February 2004 29
✪Frank Baker
THE DOUGLAS O-46A design was preceded by the Douglas O-
31, which had a similar wing, a long fuselage, and a Curtiss
Conqueror inline engine. The wing was supported by a center post
above it and had many brace wires.
The O-43 was an improved version of the O-31, and 23 O-43s
were delivered. The O-46A was the result of a major redesign of the
O-43, in which wire bracing was replaced by wing struts and the
inline engine was replaced by a Pratt & Whitney 725-horsepower
radial engine. The result was an aesthetically pleasing aircraft with
an elliptical wing; a 45-foot, 9-inch span; and a 43-foot fuselage.
The O-46 entered service with the US Army Air Corps in May
1936, and 90 were built in all. They served in the Army Air Corps
until 1940, when they were reallocated to various National Guard
units. A squadron with several O-46As was stationed at Nichols
Field in the Philippines when Pearl Harbor was attacked in 1941.
The only surviving O-46A resides in the US Air Force (USAF)
Museum at Dayton, Ohio.
In the late 1930s and the 1940s the O-46A was a popular rubberpowered
Free Flight model. When I saw the O-46A in the museum,
its elliptical wings and slim fuselage inspired me to build a Radio
Control model of it. Since I am a small-model devotee, a 54-inch
wingspan and an O.S. .25 engine were the basis of this design.
CONSTRUCTION
Wings: When cutting the wing ribs, make them their full length and
draw, with an ink pen, the centerlines on both sides. Draw the
vertical lines on ribs 8-16 to mark the location of the front edge of
the aileron hinge spar. Glue the 1⁄16 plywood doublers on the inboard
Decades-old design
with refined lines
translates into practical
RC daily flier
02sig1.QXD 11/25/03 3:04 pm Page 29
sides of ribs 3 and 6 and on the outboard
sides of rib 7. Cut a hole in rib 7 to anchor
the inboard end of the aileron hinge spar.
When cutting the holes for the 3⁄16 x 3⁄8
basswood wing spars, make sure that 1⁄8
inch of the hole is below the rib’s centerline
and 1⁄4 inch of the hole is above. From the
front view the wing is narrow at the root,
thick in the center, and thin at the tip. If
these holes are cut improperly, you will
have problems with rib positioning.
The wing is built in three sections, but
lay all three on top of the plans at the same
time. Slip all of the ribs onto the spars. Use
short lengths of 1⁄2 square balsa to block up
all of the wing spars. Note that rib 2 belongs
on the outer wing panels.
Lay the three wing sections on the
blocks, align the spars and ribs with the
plans, and secure them. Measure the height
of the front, center, and rear of each rib’s
centerline; they should be the same. Sight
along the trailing edges (TEs) to make sure
they form a straight line.
At this point all ribs in the two outer
panels should be glued in place. Spot-glue
the center-section ribs at the top of the
spars; you will slip the dihedral braces into
place later. Add 3⁄8 x 3⁄4 balsa leading edges
(LEs) to the three wing sections.
Make the TE from rib 2 to 7 by
laminating six strips of 1⁄16 square basswood
on the edge of a 1⁄4 plywood form. As an
alternative, you could cut them from 1⁄16
plywood. Install the TEs and glue them in
place.
Use a razor saw to remove the rear
sections of ribs 8-16 at the vertical lines.
Pin together two lengths of 1⁄4 x 3⁄4 balsa
separated by 1⁄16 balsa, and glue the unit to
the ribs. Install the triangular gussets that
support each rib at the aileron hinge spar.
Now you can remove the three wing panels
from the work surface.
Use a small block plane to taper the top
and bottom of the aileron spar unit to
conform with the ribs. Use the plans to cut
the aileron plate from 1⁄16 balsa, then glue it
to the aileron LE in line with the rib
centerlines. Cut the aileron ribs from scrap
1⁄16 balsa and glue them in place. Use a
sanding block to contour these ribs to match
the overall shape of the corresponding wing
ribs.
Add the soft-balsa wingtip blocks. Use a
block plane to carve the wing LE to match
the fronts of the wing ribs. Keep in mind
that the wing is thick in the middle and
tapers to the tip and to the root. You may
have to add scrap balsa to the junction of
the LE and the wingtip for continuity.
Cut the four outer wing-strut anchor
plates from 3⁄32 plywood and install the 2-56
blind nuts. Trim the notches in the bottom
of ribs 6 and 7 until the plates match the
bottom of the ribs. You do not want a blind
nut to pop loose, so rout out short pieces of
1⁄8 x 3⁄8 basswood so they will fit over the
base of the blind nuts. Drill a 3⁄32-inch hole
in these pieces so the bolt can come
through. Glue the basswood in place over
the blind nuts.
The wing’s center-section is too thin to
hold the aileron servo. Since the wing is
reasonably thick, I mounted a Hitec HS-81
microservo in the outer left wing panel and
used nylon tubing and braided wire cable to
activate the ailerons.
