Airplanes and Rockets visitor Ray L.
wrote to say he has a Biceps model that appeared in the April 1969 edition of American
Aircraft Modeler, and that he would like to have the article posted. Per Roy, "I bought
this model from one of our club members who is a team race flier and built it on a whim,
he fitted it with electric and had it test flown by one of our aerobatic pilots and after
that he lost interest it and I was happy to give it a good home, the quality of build
and finish is to pro standards." Power in the original was a Fox .59. AMA Plans Service
still prints the Biceps
A muscular, big-engine bipe for sport and exhibition control-line flying.
By Donald Yearout
Nearly every airplane nut has a soft spot for biplanes.
There is something nostalgic about a two-winger, the whistle of wind through rigging
wires and all that stuff.
When our local model club began control-line exhibition flying to stimulate public
interest and recruit new club members, I became enthusiastic about a bipe for that type
of flying. It would be an attention-grabber, an extremely stuntable and flashy aircraft.
The ship I had in mind would be large; a small plane just doesn't get attention. The
further I studied the biplane configuration, .the more intriguing it became.
It appeared that the built-in head-wind high-drag characteristic to biplanes might
be used to advantage. A large engine to turn a big low-pitch prop would give a very high
static thrust. In combination with the high-lift high-drag layout this would result in
a relatively slow-flying, extremely maneuverable craft - like a helicopter with its rotor
facing forward! And that's the way it turned out.
A few additional design features improved performance, such as: thick, blunt airfoil
sections with full-span flaps on both wings to help eliminate the wobbling or staggering
at low speeds common to biplanes during tight turns. A large elevator surface proved
effective at low speeds. A large rudder with quite a bit of turn-out kept the lines tight
at all times, since centrifugal force is not much of a factor at low speeds. There you
The theory sounded good, but the proof came in building and flying the brute. Biceps
is a spectacular performer, a real ball to fly. Although not a smooth, precision, contest-type
ship, it should give a good account of itself in any contest in the hands of a competent
pilot. For exhibition-flying, Biceps is superb, the hit of any show. It is not particularly
difficult to build. Large size and straight lines contribute to simplicity. It does take
a considerable amount of balsa.
This completed Biceps is owned by Ray L.,
of the UK. He was kind enough to send me the photo. He bought the model from a fellow
club member. I would never be able to part with such a nice airplane! (click for larger version)
high-drag, with big prop, make the bipe fly like a 'copter with rotor facing forward.
Oodles and gobs of wing area, thick airfoils
and full-span flaps add up to a precision in flight that must be seen to be believed.
You'll look good!
Tail surfaces: Both the fin-rudder and the stabilizer-elevator assemblies
are made from medium-weight sheet balsa; wood grain parallel to hinge lines. First glue
balsa sheets together edgewise to obtain enough width for entire piece to be cut from.
Then shape to a streamline section similar to that shown on the plan. I have found that
sanding boards (sandpaper of various grits glued to a flat piece of 1 x 2" pine about
8 or 10" long) make excellent tools for rough-shaping balsa wood before final sanding,
prior to painting and finishing.
After rough-shaping, thoroughly sand with progressively finer sandpaper until you
are satisfied with the job. Then seal the wood grain with a couple of coats of clear
dope. I use and recommend Aero Glass products. They cost a little more but the quality
is worth it. A couple of coats of filler should be applied next, again lightly sanding
between coats. Now cut the rudder and elevator apart from their respective assemblies
and sand the cut edges, rounding them slightly and follow by doping to seal the wood
Now glue the rudder to the fin with 1" of turn-out measured at the trailing edge.
For the elevator, recess the edge as necessary for the control-horn assembly; I used
a prefabbed unit available at most hobby stores. Cement or epoxy the horn in place. Next
attach the hinges, cloth, metal, or plastic as you prefer - I used cloth. Dope over them,
seal, and fill the surface. A drop of oil applied to the center of each hinge before
doping will keep them flexible. The final finish coats of dope will be applied after
the surfaces are attached to the fuselage.
