Shoestring

DO YOU ADMIRE the clean lines and
beauty of a particular aircraft and tell
yourself, “Someday I must model that pretty
one!”? So it has been with me and the
Shoestring. However, it would be a complex
effort if it were built using normal
construction methods. In my ongoing
investigation, two desires came forward;
luckily they could be accomplished with the
Shoestring, and I would have the pretty
model and answers to my questions.
Years ago my good friend Art Schroeder
designed a Pattern model he called the “Eye
Ball.” The reasoning behind this different
layout had merit and proved so in flight.
The theory was that if all factors were on
the line of flight (thrust, wing lift, tail,
center of gravity, etc.), it would be like
balancing the model on a pivot.
If something disturbed the aircraft
stabilitywise with such an arrangement, the
stabilizing force to overcome the
disturbance would be minimal because
stabilizing effect is direct—not through
moment arms as with other configurations.
Conversely, when rolling and/or turning
maneuvers are desired, the control forces
would be applied direct and at a minimum
force and small movement. This would
reduce drag, adding to efficiency. The
Shoestring offered the opportunity to
evaluate this theory.
In regards to this model, I also noted the
current chosen styles for aerobatics. It
seems as though the sky is full of “inline”
types, such as the Lasers, Extras, and
July 2003 31
The Shoestring is elegant in flight. Its rounded wingtips and tail tips and its fluid cheek
cowls give this aircraft true charm.
Famous Goodyear racer lends aesthetics
to high-performance design
■ Hal deBolt
32 MODEL AVIATION
Even though the paint scheme is easy to apply, it is effective in
revealing the design’s shapes. The wheel pants add class!
Scale proportions have been stretched a bit but still retain classic racing looks of original. It’s an attention-getter!
Model Aviation Hall of Famer Hal deBolt taxis the “Shoe” out for
a flight. It’s capable of doing all Pattern maneuvers.
Sukhois. Wouldn’t it be nice to see
something different?
We have learned that “comparative
investigating” allows easy judgment of
superiority. Does an inline type have an
advantage compared to a low-wing model?
We wanted a 40-size aircraft, and we were
familiar with the Live Wire Cobra—an
excellent aerobatic design. Best of all, the
Cobra parameters would match the
Shoestring. All that was needed was to
move the wing from low to shoulder height
and change to Shoestring appearance.
The next objective was to find a simpler
way to obtain those flowing lines. In the
past we had extensively investigated the use
of composite, alternate materials for the
structure. For that we developed a 1⁄3-scale
RV-4 with a 90%-plastic structure. The
rounded fuselage was made from Styrofoam
with a fiberglass skin. The fuselage was
large, and the mass of foam that was
assembled was easy to carve and sand
before applying a skin shape.
The concept worked so neatly and
effortlessly that the idea was filed away to
try with a normal-size model when the
opportunity arose. As suspected, the
Shoestring offered that chance. As the
photos indicate, the technique produced the
pretty shape, and using this style structure
was no chore; one only has to try it to be
convinced of its worth. The Shoestring
proved an excellent vehicle for our
investigations, and a bonus was that we had
a likable, noteworthy aerobatic model to
enjoy.
The Shoestring is among the top in all
respects among others of its breed.
Weightwise it is a match for the balsa
Cobra. It is quick at full power, and it easily
performs all maneuvers at two-thirds power.
The most relaxing and enjoyable flights are
made at that setting.
CONSTRUCTION
Before commencing, it would be
convenient to have some items on hand that
The basic fuselage crutch assembly is fixtured as shown for
accuracy. Robust construction makes for a durable model.
Plywood plate mounted inside wheel pant allows use of nylon
antipivot screw. The wheel pants are removable.
The fuselage crutch is ready to accept the foam outer pieces,
which will be shaped with a knife and sanding block.
The cowl is made by first carving and sanding a foam form. Note
the spinner backplate bolted in proper position.
this structure requires beyond the normal scratch-building
necessities. You’ll need some 1⁄2-inch-thick Styrofoam sheet (not
the beaded type of foam) from a building-supply center,
cyanoacrylate (CyA) and aliphatic glues, 3-ounce fiberglass cloth,
and epoxy resin. Tools you will need include a sharp, long-blade
carving knife and a 2 x 15-inch sanding block with 40-grit paper.
