BY PAT JOHNSTON
A midsize Stunt
model with the right
combination of looks
and performance
MY PURPOSE IN designing this Mustang was to develop
a midsize model that could be powered with engines in the
.46 to .53 range so handling in the wind would not be a
chore for a person my size. In stiff winds the larger 60-
size airplanes pull my 140-pound frame around more than
I prefer.
Perhaps filling my pockets with lead would help. Past
that, a smaller model is the logical solution.
I wanted an airplane that would fly competitively and
look the part of larger designs. This Mustang has a
relatively deep fuselage along with dihedral to give it a
more commanding, scalelike presentation.
The general observation is that it is a “normal”-size
CL Precision Aerobatics (Stunt) model. It is not far off;
the wing area is 626 square inches, compared with the
customary 650-700 square inches of most typical-size
Stunters. That is dimensionally close to 96% of a 680-
square-inch wing.
The Mustang is fun to fly! It has the feel and flight
characteristics of a serious competition Stunter and superior
visual appeal. The scalelike appearance is half of the
It looks like the Mustang is ready to provide chase-airplane
duties for its big brother B-25. The P-51D is a clean model!
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The wing rib patterns are glued to the
balsa for cutting out ribs.
Hinge reinforcement blocks are installed
before the top layer of TE sheeting is
glued on.
The fiberglass is laid up with epoxy over
the foam mold, and then the foam is
removed.
Landing-gear mounting blocks are
laminated from 1/16 plywood and 1/8
plywood strips.
Fully frame and box in the landing-gear
blocks before installing the LE sheeting.
Shape the foam cowl mold, which must
be undercut 1/32 inch to allow for the
thickness of the fiberglass.
The wing halves are built on fixture rods
to ensure correct alignment.
You can see the maple engine mounts,
the tank-mount area, and the rear cowl
hold-down.
The finished fiberglass cowl has adequate
vents to allow hot air to exhaust from
the cowl.
Photos by the author
equation and the color and trim scheme are the other part.
An unplanned additional component was the Magnum .53
engine and a tongue muffler. The flight judges have commented
that the sound just comes out “Mustang.”
The .53 is a Leonard Neumann (of Stuka Stunt Works)
modified engine swinging a wood 13 x 4.5 propeller at 8,500
rpm. The sound is pleasing and appropriate for the model. The
Magnum .53 is out of production, and I have replaced it with a
clone engine—the ASP .53—which Leonard still sells.
The genesis of the paint scheme were discussions my friend
Jack Pitcher and I had while observing Paul Walker’s colorful
Mustang Stunter awhile back. Jack has a terrific-looking Stunter
he has flown for years he calls the Centennial. The
workmanship and finish are top-notch, but the model is painted
mostly blue.
The result is that the Centennial earns fewer appearance
points than a flashier aircraft. Jack asserts that the airplane
needs to be “flashy” to score well, and blue is not.
As I looked at Paul’s big Mustang I envisioned a similar
paint scheme with the American eagle morphed onto the side.
Al Rabe used an eagle on one of his Bearcats from the late
1960s, and it was a striking trim scheme.
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06sig1.QXD 4/23/07 2:44 PM Page 30
I also wanted to use a bright blue and red. Brodak
Manufacturing sells a line of dope with a selection of colors that
includes “Miss Ashley Red” and “Miami Blue.” Both were just
what I was looking to find.
The Mustang is my third model to use this scheme. The first was
my big Bearcat II, followed by a classic Al Rabe Bearcat. Both
received high appearance points, and the judges always gave them
strong flight scores. With this experience I decided not to deviate
from this established theme.
For anyone who wants to do this trim scheme, I have drawn it on
AutoCAD and will be happy to send it out. I enlisted a sign shop to
cut the myriad stars with a low-tack vinyl masking material. That
worked out beautifully and saved many hours of cutting stars by
hand from frisket paper and then dealing with removing that frisket
paper. Making 36 stars is a lot of work, and the vinyl masking is
worth every penny.
Windy Urtnowski provided his cast epoxy-resin exhaust
stacks, which give the nose a nice touch of realism. He
recommends hollowing out the stacks to create the appearance
of actual exhaust openings. It does look very good that way.
