Adding Power to Plastic Scale Planes
November 1962 American Modeler
an interesting concept, but one inevitably doomed to failure simply
due to the lack of structural integrity of the airframe. You might
argue that some of the Cox plastic control line models were not
much more robust than these converted static scale kits, but at
least there was some flexure inherent in the Cox models. Author
Don Pratt approached the project from an engineer's point of view,
calculating wing and power loading, stall speed, and flight speed,
and beefing up the structure in key areas. Still, he found that
while success can be had on a limited basis, in the long run it
just is not worth the effort (IMHO). One of today's powerful, lightweight
brushless motors and a LiPo battery might work out better since
at least the severe vibration of an internal combustion engine running
a 18,000 rpm will not be trying to shake the poor thing apart.
Adding Power to Plastic Scale Planes
"Aviation Archive" researcher Don Pratt jazzes up Kansas modeling
with his glow plug engine powered "non-flying" shell scale models!
When the small styrene plastic "shelf" scale exhibition kit airplanes
appeared, I figured that eventually some control-line scale nuts
would be flying such "non-flying" models on the ends of steel lines,
but I didn't expect it to be me! Then one day in my workshop while
trying to decide what to build next, I half jokingly fitted a Baby
Spitfire .049 engine to the nose of a tiny plastic. It was not difficult
to see that there was an incompatible size differential and the
whole thing, plane and engine, felt like a lead brick in my hand.
My scale plastic control-liner dreams would have to wait until either
the engines got smaller or the plastic models got larger.
Monogram's transparent "Phantom Mustang" is powered by a
Cox .049 motor.
In another year some ready-to-fly plastic aircraft appeared.
I must say that my introduction to them was disappointing. They
were about as scale as a child's polyethylene push-toy. While single-camber
wings gave tremendous lift to overweight models that needed every
bit of it, poor penetration made them all but uncontrollable in
our healthy Mid-western breezes. Coming into the upwind side of
the circle they ballooned upward as if going for an altitude record.
Full down on the control handle was all that terminated the climb
in a mild wing-over. On the downwind side of the circle I often
found myself performing full power-on landings. I must admit that
in still air I had fun and they were practically indestructible.
Something was missing. It wasn't that they could do no more than
the simplest maneuvers. I had flown many a super-scale job so heavily
overloaded with scale detail and fittings that they could only lumber
around the circle in labored level flights. The thing that was missing
with the plastics was that I didn't build them.
Added elevator area proved too sensitive.
Here's how Revell's Douglas Skyraider looked after cowling and
decals were added. 1 1/4" dia. wheels are for off-grass flights.
Monogram Douglas Dauntless flew with Cox .020 Pee Wee &
years had passed when the non-flying shelf scale plastic kit manufacturers
decided that the public wanted larger models. Engine makers decided
there was a need for smaller engines. I began to feel that the time
was right to revive the idea of adding power to the non-flyers.
I selected Revell's $2.98 Douglas AD-6 Skyraider kit, a
Cox Thimble Drome Babe Bee .049 engine on radial tank mount, 1/32
inch, and 1/16 inch steel wire, 3/4 inch and 1 1/4 inch Perfect
rubber tire wheels, 1/2 inch Perfect tail wheel, small Perfect bell-crank,
1/16 inch Perfect spring wheel retainers, Perfect engine mounting
bolts, small Perfect control-line snaps.
turned out to be quite detailed, with bombs, rockets, "and fuel
tanks hanging below the wings. All controls were operable, dive
brakes extended, gear retracted, all from levers in the cockpit.
While all this made for a very fine exhibition shelf model, it would
require the power of a McCoy .60 to raise it off the macadam. I
decided to dispense with as much of this detail as possible to keep
the model light and simple.
Without engine cowling my converted
Skyraider weighed in at 6-3/4 ounces with its 3/4" Perfect wheels.
Model has 15" wingspan, a mean chord of 2 1/2" and 27 square inch
wing area. Thus, wing loading was just under 2 1/2 pounds per square
foot, rather high for a model, but low for a real airplane. Only
the ultra high performance sailplanes have loading below three pounds
while the average lightplane is loaded at around 8 pounds. Using
my lift tables for the nearest airfoil to that on the Skyraider
I figured the model's stalling speed at 19-mph. Again high for a
model, but very low for a full size airplane. I estimated power
loading at close to four pounds per horse power.
of the blunt nose of its airfoil section lift occurs on the scale
Skyraider rather far forward on the wing. In fact, the aerodynamic
balance point is 1/4 back from the leading edge.
tests proved that my calculations were very close. With the engine
retarded I flew several laps at 22-mph. The model lifted off after
a run of 16' from a smooth macadam surface and circled at 42-mph
with full throttle.
Over a period of two months I powered
four AD-6's and cracked up three. I learned a lot from the crack-ups.
