the tip piece. Apply one drop of thin cyanoacrylate where
the rod intersects each crossmember.
Lift the rudder and taper it to 3⁄32 inch at the tip, round the LE,
and taper the rear one-third from 3⁄16 inch to 1⁄16 inch at the TE.
Release the movable portion with lacquer thinner, add the rudder
horn, and hinge in place using your favorite method. (I use small
Klett Flex Point hinges.) Add the two .040-inch CF alignment pins
to the bottom of the rudder.
Bottom center-section and fit of main ribs and diagonals. Notice
center gussets and use of fiberglass to reinforce center of wing.
Custom contour sander used to finish-sand wing is cut from pine
block using a band saw, and then faced with fine sandpaper after
carefully smoothing.
Type: Category II and III Open 1⁄2A Gas FF
Wingspan: 330 square inches
Engine: Cyclon .049
Flying weight: 8.3 ounces
Construction: Balsa-and-plywood wing and tail,
Oliver Mk I or Mk II F1J fuselage kit
Covering/finish: 1⁄2-mil dyed Mylar or UltraCote Lite
transparent film
Horizontal, vertical stabilizers are built similarly to wing
structure, using laminated LE. Pay attention to close fit.
Covering: Cover the wing, stabilizer, and rudder so that they will be
available during the fuselage assembly. If you cover with 1⁄2-mil
dyed Mylar, use Sig Stix-It to attach it. For a slight weight penalty,
you can use Goldberg UltraCote Lite transparent film, which already
has adhesive and is much easier to apply. Pylon and Fuselage: The pylon and wing platform are built as
shown on the plans. The four strips of .007 x 1⁄16-inch CF in the
wing platform add an incredible amount of lateral bending strength.
Open the rear wall of the compartment marked “TRACKER.” If
you use the large Walston tracker, increase the pylon core width
from 1⁄4 inch to 3⁄8 inch. Add the .040-inch alignment pins to the
bottom centerline of the pylon after you dry-fit it to the fuselage.
The fuselage is made from an Oliver Mk I or Mk II F1J fuselage
kit. The Mk I is shown on the plans. The kit includes a CF
forebody, an aluminum/carbon/aluminum tailboom, a CF-reinforced
fiberglass timer mounting plate, a molded CF stabilizer mount, and
a .047-inch music-wire stabilizer mount wire.
Prepare the fiberglass timer mount. Cut a hole to suit the timer.
The Fourmost e-Timer shown requires three 1⁄16 plywood mounting
legs stepped 1⁄16 inch below the face of the timer mount.
Cut a rectangular hole in the CF forebody at the proper location
for the timer mount, and make it 1⁄8 inch undersize on all sides.
Epoxy the timer mount to the inside of the CF forebody, in
alignment with the hole. Sand the edges of the hole flush with the
timer-mount surfaces.
Install the timer and mark the rear face of the timer mount for
the rudder and flood-off lines, and drill two 3⁄32-inch-diameter holes.
Make a 3⁄32 x 1⁄4-inch slot in the opposite side of the forebody in line
with the flood-off line as shown on the plans.
Bend a piece of 3⁄32-inch-outside-diameter (OD) aluminum tube
to the shape shown, with a little extra length on each end. Feed it
into the slot, slide it rearward after the sharper portion of the bend is
inside, and then pull it forward and through the appropriate hole in
the rear face of the timer mount. Epoxy the tube in place from the
inside through the rear opening of the forebody.
Epoxy a 1⁄4-inch-long piece of the same tubing into the rudderline
hole. When the epoxy has cured, cut the tubes where they exit
the timer mount and forebody, sand them flush, and deburr their
openings.
Epoxy the tailboom to the forebody using a fixture to assure
proper fuselage alignment. The fixture will consist of a piece of the
aluminum angle used to make the wing spars and six 1⁄4 plywood
squares measuring 2 x 2 inches.
Mark corner-to-corner diagonal lines on the face of each square.
On two of the pieces, cut a 90° V notch along two adjacent
diagonals to the center. These will be used to support the angle.