I resisted the temptation to airbrush smoke residue out the
exhaust stacks and down the side of the model. It looks great on
warbirds, but this Mustang is representative of a Reno Air
Racing airplane; the pit crew would not allow its jewel to show
smudges down the sides. Constant cleaning and a fresh wax job
would be more like the real thing.
I made the ink lines with a 2.5-point-size Rapidograph ink
pen. I prefer a slightly bolder line weight than from a pen in the
1-point-width category. As in Stand-Off Scale the panel lines
need to be visible, and the 2.5-point-size pen is almost perfect.
The idea is to create the illusion and look of the Mustang
and make people believe it is one, even though it’s not scale. I
am pleased by how well this design accomplishes that goal.
Construction Overview: The Mustang’s construction methods
are relatively normal with a few exceptions. The most obvious
deviation is the double taper in the wing to make the center
All internal stabilizer structure is built on
the top sheeting piece.
A second fiberglass cowl was made for an
engine change to the Magnum .53 on the
author’s first Mustang.
The bottom stabilizer sheeting is epoxied
on, and lead shot in baggies holds it down.
The author makes his own adjustable tailhorn
hardware.
The completed tail reveals the balance
tabs on the elevators and rudder. This
adds realism.
Type: CL Stunt (Precision Aerobatics)
Wingspan: 56.38 inches
Flying weight: 52-57 ounces
Wing area: 626 square inches
Length: 42.5 inches
Engine: .46-.53 Stunt tuned two-stroke
Construction: Balsa, plywood, hardwood
Covering/finish: Heavyweight silkspan, dope
Other: 4-inch bellcrank, 2-inch spinner
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32 MODEL AVIATION
The Stalker .51RE engine installation will demand a
slightly different cowl shape than a side-exhaust
engine.
The top fuselage sheeting is rough-molded and ready to install over
formers and stringers. The stringers keep the sheeting from sagging
between formers.
With the cockpit detail installed and the canopy glued in place, the
Mustang is ready for the cowl and then finish.
wing bays that define the P-51 “D” model wing. These are not tough
to frame up, especially when using the double-sheet LE component
made from 1/8 balsa.
The 1/8 LE strips are overlapped at the intersection to produce an
integral joint with no discernable weaknesses. A double-layer LE is
not subject to the angular demands of a piece of 1/4 square LE stock,
allowing a more versatile LE shape.
Don’t overlook installing shear webs, which connect both spars,
and the webs at the front edge of the TE sheeting. It is amazing how
much this does to stiffen a wing from warps. The shear webs must
be installed with the wing in the fixture to ensure that the wing
remains true.
The relatively short nose demands that the tank compartment be
set back into the wing an inch. Don’t forget to cut the fuselage sides
with the tank setback tabs in place. I forgot this on my second model
and had to splice the tabs on.
The nose plywood doublers are 1/64 plywood. I find that to be
plenty strong.
Many times we partake in gross overkill when it comes to
“normal” construction methods. My philosophy is that until we find
an area that presents a noticeable weakness, the design is usually
overbuilt. Building the model light to begin with demands less
intense structure.
The tail surfaces are built up. The rudder is ribbed and covered
with silkspan, which shows off the structure nicely. I usually do that
to the elevator too, but the Mustang did not have fabric-covered
elevators.
The ribs are placed at 1-inch intervals to prevent any noticeable
sagging of the 1/16 balsa skins. This is working well; the tail surfaces
have the look of being shaped from solid sheet stock but retain their
lightweight qualities. The stabilizer is 1/2-inch thick at the center and
tapers to less than 3/8 inch at the tip, which keeps the “look” from
being too bulky.
CONSTRUCTION
Start this project by procuring a good supply of light balsa. That
is tough to do locally here in Idaho, so I order wood from Lone Star
Balsa. Specify the 6-pound stock. Most of the sheets are in the 5- to
7-pound range.
If you do not have a good scale, buy one! Stationery stores carry
“letter scales” that cost less than $2 and will weigh balsa as heavy as
3 ounces. These scales are calibrated in ounces and grams.
A good 16-ounce scale is handy for measuring the weight of all
the individual components. Wal-Mart carries a nice 16-ounce scale
with a dial face for a reasonable price. A quality modeler must
watch every gram that goes into an airplane.
The first priority is to decide which engine to install. We are in
an era in which the selection of world-class Stunt engines has never
been better. My first Mustang has the Neumann-modified Magnum
.53—an excellent engine. The all-up weight is 10.7 ounces, making
it perfect for this model.