With any aircraft it is convenient to have the wing and tail
available for fitting to the fuselage, so those will be assembled
first.
Wing: This wing configuration originated with the successful
Interceptor Pattern aircraft. The airfoil is progressive, with the
excellent NACA 65015 at the center and the extremely stable
65012 at the tips. True alignment is assured by assembling the
wing in “saddle fixtures,” as is done with full-scale airplanes. The
method is simple and quick. We could have used a foam-core
wing, but foam is considerably heavier than the balsa wing and we
needed to match the Cobra’s weight.
Assuming that you have produced the ribs and saddle fixtures,
assembly can commence. The wing is one piece, so mark lines on
the board at each rib station using the edge of your assembly board
and a trisquare. From the center rib front, scribe spanwise lines to
represent the leading-edge (LE) taper.
The saddle fixtures have an LE indicator. Aligning the indicator
with the LE line, erect saddle fixtures 1, 3, 5, 7, and 9, and lock
them in place with spots of CyA. The LE and trailing-edge (TE)
sheeting is tapered. Taper the eight sheets as called for on the
plans. Install the spanwise 3⁄16 square fixture strips, which support
the sheeting during assembly.
Now comes the easy part! Pin the LE and TE sheeting into the
fixtures, and align the sheeting edges with the fixture strips.
The spar is made from 3⁄32 firm sheet balsa, and it is in halves
for simplicity. They are tapered as indicated. The halves are
installed at the rear edge of the LE sheeting, and a 1⁄16 plywood
joiner is used at the center.
Install all ribs forward and rear. Forget the servo mounting in
the center rib for now. Add the LE and TE edge strips, and follow
with the bottom sheeting and the rib capstrips.
The basic assembly is completed, and the saddle fixtures ensure
positive alignment without constant attention. Remove the wing
from the fixtures.
The top of the wing is exposed. Add the needed fill-in sheet at
the tips and the top center-section sheeting, but not the bottom yet.
Contour the tip shape and blend to the sheeting. With a wide block,
sand the angle from the tip to rib 9, then install the lower tip
sheeting.
At the center-section, install the servo-compartment pieces to
fit your servo, and complete the sheeting. Attach the control horns
July 2003 33
to the strip ailerons, and fit the aileron hinges but do not cement
them. Adding the top capstrips completes the assembly.
To finish the wing and prepare for covering, shape the LE and
TE. With the ailerons in place, block-sand to perfection with 100-
grit paper. The LE attachment dowels must wait until the wing is
aligned on the fuselage.
Stabilizer and Elevator: For maximum efficiency, the tail surfaces
have airfoils. They are assembled in a similar manner as the wing.
This time the sheeting is glued together and the stabilizer outline is
sized. Using the TE, mark the rib stations on the sheeting. With the
saddle fixtures located, align the sheeting on them. The ribs can be
pressed down onto the sheeting and attached with CyA. Add the LE
and TE. Follow this with the remaining sheeting. Contour the tip
shape into the top sheeting. Block-sand from the contour to the
bottom of rib 5. Add the bottom tip sheeting.
For elevator construction, establish the outline on a balsa sheet.
Glue the LE in place on the sheeting, which is flat on the assembly
board. The ribs are simple triangles. As indicated, fabricate and
install them on the sheeting, then install the remaining sheeting.
Attach the elevator to the stabilizer with strip hinges, but do not
cement until covered. To complete, block-sand the surfaces to
perfection.
Rudder and Fin: As were the wing and horizontal tail, these are
assembled in saddle fixtures. The procedure is the same as for the
stabilizer/elevator, except that the tip contour is created on covering
sheets then drawn together when the last sheeting is installed.
Continue, and finish as with the horizontal tail.
The Shoestring’s finished cowling assembly is shown. The
fiberglass is easy to apply and to sand smooth.
Fuselage is fully shaped and ready to accept wing and tail
components. Don’t be afraid to try this unusual method.