You could just as easily mount servos in
each wing. Make the aileron servo plate
from 3⁄32 plywood and a servo mount from
3⁄4 x 3⁄4-inch aluminum angle stock. Install
the servo on the mount and use short sheetmetal
screws to attach the mount to the
plate. Install the servo unit between ribs 6
and 7 and adjust it so that the end of the
servo arm is even with the holes in the ribs.
Glue in some lengths of 1⁄8 square to
support the servo plate.
Cut the hatch support plate from 3⁄32
plywood and install it so that the 1⁄32
plywood hatch cover is flush with the
bottom of the wing. Remove the ailerons
and discard the 1⁄16 balsa spacers. Install the
Observation airplanes from the 1930s had
a style and charm all their own. Note fluid
fairing on the wing struts.
✪Type: RC sport
Wingspan: 54 inches
Engine: O.S. .25
Flying weight: 4 pounds
Construction: Balsa and plywood
Covering/finish: LustreKote paint
and EconoKote covering
02sig1.QXD 11/25/03 3:05 pm Page 30
February 2004 33
Struts are attached to the wing and fuselage with screws. There
are sheeted hard points in the wing for that purpose.
Woodwork that is executed this well borders on art. It’s almost a shame to cover this model’s airframe!
The engine mount is canted to allow the muffler to nestle into the
side of the cowl. That’s thinking ahead.
three aileron hinges—with removable pins—
at the centerline of the aileron spar. You
should be able to get roughly 45° of up and
down aileron throw. Attach a small nylon
horn to a 1⁄16 plywood plate and recess the
aileron LE to accept the plate where shown on
the plans.
Glue the four 1⁄16 plywood center-section
and 1⁄8 plywood dihedral braces in place. Put
the 1⁄2-inch square blocks under the centersection
spars and anchor the center-section
over the plans. Slip the outer panels onto the
center-section. The dihedral should be 11⁄8
inches, measured from the bottom of the spars
at rib 2 to the bottom of the spars at rib 16.
Use blocks to support the outer panels at the
correct position.
Use slow-drying epoxy to attach the outer
wing panels and sight along the spars to
ensure that they form a straight line from
wingtip to wingtip. Measure with a ruler the
heights of the rib centerlines to make sure
there is no twist to the outer wing panels.
Once the glue is dry, thread nylon tubing
from rib 12 on the left wing to rib 12 on the
right wing. Make sure that the tubing extends
beyond the aileron LE. In the servo bay,
remove approximately two inches of tubing to
provide for servo arm motion. Make a Z bend
in 1⁄16-inch-diameter wire and insert it into the
02sig2.QXD 11/25/03 11:46 am Page 33
The carved-balsa tail cone and one of the built-up elevators are
visible. This is a builder’s project.
34 MODEL AVIATION
outer hole in the servo arm. Use your transmitter to cycle the aileron
servo to ensure that full motion can be achieved. You may have to trim
the tubing a bit.
Use silicon sealer at ribs 7 and 9 to hold the tubing in place. Slip the
1⁄16 sheet that goes between ribs 11 and 12 over the tube and glue it in
place. Trim the outer ends of the tube so that an inch of it protrudes
from the 1⁄16 sheet.
Solder a short clevis coupler onto the cable, install a small nylon
clevis, and connect it to the aileron horn. Feed the cable through the
tubing to the opposite wing. Anchor the ailerons in neutral and install
the other coupler and clevis. Bind the Z-bend wire to the cable with
fine copper wire and solder it. Free the ailerons and use your
transmitter to check for proper aileron motion.
Obtain a 24-inch servo-lead extender and cut off the end that plugs
into the receiver. Plug the servo into the extender and leave roughly
three inches of wire in the servo bay. Feed the wire through the holes in
ribs 2-6 and use tape to hold the wire to the front of the rear wing spar.
Carve a rectangular pocket in the bottom of the right wingtip. Place
a narrow strip of wood that is tall enough to lift the upside-down wing
off of the work surface at the centerline of the wing. Add lead weight
in the pocket to balance the wing. Glue the weight in, close the pocket
with a piece of balsa, and sand it to conform with the wingtip. This
completes the wing for now.
Empennage: Because the fuselage has a long tail-moment arm, the tail
feathers must be as light as possible. Pin together two 17-inch lengths
of 3⁄16 x 3⁄8 balsa spars separated by a strip of 1⁄16 balsa. Glue a 1⁄16
plywood doubler to the center of the front of the stabilizer spar. Pin the
spars over the stabilizer plans.
Cut the ribs from 1⁄16 x 3⁄8 balsa strips and cut a 1⁄16 x 1⁄4-inch slot in
the rear of the elevator ribs at the centerline. Glue the ribs in place, add
the 3⁄16 square LE, and glue the 1⁄16 x 1⁄4 TE in the elevator slot. At the
elevator tips add 1⁄8 balsa from the spar tip to the outboard rib on the
top and the bottom.