Other choices for power are the RO-Jett .51 SE (side exhaust),
the O.S. .46 LA, the ever-popular SuperTigre .46 (if you still have
one around), or the current SuperTigre .51. There are many others,
but the preceding are all good choices.
I installed a Stalker .51 RE (rear exhaust) in my second Mustang.
The rear exhaust gives the airplane’s nose an extremely clean look
with no exhaust system out the side and the bonus of less fuel
residue on the model to clean up. The muffler is completely
contained in the cowl. I am starting to love the concept of a rearexhaust
engine setup.
Wing: The wing construction is a standard “D-tube” design. Shown
on the rib patterns are 5/16-inch-diameter holes for the wing fixture.
I have used two Easton 2013 arrow shafts on parallel supports
as my homemade fixture. Any good archery shop will have
appropriate shafts for a fixture. While you are there, think about
buying a half dozen 2013 shafts; you can use the extras for
pushrods.
The “20” designator is 20/64 inches in diameter, or 5/16 inch, and
the “13” is .013 inch wall thickness. This ultra-light shaft (9.01
grains per inch) is the right size for fixture rods. The extra shafts
will make two pushrods per shaft. The cost is typically less than
$20 for a half dozen.
These aluminum shafts are as light as the carbon-fiber variety
but cost considerably less. A local outlet sells extruded aluminum
06sig1.QXD 4/23/07 2:40 PM Page 32
solid stock and that works fine for fixture rods
too.
Pick higher-density balsa for the spars.
Hard spars are cheap insurance for a strong
wing. Carbon fiber could be applied to the
spars, but I don’t recommend it. Hard balsa
is much less expensive and not nearly as
hazardous with which to work.
As does balsa, carbon fiber works great in
tension. Balsa is five times stronger in tension
than compression, and carbon fiber is even
stronger under tension. However, both are
much weaker under compression, which is
the area we need to improve in the equation.
The carbon fiber will buckle under
compression unless supported with other
substances such as resins, and those cost
weight. Why not just size the balsa spar to
resist the compressive loads and quit there?
If you want to get fancy, you could make
the spar from 1/4 balsa that is 3/8 inch wide at
the root and tapers to 1/8 inch at the tip. That
weighs the same as a 1/4-inch square spar but
provides more strength toward the center of
the wing, where it is needed. No matter what
method is chosen, this wing is 21/2 inches
thick, which makes it inherently strong
anyway.
This design lends itself well to
experimenting with different methods and
materials. Feel free to do your own thing,
but always think about how much a different
part may weigh and what effect it will have
on the CG.
On my second Mustang I had a good
supply of 5-pound stock, 3/32 balsa I used for
the ribs. The weight difference was little, and
after using this rib thickness I found it easier
to work with than the customary 1/16-inch rib
stock. You can choose whichever one you
want to use.
Stack the ribs on the fixture over a copy of
the plans to align and space the ribs
accurately. I use small draftsman’s triangles
to square everything up. Install the spars, the
1/8-inch LE, and the TE assemblies.
The landing-gear mounts are shown built
from 1/8 and 1/16 plywood supported with
plywood rib doublers. I prefer this method to
conventional hardwood landing-gear blocks,
which are heavier. A 1/16 plywood gear cover
laminated to a balsa block for shaping to the
airfoil contour secures the 1/8-inch wire gear
with 4-40 Allen-head screws.
As I mentioned in the overview, the wing
LE is made from two layers of 1/8 balsa. The
first layer holds the ribs in alignment and is
trimmed to conform to the airfoil.
The 1/16 LE sheeting is installed, and the
front of the wing is block-sanded flat to
receive the front piece of the 1/8 inch LE
layer. This construction “locks” the sheeting
into the LE and allows a nice radius to be
sanded into the LE.
I have drawn a wing-section view to show
the components of the wing construction.
Make sure the wing is still in the fixture when
placing the vertical shear webs at the front of
the TE and between the spars. The shear webs
lock the wing into position and heavily resist
twisting forces. The wing must be held
straight in the fixture at this stage.
Each wing-panel tip is raised 1 inch at the
centerline of the root and tip ribs to establish
the necessary dihedral. Join the wing halves
and install the 4-inch bellcrank.