The converted AD-6 is hot. Elevator proved ultra sensitive so I
mounted restricting bars so full travel did not exceed 1/4"- 1/16"
down, 3/16" up. I lost two models in high speed stalls caused by
too abrupt pull-ups, but found them to be quite controllable under
smooth control action. I have over 1,000 hours in the air, flying
all sizes and weights of real aircraft, and I had the impression
that the AD-6 model performed more like a real plane than any control
line model I ever flew. Non-pilot control-line fliers thought she
acted very much like a speed job.
in converting Revell's AD-6 shelf-scaler to flying control-liner
starts (top) when Pratt cuts scale control fitting mounts off inside
right fuselage. Larger firewall from sheet styrene is drilled for
mounting engine and tank. Special assembly is made for tail wheel.
Assembled fuselage - note inspection opening below stabilizer mounting
hole and angle of stab slot. Landing gear installed in center wing.
Engine, horizontal fin and elevator mounted on Skyraider
fuselage. Bellcrank, wingtip line
guides and control line lead-outs
are in place. AD-6 as first test flown without cowl and
size wheels (3/4" dia.) for takeoffs at paved field. Final configuration
shown on opposite page. Wing airfoil was something
near to a neutral laminar flow...
crude but air-worthy. As on
the real plane, horizontal fin had thrust line adjusted to
for considerable tail down-load. The wing I would leave as is, the
fin would have to be fudged.
to be a one-chance proposition. When the engine stopped, the model
made for terra ferma. There was just time enough to get the elevator
in full up position before contact with the ground. Every landing
was made in the three point attitude. There was no tendency to skip
or balloon back off after contact. The model stayed down for a short
After several test flights I decided to
do some additional finishing work on an AD-6. I removed prop needle
valve, cylinder, and head from the engine in order to slip the assembled
engine cowling over it. I glued all cowling parts together per the
kit plan. After the glue had set about two hours, I cut holes in
the cowling assembly for the engine cylinder, needle valve, air
and fuel filler vents. Then I slipped the cowling into place and
glued it. Through the openings in the cowling I re-assembled the
engine, and the model was again ready to fly.
discovered that neither the decals furnished by Revell nor the decorative
enamels normally used to paint this type of model are hot fuel proof.
I fuel proofed the decals-by spraying (not brushing) a fine thin
coat of clear butyrate dope over them 12 hours after their application
to the model. The enamel was something else! I solved the painting
problem by first covering the area I wished to paint with two coats
of white decorative enamel, sanding lightly, then painting with
three coats of colored butyrate dope. This worked very well. Butyrate
lacquer cannot be applied directly to the plastic surface of the
model in any concentration... the solvent in the lacquer will soften
and distort the styrene.
On my third model I attached the
rockets, bomb, and drop fuel tanks from the kit, mounted the landing
gear doors in down position. While the plane did fly in this form,
performance and handling were poor and I did not repeat this on
model number four. Model # 3 weighed 7-2/3 ounces.
the AD-6 models I decided to see if I could use a smaller engine
and model kit with equal results. I mounted a Cox Pee Wee .020 engine
in Monogram's $1.49 Douglas Dauntless Dive Bomber. This tiny ship
spans just 10" and weighed 4 ounces when converted to a control-line
model. Its flying characteristics can be best described as H-O-T.
I flew it on 20 foot lines, got it up and down in one piece and
never chanced it again. Too tricky and sensitive.
hope that some day we can drop in at the hobby store and pick up
a plastic kit of the AD-6 in 30" span for about $6.95 including
all goodies except the paint and power plant. By adding a McCoy
.35 we would find that the all-up weight would come out right at
28 ounces. The wing loading would be just over 2 pounds per square
foot, power loading would be 3-3/4 pounds per horse power. Once,
many years farther back than I would like to admit, I won a scale
event flying a P-40 that weighed just under three pounds, powered
by an O&R .29 with ignition, and gasoline for fuel. We had stiff
competition in those days too. Now is the plastic idea starting
to look promising?
am presently converting a twin engine kit to power. If I am successful,
I will prove that it can be done. Now it's up to the kit manufacturers
to pick up the ball and carry it from here.
from 6 to 60 who builds a scale plastic kit model this year is a
potential purchaser of a scale. plastic flying control-line model
next year. It's the next logical step for the person who lacks the
necessary skill and experience to make the big jump to balsa built-up
scale planes. How many millions of builders have stared longingly
at the plane they had just completed as it sat there on the shelf...
and wished that it could fly?
ADDING POWER TO AD-6
Meantime, want to try your hand at powering Revell's
Skyraider? Here's my construction sequence along with changes made
in the scale "shelf" kit: With X-acto knife cut away all fittings
from inside right fuselage half so nothing will obstruct free operation
of elevator control rod. Glue rudder halves together, cement rudder
onto right fuselage post off set 100 right, allow to dry.