When the glue is dry, use a sanding block to achieve a symmetrical
airfoil. Do not round the LE because sheeting needs to be added. Shape
the tips of the tailplane. Install the four elevator hinges using those with
removable hinge pins.
Sheet the bottom of the stabilizer with 1⁄32 balsa. Cut the two 3⁄32
plywood tail strut plates, install 2-56 blind nuts, and cover them with
routed-out pieces of 1⁄8 basswood. Install the plates flush with the
bottom sheeting. Stick a pin through the 2-56 blind nut and the bottom
sheeting. Enlarge the hole so that a 2-56 bolt will go in.
Sheet the top of the stabilizer and then sand the LE to shape. I used
a MAPP gas torch to braze a 1⁄16 x 1⁄4-inch strip of brass elevator horn
to a 3⁄32-inch-diameter length of copper-coated welding rod. I like to
use welding rod because it bends easier than music wire and can be
adjusted later if necessary.
Install the U-shaped elevator-horn assembly. Cut 3⁄32-inch grooves
in lengths of 3⁄16 balsa and glue them over the rear pointing part of the
rods to anchor the assembly to the elevators. Remove the part of the
elevator LE that is between the two inner ribs.
Construct the vertical fin/rudder assembly in a similar manner. The
vertical fin’s rear spar extends down into the slot in former 11. At this
point I covered the rudder with silk and applied four coats of clear
butyrate dope followed by two coats of white. I masked and sprayed
the vertical blue LE and the red flag strips. I did this now because
access to the rudder is limited when it is installed. Set the tail feathers
aside.
Fuselage: I took the plans to the copy shop and had a copy made of the
formers. I cut out the paper formers somewhat oversize and used spray
adhesive to attach them to the proper materials. Former 5 is 1⁄8 balsa,
faced on the rear side with 1⁄32 plywood.
Before removing the paper from the cut formers, mark where the
horizontal and vertical centerlines are located. Once the paper is
removed, draw these centerlines on the formers. Place the plans on the
work surface and pin the right-side 3⁄16 square stringer to them. Glue
formers 2-7 to the stringer and add the left-side stringer. Use a triangle
to ensure that the centerlines are correct and that the formers are
perpendicular to the work surface.
When the glue is dry, remove the assembly from the work surface.
The Douglas’s wing struts are fitted and then attached before
the cosmetic fairings are made.
Balsa gussets add strength to the rib/trailing-edge interface. The
dihedral braces are visible too.
02sig2.QXD 11/25/03 11:47 am Page 34
February 2004 35
02sig2.QXD 11/25/03 12:47 pm Page 35
36 MODEL AVIATION
Full-Size Plans Available—see page 199
02sig2.QXD 11/25/03 12:48 pm Page 36
February 2004 37
02sig2.QXD 11/25/03 12:49 pm Page 37
Use contact cement to glue the 3-inch-wide
1⁄32 plywood doublers that go from F1 to F6
onto two 36-inch-long sheets of 3⁄32 balsa. Cut
the fuselage sides from these with the 1⁄32
plywood facing the inside of the fuselage.
Mark the location of the 3⁄16 square
stringers on the inside of each side. Glue just
the stringer (with F2-F6 attached) to the right
fuselage side and let the glue dry. Turn the
fuselage over and repeat this process for the
left fuselage side.
Check to make sure that the stringers are
aligned with the marks and that the tops of the
sides meet correctly at the tail end of the
fuselage. Glue the fuselage sides to F2-F7.
When the glue is dry, add formers 8-11.
Install the 3⁄16 square stringer at the top of
these latter formers. Do not add the top or
bottom 3⁄16 square stringers that go from F1 to
F5.
It’s time to construct the firewall
assembly. Lay the fuel tank on K&S
Engineering number 254 tin plate and cut out
the three pieces. Make the 90° bends in the
body, and then solder the tab onto the side.
Cut the slits in the end caps, and then bend the
3⁄16-inch strips 90° to the cap. Solder the front
end cap onto the tank. Bend the short 3⁄16-inch
pieces that stick up flush to the sides of the
tank and solder them. Drill three 1⁄8-inchdiameter
holes in the front cap per the plans.
Looking at the front of the tank, the left
hole is the fuel line and the other two are
vents. Cut a length of soft-copper tubing that
protrudes roughly 11⁄2 inches from the front of
the tank, and bend the rest down to touch the
bottom of the tank approximately 3⁄8 inch from
the back of it.
Insert the tube and solder it where it
touches the bottom of the tank. Cut one vent
tube so that it goes to the back of the tank
where it is soldered to the top of the tank.
Bend one tank vent tube at a 90° angle and
insert it so that its end is roughly 1⁄8 inch from
the side of the tank near the front of the tank.
All three tubes should protrude the same
11⁄2 inches from the front of the tank. Solder
all three tubes to the front cap of the tank.