I installed a flap-horn assembly I made
from 3/32 music wire and brass stock. Don’t
worry about the dihedral. Just make sure the
flap-horn wires are in line with the centerline
of the flaps.
The flaps will have hard-point horn boxes
to receive the 3/32-inch-diameter wire horns.
These allow enough lateral movement to
permit free controls. I have experienced no
problems using a one-piece control horn and
dihedral. The center-section sheeting,
capstrips, and wingtips complete the wing at
this stage.
Install an adjustable leadout guide of your
choice. I like the Paul Walker/Bill Werwagetype
unit, which is 1/4 basswood with 1/8-inchdiameter
holes drilled at 3/16-inch increments
and a slot connecting all the holes. An eyelet
may be pulled out, the leadout wire slid to the
desired position, and then the eyelet pushed
back in. This simple unit weighs roughly 4
grams.
The flaps are solid 1/4 sheet stock, shaped
to a taper. I also prefer to install a 1/64 x 1/4
plywood piece in the TE to establish a taper
line, which also makes a hard TE that resists
dents. A Dremel tool saw blade held in the
Dremel router table is used to make a perfect
slot for the plywood.
As I mentioned, “lucky boxes” are
installed in the flaps. These are plywood
34 MODEL
Full-Size Plans Available—See Page 199
June 2007 35
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Avia
SIG
Av
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06sig2.QXD 4/23/07 12:33 PM Page 36
boxes with a slot built in to receive the flap
horn. They allow the flaps to be “tweaked” for
trim purposes without risking damaging them.
The composite weight of the flaps should
be less than 1.5 ounces. The wing (complete)
at this stage should weigh approximately 11.5
ounces.
Tail: The stabilizer is shown built up with the
airfoil section shown on the plans. The overall
thickness is 1/2 inch at the root and tapers to 3/8
inch at the tip.
The elevators are built similarly, with a
straight, tapered section. I glue all the internal
components to the top skin and use thin epoxy
to glue on the bottom sheeting. For this
operation I put lead #8 birdshot in Ziploc-type
sandwich baggies in 1-pound increments. I
place the shot bags over the bottom sheeting to
hold it down evenly while the epoxy is setting.
You can buy an inexpensive 25-pound bag
of birdshot at firearms stores that carry
reloading components. This shot-bag method
works well for applying even weight to
components that are being laminated.
The rudder is built up with ribs and is
covered with silkspan. It is the only control
surface that has fabric covering. This little
feature adds a nice touch of realism, along with
the balance tabs on the elevators and rudder.
An Al Rabe rudder is shown. If you have
not tried one, do so on this model. It helps
counter gyroscopic precession caused by large
propellers. The Rabe rudder helps maintain
line tension on the top portions of outside
square maneuvers.
Keep the rudder assembly as light as
possible. Excessive weight in the tail will have
to be made up with lead or some form of
heavier components in the nose.
Fuselage: Start by building the motor crutch
assembly. Epoxy 1/2 cross-grain balsa
between the 1/2 square maple motor mounts.
This ties the nose components together,
clamping the vibration from the engine.
The front firewall is built from five layers
of balsa laminated at 45° to each other. The
first and last layers have horizontal grains.
Cut the 3/32-inch fuselage sides, and epoxy
the 1/64 plywood doublers to them. Glue the
completed sides to the crutch assembly.
Make fuselage formers from 1/8 x 1/4 balsa
strips capped with the top sections of each
former.
The top view of the fuselage shows the
crossbracing between the formers. This adds
considerably to the torsional stability for
little weight. Shape, hollow, and install the
bottom air-scoop blocks that transition into
the rear of the fuselage.
Install the removable tail-wheel-assembly
mounting bracket. This is a nice feature when
it comes to finishing or even to alter the
weight of the tail-wheel assembly for the CG
adjustment. Pushrods will be installed at the
appropriate stage.
The top sheeting is premolded over
available forms. I use a 3-inch-diameter piece
of PVC irrigation pipe to form the front
sheeting. When wetted on the outside curve
with hot water, the 3/32 sheeting will be easy
to wrap around a form. Tape it to the form
and allow it to dry.
Hardware stores sell PVC in short lengths.
A baseball bat is a good source with which to
form the rear fuselage tapered sheeting.