The wing is assembled in a form-fitting fixture. The result is a
strong, warp-resistant structure. A deBolt trademark!
Type: Semiscale RC sport Pattern
Wingspan: 60 inches
Engine: .40-.46 two-stroke
Flying weight: 76 ounces
Construction: Balsa, plywood, foam
Covering/finish: Heat-shrink film, model
epoxy paint
34 MODEL AVIATION
Fuselage: With its futuristic structure, this
is where the difference is and perhaps some
new lessons will be learned. Understanding
the stress factor can assist one’s thinking.
There is a plywood rectangular “box,” and
foam is attached to it. All of the major
stresses the model encounters are absorbed
by this box. The engine, landing gear, wing,
and servos are connected to it. In effect, all
the foam does is transform the box into the
desired shape of the fuselage.
Use only aliphatic glue (Titebond,
Elmer’s, etc.) for fuselage assembly. There
will be joints in the foam sheeting. When
joining the foam, keep the glue away from
the outside or the inside edges of the joints
because glue seams are a detriment when
sanding foam. The joints will be further
secured when the covering is attached with
epoxy resin.
Produce the plywood parts for the
substructure box and assemble it. As there
would be with a normal structure, there are
two sides of foam. Scribe a long line on the
assembly board, and center the substructure
on it. Spot-glue with CyA to hold the
substructure in place. Cut two sides from
foam sheet and glue them to the
substructure using the line to assure
alignment. Install the two balsa bulkheads
between the sides. There is an angled wingfairing
joiner at the wing TE. The wing
fairing and fuselage have matching balsa
surfaces.
Install the top deck. As indicated, two
tapered lengths of foam extend from the
joiner to the stabilizer LE. The bottom edges
are angled so that they “lean in” a bit. Glue
them in place, and flatten their top edges
with a sanding block. Glue an appropriate
strip of foam to the top edges.
At the joiner position, create the angle
and glue the already shaped balsa surface to
the foam. On the top of the substructure at
the wing LE, position and glue the balsa
former in place.
On the substructure where the wing LE
will be attached, use some 1⁄8 balsa sheet to
cover the area and shape the top to suit the
former.
With the carving knife and long sanding
block, and using the joiner-plate shape as a
guide, carve and sand the top deck to shape.
Place the wing-joiner plate in position, and,
as with the rear top deck, fabricate two foam
side pieces as indicated. These should have
an angle on the lower edge. Glue these front
and back to the balsa facings. Block-sand
the edges flat, and glue an appropriate strip
of foam to them.
Fill the area on top of the substructure, in
front of the LE position and between the
formers, with foam. Glue the properly
shaped 1⁄4-inch balsa former onto the bottom
of the engine-mount bulkhead and another
to the front of the landing-gear mount. Fill
the area between with foam.
The bottom of the fuselage is a long
length of foam; leave this off until after the
equipment is installed.
Now comes the fun part: producing those
pretty flowing lines. There is a slight
curvature to the main sides. To begin this
shape, work in the lengthwise direction with
the long sanding block. Work carefully
because the foam comes off easily. With
that finished, create the rest of the shape,
carving and sanding so that the shapes flow
into the curved sides.
When the “rough” sanding is done, put
some sizable pieces of 100-grit sandpaper in
the palm of your hand and blend to the
various shapes, sanding them to perfection.
Remove the wing fairing. The lower
There is plenty of room for radio-system
components. Be sure to cushion the
receiver and battery pack.
The control linkage is a normal
configuration with nylon horns and wire
pushrods.
Removable fiberglass cowling allows
plenty of cooling air to get to the engine.
Note simulated scale lower intake.
The canopy section is attached to the
wing. Note single bolt hole to allow holddown.
Dowels secure the front.
Tank fits snug in forward fuselage. Be
sure to check fit during construction.
This is the finished cheek-cowl assembly
as seen from behind. Notice the cutouts
to fit around the engine.
July 2003 35
36 MODEL AVIATION
airfoil curvature is in the substructure.