Clean the inside of the tank, and then solder
on the rear cap. Be sure to test the tank for
leaks.
Cut the firewall from 3⁄16 plywood. From
the top view the right and left sides are
perpendicular to the front of the firewall.
From the side view the top and bottom edges
are at an angle. Use a sanding block to
achieve the transition between these angles.
Drill the mounting holes for a Dave
Brown Products 2025 nylon engine mount.
Insert the four 3⁄4-inch-long, 4-40 bolts in the
holes from the rear of the firewall. Align a
pair of bolt slots and solder a length of thin
music wire in the slots so that the bolts cannot
turn. Do this for the remaining pair of bolts.
Cover both pairs of bolt heads and wires with
electrical tape.
Install the three 2-56 blind nuts that will
hold the cowl supports, and back them up
with routed-out pieces of basswood. Install
the fuel tank by pushing the three copper
tubes through the holes in the firewall. Bend
the tubes until the front of the tank is parallel
to the firewall.
Test-fit the firewall to the fuselage. You
will probably have to modify the hole in F2 to
accept the tank. Make sure that the tank does
not cover the hole for the motor control nylon
tube. Block up the fuselage and use a level to
ensure that the top of the fuselage is at 0°
longitudinally and laterally. Epoxy the
firewall assembly to the fuselage. Add
triangle stock to support the junction of the
firewall and the fuselage sides.
I like to use Du-Bro nylon antenna tubes
with braided cable for the rudder and elevator
pushrods. Feed the nylon tubes through the
holes in F10-F6 and let them extend two
inches forward of F6. Solder 1⁄16-inchdiameter
Z-bend wires to two lengths of
cable. Insert the Z wires into the brass rudder
and elevator horns.
Feed the elevator cable into its nylon tube
and position the stabilizer in its approximate
position. A length of cable should protrude
three inches from the front end of the tube.
Mark the cable for cutting at that point.
Follow the same procedure for the rudder
pushrod.
Remove the tail feathers. Install the 1⁄16 x
1⁄4-inch top stringers from F7 to F9. Sheet the
top of the fuselage from F7 to F9 with 1⁄16
balsa.
Make the servo rails from 1⁄8 birch
plywood and the servo-rail supports from 1⁄8
light plywood. Install small nylon clevises and
38 MODEL AVIATION
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02sig2.QXD 11/25/03 12:49 pm Page 38
parallel to the work surface. Cut the cable at
the servo end and solder on the brass
connector. Glue the stabilizer to the fuselage.
Recheck the stabilizer’s alignment before the
glue dries.
Attach the rudder to the vertical fin and
insert the permanent hinge pins. Slip the
rudder cable into its tube. Insert the fin post
into the slot in F11. The bottom of the
vertical fin should rest on the top of the
stabilizer. Cut the pushrod cable to the proper
length and solder the clevis connector to it.
Glue the vertical fin in position. Use your
transmitter to check the motion of the rudder
and the elevator and for the proper throw.
You may have to carve out a bit of F11 to get
good clearance around the rudder horn.
Use soft-balsa blocks to fill the junction
of the vertical fin and the stabilizer. These
blocks should go one inch up the vertical fin
and forward to halfway between F8 and F9.
The sides of the vertical fin flow into the
fuselage in a smooth, concave fashion. Look
at a picture of the O-46 to see how this
should look.
Install the two 1⁄2-inch square basswood
blocks that support the stabilizer struts. Drill
a 1⁄16-inch pilot hole in each block after it is
glued in place. Fabricate F12 using 1⁄16-inchdiameter
music wire and a 1⁄16 birch plywood
sandwich. Glue F12 to F11.
Feed a length of Du-Bro antenna nylon
tubing through the holes. The front end of the
tube should be approximately 1⁄2 inch
forward of the front servo rail. Leave three
inches of tubing sticking out of F12. Use
silicone sealer at F6 and F10 to secure the
tubing.
Turn your attention to the front of the
fuselage. You will make the landing-gear and
cabane-strut assemblies using the “sandwich”
technique. Bend the forward landing-gear
strut from 5⁄32-inch-diameter music wire.
Bend the rear landing-gear strut from 1⁄8-
inch-diameter music wire. Make sure that the
upper horizontal parts of both landing-gear
struts are the same width.
Cut four rectangular pieces of birch
plywood (three 3⁄32 and one 1⁄16) that are
roughly 1⁄4 inch larger than F3b. Cut a slot in
the 3⁄32 and 1⁄16 plywood pieces to fit the top
of the 5⁄32-inch strut. Glue all four pieces
together with epoxy and clamp them.
When the glue has set, cut F3 from the
plans and paste it onto the plywood sandwich
so that its top is parallel to the line of the
axles and the landing-gear legs exit the
former where shown on the plans. Cut the
sandwich to shape. Use this technique for the
rear landing-gear strut and the cabane struts.