Drill a hole exactly at the center of each of the remaining four
squares where the diagonals intersect, as follows: two at 11⁄8 inch
diameter, one at 7⁄8 inch diameter, and one at 3⁄8 inch diameter.
Chamfer one corner of each square to clear the inside of the
aluminum angle. Slip the 11⁄8-inch-diameter squares over the
forebody at the front and rear. Use masking tape around the body, if
required, to get a close fit. Slip the 7⁄8-inch-diameter square over the
tailboom, and slide it forward until it fits snugly. Do the same with
the 3⁄8-inch-diameter square.
Apply epoxy, join the forebody and tailboom, and set it into the
angle. Carefully inspect to ensure that the fuselage is straight in the
horizontal and vertical planes. Make sure that the timer-mount face
is vertical.
Horizontal stabilizer shown in full DT position. Notice stabilizer
slide adjustment screw in rear of fuselage.
Stabilizer and rudder in power mode. Rudder is straight; rudder
stop screws are visible. Note stabilizer attachment device.
Fuselage tubes are glued together in fixture to ensure
straightness—a simple, accurate method to align front, rear tubes.
Close-up of Fourmost e-Timer; see attachments to auto surfaces
and flood-off. The e-Timer has been reliable and accurate.
While the structure is still in the fixture, use a fine-tip permanent
marker to indicate where the vertical diagonal on each square meets
the top of the fuselage. Remove the
fuselage from the fixture pieces, and draw a
centerline on top of the fuselage using those
marks.
Cut an opening in the top center of the
tailboom for the VIT assembly. Install the
3⁄32-inch OD aluminum tube into the
tailboom, and drill a clearance hole for the
rear 0-80 mounting bolt. Slide the stabilizer
mount onto the tailboom, to a position
where the rear hole in the stabilizer lines up
exactly with the glide/DT post, and mark
this position. Sand the inside of the
stabilizer mount ring, if necessary.
Drill two .040-inch-diameter holes into
the fuselage, on the top centerline, in a
location that will put the pylon in the
position shown on the plans when the
alignment pins are engaged. Reinstall the
fuselage in the fixture using only the front
11⁄8-inch-diameter and rear 3⁄8-inch-diameter
squares, and epoxy the pylon in place.
Mount the wing to the pylon with rubber
bands. Clean the area where the stabilizer
mount will be glued and apply a thin layer
of epoxy. Whenever you are gluing to
aluminum, use 3M Scotch Weld DP-460
epoxy. You should scuff the area to be
glued with sandpaper to give it “teeth” and
clean it with acetone or lacquer thinner.
Slide the stabilizer mount back into
position and clean up the excess epoxy with
an alcohol-dampened paper towel.
Temporarily mount the stabilizer with
masking tape. Align the stabilizer mount so
that there is no stabilizer tilt relative to the
wing. Remove the rear square for this step.
Position the rudder as shown on the plans,
and mark and drill the two .040-inch
alignment-pin holes. Epoxy the rudder into
place.
Mount the bladder assembly on the
.078-inch-diameter skid, insert the skid into
the engine mount, and install the engine.
Mount the flood-off bracket to the bottom
of the right-hand engine-mounting lug.
Install the timer (with appropriate launch
switch) and the VIT assembly.
Mount the wing and stabilizer, and
insert the engine assembly into the fuselage.
Check the CG position and add weight to
the nose or tail as required. (Nose weight
can be put inside the bladder compartment.)
A CG between 54% and 59% of the main
chord is acceptable; 56% is optimum.
When you are satisfied with the CG
location, completely disassemble
everything and sand the engine-offset
angles into the front of the carbon forebody.
Drill clearance holes on each side of the
forebody for the fuel lines to exit.
Install the bladder-compartment
bulkhead (1⁄16 balsa) and fuel-proof it. Mask
off the tailboom and paint the forward
section of the fuselage with Klass Kote
two-part epoxy primer and paint, per the
manufacturer’s instructions, for a beautiful,
fuelproof finish.