Wings: There isn't anything unusual in their construction except
for the wing-strut-attachment method, which will be explained in detail later. First
cut out the ribs. You may think there are a lot of them and there are, but the close-rib
spacing really looks nice on the completed ship; it serves a real purpose too. On a thick
airfoil, close-rib spacing is necessary to prevent the covering from sagging, which would
defeat the purpose of the thick airfoil. Cut the ribs from medium-soft 1/16 sheet balsa.
I made metal templates for the W1 and W2 ribs, so I could cut out about a dozen at a
time which helped somewhat.
Make up the wing spars from hard, straight balsa - 1/4 x 1" for the front spars and
1/4 x 3/4" for the rear. Splice as necessary to gain the proper lengths. Slide the full-length
ribs, one at a time, onto both the front and rear spar making sure you do not forget
the W3 center rib in the top wing and the W7 rib in the bottom one. When the ribs are
in their approximate position, pin the spars down directly over their corresponding plan,
blocking them up to clear the ribs with balsa scraps - 1/8" thicker scrap under the rear
spar than the front. Next, slide the ribs into their exact positions and spot-glue them
in place. Later you can cement more thoroughly after removing the wing from the plan.
The trailing edges can be installed next. They may be shaped either before or after
attaching. I used a razor plane on the original model which worked very well. Next put
on the 1/4 x 1 leading edges after beveling one edge of each so they fit together nicely.
Before trimming the leading edges to final shape (again using a razor plane or substitute),
install all the false ribs (W2, 5 & 6) in their proper positions and cement thoroughly.
Make darned sure you don't forget to put in the wing strut mounting blocks (made from
1/2" hard sheet balsa). Cement them thoroughly in their proper locations - bottom of
the top wing, and top of the bottom wing.
the wing tips and other finishing pieces such as top wing center-section trailing edge
and bottom wing center-section trailing edge and bottom wing center-section sheeting.
Carefully sand entire structures, dope and sand again. The wings should now be ready
for covering. I used silk on the original ship for durability although other covering
materials may be used as well. Dope with three coats of clear and two of color for reasonably
good finish, substituting a coat of filler for the third clear coat if desired.
Fuselage: Make the engine mounts first, using very hard wood (maple
was used on the original Biceps). Each mount measures 3/8 x 3/4 x 12 1/8". Drill the
mounting holes to fit the engine you are going to use. A Fox 59 was used in the first
Biceps and worked out beautifully - a very powerful, dependable engine. Also drill the
cabane strut in the location shown on the plan. Now mount the engine on the mounts using
#6-32 machine screws. A small amount of out-thrust can be added at this time if desired,
although it is not absolutely necessary.
Next cut out the 1/16 plywood doublers and the 1/8 ply firewall (patterns are shown
on the plan). Place the firewall in position on the mounts and apply epoxy glue to the
joined surfaces. Lay the mounts upside down on a flat surface with the engine and firewall
top hanging over the edge. Use a piece of wax paper or plastic wrap under the mounts
to keep the epoxy off the surface. Glue the 1/16 plywood doublers to the engine mounts
and firewall with more epoxy and then let everything cure.
Now cut out the 1/8 fuselage sides and 1/16 balsa doublers (grain-vertical on the
latter) from medium-weight sheet balsa. Then cement the 1/16 doubler pieces against the
inner surface of the 1/8 side parts; be sure the cutout for the lower wing is cut out
through both parts. Now cement fuselage side assemblies to the plywood doublers, putting
on formers F1 and F2 at this time. After allowing the glue to dry overnight, bring the
aft ends of the fuselage together and cement them with a wedge of scrap balsa for reinforcement.
Install the rest of the fuselage formers, bellcrank and mount assembly and landing gear
Attach a couple of 6-32 nuts to the upper surface of the gear mount while you can
still get at it. Do not put the top and bottom sheeting on yet. You can, however fit
the fuel tank in and build up a supporting plate of balsa underneath and on top. I used
a plastic "clunk" R/C tank in my Biceps, but a metal control-line tank could be used
as well. One advantage to the plastic tank is that it is removable for repair or replacement
through the round hole in the firewall. If a metal tank is used, the firewall hole may
be eliminated. No fuel tank fill and vent lines are shown on the plans because of the
option of tank type used.