To install the landing gear, block up the
fuselage upside-down on the work surface
with the cockpit area parallel to the surface.
Use slow-drying glue and clamp F3b to F3.
Adjust F3b until the axles are the same
distance from the surface.
Bend the rear landing-gear legs
forward and put F4 in place. Mark where
the legs touch the forward legs and bend
the rear legs to match the angle of the
front legs. Use masking tape to bind the
legs, and then make sure that F4 goes up
40 MODEL AVIATION
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brass connectors on the servo arms. Mount
the three HS-81 servos on the rails and
position the assembly in the fuselage.
Use your transmitter to cycle the servos,
and mark the most rearward reach of the
connectors. Cut off the nylon tubes 1⁄8 inch to
the rear of that point. Use silicone sealant at
several formers to hold the nylon tubing in
place. Glue the servo-rail assembly into the
fuselage.
Slide a length of nylon tubing through the
holes in F1-F3 and F5. Install a clevis and
connector on the throttle servo arm and adjust
the tube until adequate clearance is obtained
and one inch of tubing protrudes from the
firewall. Use silicone sealant at the back of F1
and the front of F5 to hold the tube in place.
Since the rear of the fuselage is so narrow,
there is little support for the stabilizer. To
provide a better platform, use 1-inch-thick
soft-balsa blocks to fill the area from F9 to
F11 on each side of the fuselage. Leave a 3⁄8-
inch slot between the blocks and rough-carve
them to shape. Hollow the blocks, but leave a
thick area at the top to act as a base for the
stabilizer. Glue the blocks in place and finalsand
them. Carve the top of the blocks to the
shape of the bottom of the stabilizer.
Block up the fuselage so that the top of it
is at 0°. Lay a level across the cockpit area
and make sure it is parallel to the work
surface. Insert the elevator cable into its tube.
With the elevator held in neutral, pin the
stabilizer to the fuselage. Make sure that it is
02sig2.QXD 11/25/03 12:49 pm Page 40
February 2004 41
into the bottom of the fuselage sides 1⁄8
inch.
Install the 3⁄16 square bottom stringer. The
section from F1 to F2 will have to be cut from
3⁄16 sheet and spliced onto a 36-inch length of
3⁄16 square. Remove the masking tape, bind
the landing-gear legs with copper wire, and
solder them together. Now you can remove
the fuselage from the bench.
Cut the wing-strut tab rails from 1⁄8 birch
plywood and make the four 1⁄32-inch
aluminum tabs per the plans. Position the rear
tabs on the rail so that the 5⁄64-inch-diameter
hole is 3⁄16 inch beyond the outside of the
fuselage. Use two 3⁄32-inch flush rivets to
attach the tabs to the rear rail. Cut a 1⁄32-inch
notch in the bottom of the fuselage sides and
install the rear rail with the tabs facing the
bottom of the fuselage.
Attach the tabs on the front rail using #2 x
1⁄2-inch sheet-metal screws through the
plywood into the tabs. Place the rail, tabs
down, on the top of F3b and run the screws
into the former.
Build the battery tunnel from 1⁄8 balsa. The
floor should be glued to F3, F4, F5, and both
tab rails. Bend the 1⁄8-inch-music-wire wing
cabane struts per the plans.
Use the “sandwich” technique to fabricate
F3a and F5a. Block up the fuselage so that it
is parallel to the work surface lengthwise and
crosswise. Insert these two formers into the
fuselage and use slow-drying epoxy. Use an
incidence meter to ensure that the wing will
be at 1° incidence. Check to see that the wing
center-section will be parallel to the work
surface.
Install the fuselage top 3⁄16 square stringer.
The section from F1 to F2 is cut from 3⁄16
sheet and spliced onto a 8-inch length of 3⁄16
square. Install the 1⁄16 x 1⁄4 stringers on the
bottom of the fuselage. Install the two upper
stub ribs and the four lower stub ribs between
F1 and F2. These ribs will be sanded to shape.
Sheet the top and bottom of the fuselage from
F1 to F5 with 3⁄32 balsa sheet.
Cut thin poster board to the approximate
size needed to cover half of the fuselage
bottom from F5 to F12. When you achieve a
reasonable fit, cut a right and a left copy from
1⁄16 balsa sheet. Trial-fit the sheets to the
bottom of the fuselage.
To accommodate the compound curve,
make a 11⁄4-inch-long V-shaped cut at F7
from the centerline downward along F7. Start
with a narrow slot and increase it until the
sheet conforms to all of the formers.
After the bottom is sheeted, rough-carve a
block of balsa for the tail cone. The upper part
of the tail cone conforms to the bottom of the
rudder. Hollow out the tail cone to a thickness
of approximately 3⁄16 inch.
Drill a 1⁄8-inch-diameter hole to
accommodate the antenna tube, slide the
block onto the tube, and then glue the block to
F11 and F12. Fill the rectangular space
between F11 and the bottom of the stabilizer
with 3⁄32 balsa. Final-sand the rear of the
fuselage and the tail cone.