Remove the side foam in the mount area
and prepare to install the wing. It will be
more convenient if the landing gear is
installed at this point, so attach it now.
There are two pieces of hardwood
attached to the substructure at the wing LE
to accept the wing-attachment dowels. Cut
the foam away in that area. Measure
carefully, and install the two birch dowels in
the wing. In slightly oversize hardwood
pieces, drill the needed holes for the dowels.
Place these hardwood pieces on the dowels
and center the wing in its saddle. Fit the
hardwood to the substructure and cement it
in place. Fill around it with foam scraps.
The rear wing attachment is a 10-24
nylon screw. Install a piece of 1⁄4 plywood in
the substructure level with the wing mount
at the TE. Drill a small pilot hole through
the wing, centered on the plywood mount.
Measuring from the fuselage rudder post
to each wingtip, make sure the wing is
aligned with the fuselage. Using the wing
hole, tap-drill the plywood mount.
Remove the wing and put the wing
fairing in place. Using the rib pattern
aligned with the fuselage wing saddle, mark
the fairing, and, with the wing back in place,
fit the fairing to the wing and fuselage.
Cement the fairing to the wing.
Enlarge the rear wing-attachment hole
through the fairing to 3⁄8 inch in diameter.
Center-drill a 1⁄2-inch length of 3⁄8-inchdiameter
dowel. Cement the dowel in the
hole so it rests against the wing. Tap the
plywood mount for a 10-24 thread. Attach
the wing with a nylon screw.
The stabilizer saddle is next. Notice that
the fuselage sides are parallel with the
thrustline and that the stabilizer is set so that
the lower side is 1⁄4 inch lower at the rear
edge, positive incidence. With the saddle
created, align the stabilizer horizontally by
adjusting the saddle so that it is parallel with
the wing.
Nothing has been done with the engine
or its cowl yet. Experience has shown that
cowl production is best done after the
fuselage has been fiberglassed.
It’s time to install the servos. A twoplus-
one tray fits nicely. Temporarily attach
the tail so that control-horn locations can be
ascertained. With a yardstick, mark the
pushrod path from the servo output to the
control horns on the fuselage. Locate places
on bulkheads 3 and 4 for pushrod guide
holes. At the rear the pushrods will exit
through the foam at shallow angles. The
passage holes can be created with 3⁄32-inchdiameter
music wire with a sharpened end.
Once all of the holes are made, insert the
rods and make sure they remain straight and
have no areas that bind or rub. Where the
rods pass through the foam, enlarge the
passages enough to accept plastic tubing.
Glue the tubing in place.
Center-drill a 1⁄4-inch-diameter dowel
with a 1⁄16-inch-diameter hole. Fabricate the
tail-wheel assembly from 1⁄16-inch-diameter
music wire and insert it into the dowel.
Glue the dowel to the fuselage end.
Full-Size Plans Available—see page 183
The fuselage is ready to be fiberglassed.
This process is easy to do, but it can be
messy; try to do it neatly. With foam you
must use epoxy resin; other types will melt
the foam. The fiberglass cloth I use is 3
ounces per square yard in weight. Cut a
piece of cloth large enough to wrap around
the entire fuselage. Arrange to have the
fuselage stable when upside-down.
Wet the entire bottom of the fuselage
with epoxy resin. Center the cloth laterally,
lay it centered on the bottom, and stretch it
tight lengthwise. With your fingers, press
the cloth into the resin, working from the
centerline outward until the cloth is attached
in all of the resined area.
Lay the fuselage on one side, and coat
the exposed side with resin. Gently move
the cloth onto the resin-coated area; don’t
pull hard. Working with your fingers from
the bottom upward, press the cloth into the
resin. The objective is to have the cloth
attached halfway around the top. Trim any
excess cloth with scissors.
With that finished, coat the other side
with resin. Working from the bottom, press
the cloth into the resin with your fingers.
When you’re close to the top, trim the
excess so that there will not be a large
overlap.
Set the fuselage aside to allow the resin
to set; it should take approximately 24
hours.
Cowl: The cowl is complex in shape, but it
can be produced easily with this procedure.
It’s something you should remember for
future projects.