Borrow the bat from your neighborhood
teenager, and then teach the kids how to fly as
payback.
Once preformed, install the sheeting over
the formers. I prefer to split the molds down
the center and install them a half at a time. This
makes an extra joint on the top of the fuselage
but is easier to do.
Notice the strip of 1/8 x 1/4 balsa on the top
of the formers, to facilitate the sheeting
installation and provide a glue ledge for the
sheeting. A front top nose block is carved to
shape and hollowed.
You can carve and hollow the cowling
from balsa or form a fiberglass unit. I use blue
construction foam as a cowl mold and lay up
fiberglass over the top to make an epoxyfiberglass
cowl. Then I remove the foam and
install all necessary mounting parts in the
cowl.
I use a Sig 11-inch canopy and a Hangar 9
1/10-scale pilot. The pilot figure has nice detail
and is painted with water-based acrylic paints.
The plans have a detail for the
instrument panel. It is applied to the back of
a flat-black painted panel for the instrument
cutouts. All this is well laid out on the
plans.
Finishing: I recommend a dope finish; I
use the Brodak brand throughout the
finishing process. I won’t go into details
of the finish since it is redundant in many
construction articles.
Just remember that nothing but elbow
grease can create a beautiful, light finish.
The nice thing about elbow grease is that
it doesn’t fish-eye the color coats.
Apply the color scheme of your choice,
ink lines, or whatever else with a clear
coat sprayed overall. I rub out my clear
with a regular automotive rubbing
compound followed by progressively finer
polishes. Wax the whole airplane after the
dope has had time to cure.
Consult Windy Urtnowski for a
complete line of videotapes and DVDs that
cover finishing, building, trimming, etc.
Trimming and Flying: The fuel tank is
wedged in with foam to resist vibration. Fuel
vents are permanently mounted in the
fuselage, and silicone fuel lines connect them
to the tank. This provides great isolation
between the tank and the vents.
Mount the engine on aluminum pads.
Position the CG as indicated on the plans. It is
approximately a 20% CG, which is a good
starting point.
Balance the model laterally by supporting
it on the tail wheel and glow plug. The right
wing should measure 1.5 ounces heavy.
Install enough lead in the tip box to achieve
this figure.
The leadout wires are swept back 1 inch
behind the CG as projected to the tip. My
38 MODEL AVIATION
handle is adjusted for roughly 3 inches of
line spacing. This controls the turn rate and
makes the exits on the square corners easier
to control, helping to eliminate the usual
bobble.
Many people may want to start with 4-
inch spacing on their first flights. If clean
squares are difficult, reduce the spacing
until the squares exit flat and smooth. Use
a propeller that provides 5.0- to 5.2-second
lap times.
I set up my Mustang with lightweight
23/4-inch-diameter wheels to handle the
sometimes poor grass fields, but many
people will install lightweight 21/2-inchdiameter
wheels. I think the larger wheels
give more consistent landings on pavement.
All the preceding adjustments are safe
starting points. I expect that all builders
will make adjustments to suit their flying
styles.
I hope you enjoy this airplane as much as I
do. It is fun to fly and competitive. I feel
that the Mustang can fly with the best of
them in the hands of top competitors. The
bonus is its scalelike presence.
I thank Al Rabe for providing lots of
inspiration from my youth onward. Dee
Rice has been a great friend to bounce
ideas off of and a good source of ideas and
improvements to the design approach.
The Stunt community has a bunch of
terrific guys who are ready and willing to
help out whenever they are asked. Thanks,
all! MA
Pat Johnston
4675 N. Oxbow Pl.
Boise ID 83713-9501
(208) 376-6177
[email protected]
Sources:
Brodak Distribution
100 Park Ave.
Carmichaels PA 15320
www.brodak.com
Lone Star Balsa
115 Industrial
Lancaster TX 75134
www.lonestar-models.com
PAMPA (Precision Aerobatics Model Pilots
Association)
158 Flying Cloud Isle
Foster City CA 94404
www.control-line.org
RSM Distribution
21899 Heliotrope Ln.
Windomar CA 92595
www.rsmdistribution.com
Stuka Stunt Works
1504 N. Cherry
Mount Carmel IL 62863
www.clstunt.com
Windy Urtnowski
93 Elliott Pl.
Rutherford NJ 07070
www.windyurtnowski.com
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