On the left side of the fuselage just behind
the rear cabane strut, drill a 5⁄32-inch hole
diagonally down through F5a and F5 so that it
exits F5 1⁄4 inch below the top of the fuselage
side.
Carve two 1-inch-thick, 3-inch-wide softbalsa
blocks to fit the side of the fuselage
from F1 to a point 11⁄4 inches to the rear of
F2. Rough-carve the blocks to match the
shape shown in cross-section A-A on the
plans.
You will have to add blocks at the top and
bottom of the side blocks. The top and bottom
of these side blocks curve around and
smoothly join the top and bottom of the
fuselage. Behind the A-A cross-section the
blocks taper linearly to the side of the
fuselage. This is a result of replacing the O-
43’s inline engine with a radial engine.
Landing Gear: The landing-gear legs begin
as inside and outside 3⁄8 balsa pieces. Roughcarve
the fuselage ends of them to match the
fuselage’s curvature. Press each half against
the landing-gear wires, and then carve a half
circle in each along the depressions made by
the wires. The halves should meet when
placed over the wires.
Final-fit the balsa to the fuselage
curvature. Solder the 13⁄8-inch-diameter .010
brass plates to the main landing gear. Cut the
balsa to fit the brass plate at the lower end of
the leg. Rough-carve the balsa to the airfoil
shape.
The junction of the top of the landing-gear
leg and the fuselage forms a straight line
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42 MODEL AVIATION
parallel to the fuselage centerline. On the
inner side the landing-gear leg flows into the
bottom of the fuselage for its full length. At
the wheel end the landing-gear leg curves out
to the brass plate in all directions. You will
need to add balsa to make both junctions.
I have inspected the full-scale O-46A.
The inner panels that flow to the bottom of
the fuselage are not attached to the fuselage
and move downward when the landing gear
spreads. The halves of the landing-gear legs
can be glued to the wires and the fuselage,
and then the final carving and sanding can be
done. Use a cutoff wheel to trim the landinggear
axles to accept the Williams Bros. 31⁄4-
inch-diameter smooth-contour wheels.
Cockpit Hatch: Glue 1⁄8 square basswood
strips to the inside of the fuselage along the
top of both sides of the cockpit area. Cut a
piece of 1⁄8 balsa sheet to fit the top of the
fuselage from F5 to F7 for the cockpit floor.
Lay plastic wrap over the cockpit area.
From the outside, pin lengths of 1⁄8 square
basswood to the insides of the previous strips
so that the plastic wrap is between them. Add
glue to the tops of the strips, and lay the 1⁄8
balsa cockpit floor on the glued strips. Place
weights on top to hold down the balsa.
When the glue is dry, remove the pins, the
floor, and its strips. Add three 1⁄8 x 1⁄4 cross
strips of basswood. At the rear add a 3⁄4-inchwide
piece of 1⁄8 plywood across the bottom
between the two 1⁄8 square pieces. On top add
a 3⁄4-inch-wide piece of 1⁄16 plywood across
the full width of the floor. Drill a 11⁄64-inch
hole in the center of this piece that goes all
the way through. Cut a piece of 1⁄8 plywood
that fits across the fuselage under the 1⁄8
square side strips and against F7, and then
glue it in place.
Cut the hatch tab from 1⁄32-inch
aluminum, drill several holes in it, and epoxy
it to the front bottom of the hatch so that the
tab sticks out roughly 1⁄8 inch. Cut a slot in F5
to accept the tab.
Rough-cut a balsa block to form the front
of the cockpit that is under the windscreen,
and glue it to the cockpit floor. Put the
cockpit hatch down in the cockpit area so that
the tab goes into the slot and the rear of the
hatch slides down F7. You will have to sand
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the bottom of the rear of the hatch to half
round for it to slide into place. Final-shape
and sand the front of the cockpit.
With the hatch in place, drill a 7⁄64-inchdiameter
hole in the bottommost 1⁄8 plywood
and tap it for an 8-32 nylon bolt.
Wing Installation: Because there is little
space between the wing and the top of the
fuselage, I covered the top of the fuselage
from an inch in front of F2 to F5 with Sky
Blue covering material. Cut four cabane-strut
retainers from 1⁄8 plywood and four caps from
1⁄16 plywood. Slip the wing onto the four
struts and put on the retainers and caps. Hold
them in place with clamps.
Block up the fuselage so that the cockpit
floor is parallel to the work surface
lengthwise and sideways. Use your incidence
meter to ensure that the wing, measured at rib
7, is at 1° positive incidence.
Attach a length of white thread to a pin
and stick it in the top of the vertical fin. Use
the thread to make sure that the distance to
the rear spar is the same at both wingtips.
Measure the vertical distance from the work
surface to the bottom of the wingtip and
make sure they are the same. Adjust the wing
on the cabane struts until all of these
measurements are correct.