Rigging: Slide the rudder-adjust nuts onto
the rudder stop post, and position them so
that the rudder has no left offset and can
move 15° to the right. Glue a .025-inch
music-wire hook in the position shown.
Install a 5⁄8-inch dental band between the
hook and the rudder horn.
Rig the three VIT control lines. Stretch
in the lines provides the force necessary to
hold the surfaces firmly in position.
It’s easiest to install the rudder line by
feeding a piece of .015-inch music wire
through the tube in the rear timer-mount
face, then out the rear through the VIT
cutout in the tailboom. Feed the rudder line
through its hole in the tailboom, at the front
of the rudder, and out the VIT cutout.
Cyanoacrylate-glue the rudder line to the
end of the music wire, and pull it forward
through the timer-mount hole.
You install the DT and power lines in a
similar manner. The only difference is that
the forward portion is 20-pound
monofilament and the rear 10 inches is 30-
pound braided Dacron, to provide smooth
action where the lines make a 90° turn
around the 3/32-inch-diameter guide tubes.
Route the lines through the VIT assembly
(see the isometric drawing), remount the
assembly, and complete the rigging.
Final Assembly and Checkout: Install the
engine mount with DP-460, holding it in
place with masking tape to make sure it is
fully seated against the forebody at the
proper offset angles. Redrill the fuel-line
exit holes through the engine-mount
skirt. Install the engine flood-off line as
shown on the plans.
Adding a flood-off fitting to the
Cyclon engine is straightforward. Fill the
cast round emblem on the right side of
the venturi with J.B. Weld epoxy. Insert
a snugly fit wooden dowel fully into the
venturi, and drill through the venturi
wall and tap for a 2-56 thread. Remove
the dowel and clean the area. Install a 2-
56-size pressure fitting. Reassemble the
model and make certain that all of the
systems are functioning properly.
Spend some time running the systems
repeatedly until you are satisfied that
everything operates smoothly and
reliably.
Flight-Testing: With the power and
rudder lines disconnected, hand-glide the
model from shoulder height, at glide
speed, over the proverbial tall grass. You
are looking for a light, floating glide
with the beginnings of a wide right turn.
Continue to adjust the rudder- and glideadjust
nuts until your model has a
smooth glide.
Set the power-adjust screw so that
there is a 3⁄32-inch gap between the glideadjust
nut and the upper surface of the
stabilizer. Hook up the rudder and power
lines.
Hand glide as before with a little
more speed. You are looking for a fast
glide that goes exactly where you point
the model, with no tendency to float,
nose over, or turn. Adjust the power
screw and rudder nut until you arefully seated against the forebody at the
proper offset angles. Redrill the fuel-line
exit holes through the engine-mount
skirt. Install the engine flood-off line as
shown on the plans.
Adding a flood-off fitting to the
Cyclon engine is straightforward. Fill the
cast round emblem on the right side of
the venturi with J.B. Weld epoxy. Insert
a snugly fit wooden dowel fully into the
venturi, and drill through the venturi
wall and tap for a 2-56 thread. Remove
the dowel and clean the area. Install a 2-
56-size pressure fitting. Reassemble the
model and make certain that all of the
systems are functioning properly.
Spend some time running the systems
repeatedly until you are satisfied that
everything operates smoothly and
reliably.
Flight-Testing: With the power and
rudder lines disconnected, hand-glide the
model from shoulder height, at glide
speed, over the proverbial tall grass. You
are looking for a light, floating glide
with the beginnings of a wide right turn.
Continue to adjust the rudder- and glideadjust
nuts until your model has a
smooth glide.
Set the power-adjust screw so that
there is a 3⁄32-inch gap between the glideadjust
nut and the upper surface of the
stabilizer. Hook up the rudder and power
lines.
Hand glide as before with a little
more speed. You are looking for a fast
glide that goes exactly where you point
the model, with no tendency to float,
nose over, or turn. Adjust the power
screw and rudder nut until you are