Cement elevator halves together. On right horizontal fin halves
with X-acto knife and metal file, cut out elevator hinge guides
for free up-and-down elevator movement. Glue right horizontal fin
together with right elevator in position in hinges. Repeat for left
horizontal fin and elevator.
To correct tail download,
file slot where right horizontal fin fits into fuselage until fin
thrust line is altered 5° upward at leading edge from scale position
shown by Revell. Repeat on left fuselage half.
joining elevator halves passes through fuselage and glues into styrene
control horn. This horn was sufficient for operating elevator on
my control-line model. Fuselage openings were large enough for axle
to move freely. For access to control horn and rod end after fuselage
is assembled cut generous opening in left fuselage half under left
Glue right horizontal fin into right
fuselage half slot, passing elevator half-axle through hole in fuselage.
Glue control horn onto half-axle inside fuselage. Glue left horizontal
fin into slot in left fuselage half. When these assemblies dry glue
left and right fuselage halves together. Carefully guide left half-axle
of elevator into slot in control horn already glued to right elevator
half-axle. Check elevator and horn for free up-and-down travel.
From a 1/8" thick white model-makers sheet styrene, purchased
from a local plastics house, I cut a firewall with a coping saw
that is 1/16" larger all around than kit one. Using metal file I
hand fit it into fuselage just behind mounting lips moulded into
fuselage halves for holding kit firewall. After fitting I drill
firewall holes for radial tank mount of Cox engine .
I mounted the Cox inverted, later to upright position after tearing
one up during a hard landing. I use the regular mounting bolts for
the Cox .049, offsetting it up and to the right with two washers
at upper right hole viewed from front and four washers at lower
right hole. This results in a desirable thrust line differential
between engine and wing which offsets the rather high angle of attack
of Revell kit wing.
For tail wheel bend 1/32" steel
wire as the axle and leg; bend upper leg end into flat "S." Cut
two 1/2" squares from 1/8" thick white styrene. File grooves in
these so "S" can be imbedded between the styrene pieces as they
are glued together. File styrene assembly to tear-drop shape, glue
tail-wheel into well molded into the kit fuselage.
Build wing center section lower half and landing gear as one assembly.
Using kit's scale landing gear as a guide bend 1/16" steel wire
as shown. Carefully shape so wire leads through cradles already
fitted in lower wing center section for kit's retractable landing
gear. Use U-shaped styrene plastic kit parts for capping cradles
and securing gear.
Since I fly off a hard surface
I mount 3/4" diameter Perfect wheels onto the gear axles, secure
them with 1/16" spring retainers.
Layout wing panel
halves, ailerons, and flaps, and glue together. After these dry
sufficiently to set, quickly glue wing panels to lower half of wing
center section, and before that glue can set, glue wing center section
upper half to lower half of wing center section then adjust entire
assembly for alignment. Fit ailerons and glue securely in place,
follow with wing flaps.
To mount bell-crank to wing
center section, cut 1/8" thick styrene 1" x 3/4" and drill for bell-crank
mounting bolt. On upper wing center section is a rectangular opening
3/4" x 1-1/2". Positioned bell-crank pivot mounting bolt in this
1" back from leading edge and 1/4" in from left side. Bolt bell-crank
to styrene platform piece; glue platform to upper wing center section
so bell-crank mounting bolt fits specified location.
Form 1/16" steel wire elevator control rod so one end travels down
through hole in bell-crank arm and other end to the right through
elevator control horn. Secure rod to bell-crank with washer and
1/16" Perfect spring wheel retainer. Secure other end to elevator
control horn by making "L" bend to right of sufficient length so
rod snaps into horn and is held there by slight pressure of the
rod rubbing against inner wall of left fuselage. From "L" bend to
"L" rod is exactly 6". At the moment attach rod only to bell-crank.
Before joining wing and fuselage assemblies attach
control line lead-outs to bell-crank, cut notches in left fuselage
for these to travel through, make wingtip line guide from 1/8" styrene,
drilled for lines to pass through, glue it on wingtip. With nylon
lead-outs tied to bellcrank, drop fuselage onto wing center section
and glue securely in place. Tie small Perfect line snaps on line
lead-outs just outboard from wingtip line guide. Note that your
model is beginning to take shape and display some character.
Snap control rod into elevator control horn and check controls for
free movement. If you feel some drag, apply Vaseline to bell-crank
and elevator horn.
When satisfied with control linkage,
glue windshield, canopy, and fuselage dive brake outer panels into
position. To cover holes in wing underside where landing gear came
through glue gear doors on in "up" position. Bolt on 4-1/2 inch
Cox Nylon Prop, fill tank with Cox Thimble Drome fuel, and make
test flights on 20' nylon lines.
Posted December 3, 2011
(Seize the Day!)
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which all began in Mayo, MD. There
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