Unclamp one of the struts. Coat both
sides of the retainer and its slot with quicksetting
epoxy. Replace the retainer, the cap,
and the clamp. Repeat all of the
measurements and check the incidence. Once
the epoxy is set, repeat the process for each
strut.
Cut pieces of 3⁄32 sheet to fit the bottom of
the center-section. Make a cutout at the rear
left corner for the servo lead to exit the wing.
Feed the servo wire through the slanted hole
in F5 and pull it until it will fit against the
rear cabane strut. Solder the servo connector
onto the wires.
Glue the bottom sheeting on the centersection.
Glue the 3⁄32 sheeting on the top of
the center-section. These sheets provide a
strong place for one to pick up the model.
Cut a piece of 3⁄4 balsa to fit between the
number-2 ribs and behind the rear spar. Cut
the U-shaped rear contour with a band saw.
Rough-carve the balsa to shape, glue it in
place, and final-sand it. Add strips of 1⁄16 x 1⁄2
02sig2.QXD 11/25/03 12:50 pm Page 42
February 2004 43
balsa along the outside of the bottom edges
of the number-2 ribs. This will aid in
covering.
Wing Struts: This model’s wing struts are
functional and will be attached using 2-56
bolts. An unusual feature is the large fairings
at each end of a strut.
Cut 3⁄16 x 5⁄8-inch strips of basswood a bit
longer than shown on the plans. Carve and
sand them to an airfoil shape. Leave the
fuselage end square and position it against
the end of a strut tab. Mark the wing end at
an angle to match the bottom of the wing as
the center of the strut is positioned over the
2-56 blind nut. Cut the strut approximately
1⁄32 inch shorter than the mark. Use a saw to
cut slits in line with the long axis of the strut
airfoil, roughly 3⁄4-inch deep at both ends of
the strut. Cut the upper strut tab from a piece
of .020 sheet aluminum and bend the end of
it to match the angle of the strut end. Fill the
outer strut slot with epoxy and slip the tab in
place. The bent end of the tab should fit tight
against the slanted end of the strut.
Cut the lower strut tab from .016 brass
sheet, drill a 3⁄32-inch-diameter hole at the
end, and solder a 2-56 nut to the tab. Insert
the tab in the slit at the fuselage end until the
nut barely touches the end of the strut.
Recheck the strut’s fit between the
fuselage and the wing. When it fits
reasonably well, use a short 2-56 bolt to
attach the strut to a front fuselage tab.
Repeat this process for the strut that goes on
the rear tab on the same side of the fuselage.
Block up the fuselage so that the top of
the fuselage in the cockpit area is parallel to
the work surface. Use your incidence meter
to check that the wing at rib 7 has 1° of
incidence. Mark the outer end of the strut
where the 2-56 bolt will go through and
screw into the blind nut. Remove the strut
and drill the 5⁄64-inch-diameter hole
perpendicular to the slanted end of the strut
and through the aluminum tab.
Drill 1⁄16-inch-diameter holes in each end
of the struts that will go through the metal
tabs. Run #2 x 3⁄8-inch sheet-metal screws
through the struts and the tabs. Use a cutoff
wheel to grind the pointed ends of the
screws flush with the struts. This will ensure
that the tabs will not pull out. Reinstall the
strut and put in both bolts. The incidence
should not change. Repeat this process for
the rear strut and the two struts on the
other wing. All four wing struts will
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remain bolted to the airplane.
Cut eight 21⁄2-inch-wide x 23⁄4-inch-long
balsa blocks with the grain running the long
dimension. Draw a centerline 11⁄16 inch from
the front of the blocks. Position a block on the
side of the fuselage and under a wing strut so
that the centerline is underneath the fuselage
bolt head. Use a pencil to trace the wing strut
onto the block. The lines will angle toward
the rear. Copy these lines onto the inside of all
of the blocks.
Pair up the blocks and carve half an airfoil
in each member of the pair between the
slanted lines. Do not make these grooves too
deep; that will complicate making the
fairings. The halves should just fit together.
Start with the fuselage end and slip a pair
of blocks onto the strut. Use a soft pencil to
mark the fuselage curvature on the blocks.
Remove them and use a band saw to cut the
curve. Slide the two blocks back onto the
lower end of the strut and carve a space out of
the ends of the blocks to accept the strut tab
and bolt. Glue the pair of blocks onto the
wing strut. Make sure that they are tight up
against the fuselage.
Repeat this process for the fairing at the
upper end of the wing strut. Notice that the
upper end of the strut and the blocks are flush
with the bottom of the wing.
Locate the strut bolts, cut small holes to
back out the bolts, and remove the strut from
the wing. Cut the airfoil-shaped fairing ends
from 1⁄64 plywood, as shown on the plans. Cut
a rectangular hole in a lower end fairing piece
so that the strut and fairing can slip onto the
fuselage strut tab. Epoxy the plywood to the
strut’s balsa. Press the plywood hard against
the balsa to get a tight fit. Repeat this at the
wing end of the strut.