Landing gear: This is made from 1/8 spring aluminum stock. A Sig
landing-gear blank was used on the original model. The aluminum spring stock is cut to
size and bent to shape as shown on the plan, being very careful to use large radius bends.
A small radius bend will very likely crack the hard aluminum stock, in which case, you've
had it. Nothing to do but start over again with another blank. The wheels are attached
with No. 8 machine screws (see plan for details).
The wheel pants are built up from three laminations of balsa with a plywood insert
in the side between the wheel and gear leg. Use epoxy glue between the plywood insert
and landing gear leg, and epoxy putty to fillet in for extra strength to finish off the
job. You will probably have to drill out the wheel hubs with a #19 drill to make them
fit a No.8 machine screw since most wheels have smaller hub holes than that. Mount the
gear on the fuselage with two No. 6 flat head machine screws, countersinking the heads
flush with the surface.
Miscellaneous assembly: Install the lower wing. Before you slide
it through the holes in the fuselage however, put wing flap horn into position. The flap
horn is made from 3/32 music wire. The sheet metal lever arm is silver-soldered to it.
Glue the wing in position and connect the bellcrank to the flap horn. Install the flaps
by attaching the hinges (fabric or metal).
Now install the stabilizer and attach the elevator to the flap horn. Thoroughly check
out the control system and modify as necessary before going further. Next, install the
tail wheel and mount. Before going further, all four wing struts must be built and attached.
The struts are made from music wire as shown on the plan. Spiral wrap all tips with copper
wire and solder all joints. Now see if you can fit the bottom ends of the cabane struts
into the corresponding holes in the engine mounts. If you can - great! If not, bend the
struts to fit or enlarge the holes in the engine mounts.
Place the interplane struts into position in the lower wing and fit the top wing onto
all the strut tips. Make a couple of cardboard jigs (shown on the plan) to align the
top wing properly. When everything fits, epoxy the lower ends of all the struts into
their mating holes and allow to cure undisturbed. Remove the top wing and install the
top and bottom sheeting on the fuselage. Round all corners and edges, install the rudder,
and start applying the finish. At least two coats of clear followed by filler and color
coats. Attach the top wing again, using the cardboard jigs and the old faithful epoxy.
After attaching the top wing flaps, install the flap linkage rods and make sure they
work without binding.
Build the flap linkage rods from 1/8 brass tubing flattened slightly to an oval shape.
Cut length carefully so the flaps will be even and parallel. Drill 1/16 holes through
the rod tips so they will end up at least 1/16 below the flap surface. The hinge pins
at both ends of the flap rods are 1/16 music wire, 7/8" long. The tubing pieces that
complete the assembly are 1/16 ID x 1/16" long, either brass or aluminum. Grease the
rod tip, hinge pins and tips of the tubing (not the center) with Vaseline.
Place the assembly in position in the grooves in the flaps and cover with epoxy. Try
to keep the epoxy away from the oval rods to prevent binding later. It is much easier
to do the lower wing hookup first, then after that is cured, turn the ship over and repeat
the hookup to the top wing. Be certain that the flaps are even and parallel before the
last hookup is made. It is darned hard to correct later.
Build the cowling from sheet balsa, being sure to dope generously inside. The cowling
may be attached using a couple of screws through the side into the engine mounts. After
completing all the little details like the spinner, windshield, line guides, etc., it
should be completed. Nothing left but some spectacular flying.
Biceps Plans Sheet 1
<click for larger version>
Biceps Plans Sheet 2
<click for larger version>
The AMA Plans Service offers a full-size
version of many of the plans show here at a very reasonable cost. They will scale the plans any size for you. It is always
best to buy printed plans because my scanner versions often have distortions that can cause parts to fit poorly. Purchasing
plans also help to support the operation of the Academy of Model
Aeronautics - the #1 advocate for model aviation throughout the world. If the AMA no longer has this plan on file, I
will be glad to send you my higher resolution version.
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Posted July 16, 2012