Make sure that the airfoil shapes are
pointed forward. Use a sharp knife to carve
the fairings to shape. The front and back final
curvatures start at the centerline of the nose
and tail of the wing strut and end at the
centerline of the plywood end pieces.
Carve and sand the top and bottom blocks
to the shapes shown on the plans. The fairings
should flow smoothly onto the struts with no
discernable joint. The ends of the fairings
should be approximately 1⁄2 inch from the bolt
holes. You will repeat this tedious process for
the three remaining struts. Although they take
considerable time to make, they are one of the
aircraft’s distinguishing features.
When the struts are installed for good, be
sure to put lock washers on the four 2-56 bolts
02sig2.QXD 11/25/03 12:50 pm Page 43
at the fuselage sides. You do not want these
bolts to loosen and fall out.
Cowling: I fabricated the cowl from
fiberglass cloth and epoxy resin using the
“balloon” method. However, the cowl is
simple and could easily be constructed from
1⁄32 plywood wrapped around balsa rings.
I used a lathe to make a cowling form that
was 3⁄4 inch longer than the cowl and had a
30° slant to have the cowl flaps open. Use a
band saw to cut off this slanted part so you
have two forms. When the resin has
hardened, cut the two cowl pieces to length.
Epoxy the slanted ring to the rear of the front
part of the cowl. Lay some fiberglass cloth on
the inside of the joint to reinforce it.
Cut holes in the cowl to fit the O.S. .25 FP
or other engine this size, and support the cowl
with 3⁄8-inch-wide aluminum angles bolted to
the firewall at the three blind-nut positions.
Eliminate the pinholes with filler and sand
the cowl. Now that the engine is mounted,
finish fabricating the throttle pushrod. Use
your transmitter to check the high and low
positions.
Finish: An excellent source for the O-46A’s
color scheme and markings is the
Squadron/Signal booklet Air Force Colors
Vol. 1, 1926-1942. It contains good color
schemes and excellent pictures.
The O-46A in the USAF Museum has a
sky-blue fuselage with yellow wings and tail
feathers. Most pictures of the aircraft show a
metal fuselage with the rest of the airplane
painted silver. I used a 1930s color scheme as
depicted in the booklet.
I sprayed three coats of LustreKote primer
on the cowl, landing-gear legs, wing struts,
and fuselage forward of the front cabane
struts. Then I sanded those areas and sprayed
on several coats of LustreKote Sky Blue.
I covered the rest of the fuselage with Sky
Blue EconoKote and covered the wings and
tail feathers with Yellow EconoKote. When
the ailerons and elevators were covered, I
installed the permanent hinge pins. I cut the
star insignia and the “US Army” on the
bottom of the wing from trim film. I used few
other markings.
I glued strips of 1⁄8 x 1⁄4 balsa behind each
cabane strut and sanded them to an airfoil
shape, and then I covered the struts with the
Sky Blue film. I painted the cockpit floor with
gray butyrate dope.
Although the O-46A has a long, enclosed
cockpit, I opted for open front and rear
cockpits. I molded the front windscreen and
the central enclosure over balsa forms using
.040 acetate sheet and 250° in the kitchen
oven.
I used Williams Bros. 1⁄8-scale standard
pilots for the pilot and the rear gunner.
Fabricate the stabilizer struts from 1⁄4-inch
streamlined aluminum tubing that is flattened
at each end. Use #2 x 3⁄8-inch screws at the
fuselage end and 2-56 x 1⁄4-inch bolts at the
outer ends. I think you will find the finished
model appealing.
Flying: After the first test flight I reflexed the
ailerons up approximately 1⁄8 inch, which
eliminated the aileron sensitivity. I added
roughly 3⁄4 ounce of lead to the nose to reduce
the elevator sensitivity. The engine-mount
bolts that point forward serve as mounting
points for the weights.
Takeoffs are accomplished by letting the
model run on the ground until it achieves a
high rate of speed. A touch of elevator will
cause it to take off and initiate a shallow
climb. Because of the model’s high parasol
wing and narrow landing gear, it does not like
crosswind takeoffs. Most important, do not
try to “horse” it off of the ground before it
attains adequate speed.
The O-46A is fast in the air, and you can
back off on the throttle once it reaches a
reasonable altitude. When this model passes
overhead, the sight of the yellow elliptical
wings against the blue sky brings back the
days of the 1930s Air Corps.
Landings are beautiful. Throttle back to
idle on final approach and let the model glide
in. It will have a flat but slightly nose-down
glide, and once a few feet above the ground, a
light touch of elevator will result in a perfect
three-point landing. MA
Frank B. Baker
5301 Burnett Dr.
Madison WI 53705
44 MODEL AVIATION
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02sig2.QXD 11/25/03 12:56 pm Page 44