The Enya 46 four-stroke engine proved
to be an excellent match with the original
Shoestring. However, any engine roughly
that size should do well. Do not overpower
the model; it is a very low-drag design that
does not require excess power.
Install your engine and mount on the
firewall so that the crankshaft aligns with
the required thrustline. Plug all engine-inlet
holes with tissue paper to keep dirt and dust
out. Produce the 3⁄32 plywood cowl former.
Be sure that the outline is approximately 1⁄64
inch smaller than the fuselage firewall. Pin
the former securely to the firewall with a
piece of Saran Wrap between them.
Produce the cowl-spinner former from
1⁄16 plywood. This is secured against the
engine drive washer. To help you when
removing the finished cowl, make the
former hole 5⁄16 inch in diameter if your
engine has a 1⁄4-inch-diameter crankshaft.
Wrap masking tape around the shaft to fit
the former hole. When you are ready,
removing the tape will make it easier to
remove the cowl the first time.
Fill all around the engine with pieces of
foam. Use a bit of glue on each piece. The
object is to make the foam larger than the
finished cowl will be. Face off the front of
the cowl foam. Reduce the top foam until it
is close to flowing into the fuselage top
curvature. Mark a centerline on this surface
to use as reference, keeping the cowl
symmetrical. Produce the air-inlet holes.
This effort will teach you to be a
sculptor (if you are not one already). Begin
the sculpting by creating the cowl cheeks;
the inlet holes serve as guides. Don’t go too
far inward with them. Carving, flow the
bottom curvature into the spinner ring.
Blend that curvature into the cheeks. Do the
same for the upper curvature.
The cowl cheeks flow into the wing.
Install the wing. With the rib template,
create the required openings in oversize
pieces of foam. Fit those slots onto the
wing, and trim the foam close to the cheek
size. Cement the foam to the fuselage but
not to the wing.
Remove the wing, and shape the foam to
match the cowl cheeks. At the wing slots
the upper portion of the slot is cut loose at
the LE, and is then glued to the wing. The
lower portion remains on the fuselage and
unattached to the wing.
Cover the cowl foam with 3-ounce
fiberglass cloth. When that resin has set,
remove the cowl and sand it thoroughly.
Apply a layer of 3⁄4-ounce fiberglass cloth.
This layer will create a smoother surface.
Complete this process by fiberglassing the
wing fairing and the cheek fairings.
Sand all fiberglassed areas with 60-grit
sandpaper. Then apply a thin coat of
resin. When the resin has set and has been
sanded smooth, the Shoestring is ready
for covering and painting.
Covering and Finishing: Choosing a color
scheme is a highlight of a big project. The
Shoestring offers a variety of authentic
choices because of its full-scale racing
career. The Shoestring was top dog in
midget racing for a number of years, and
its ability was shown as it won with several
different owners and pilots. It seemed that
each ownership brought a new color and
scheme.
The first featured a yellow-green hue
that would be hard to duplicate. It’s widely
believed that the second time around the
trim scheme was kept, but the base color
was changed to blue. That’s the design I
chose for my Shoestring. A later Circus
Circus sponsorship inspired a wild scheme
of bright colors which would stand out on
any flightline!
The wing and tail are covered with ironon
film. I have found Coverite film easy to
use and durable. The tail is installed after
it’s covered. Before gluing the tail on,
check its alignment with the wing.
Paint the fiberglassed fuselage.
Preparation is essential. The easiest
procedure is to sand it out with 60-grit
paper until overlaps are blended, then add a
coat of thinned resin. (Acetone thins it
fine.) Make sure there are no
imperfections. Lightweight spackling
compounds in paint departments are
excellent for filling small dings and dents.
The same compound makes an easy primer
base coat. It fills well and sands
effortlessly. Put some compound in a cup,
and thin it with water to brushing
consistency.
There are numerous “hobby epoxy”
paints which may be fine for the color.
However, Rust-Oleum works fine and is
available in local hardware stores.
Installation: I would think that most
builders involved with this project have
been down this road a number of times.
There is nothing unusual about it. The
engine mount is held with T-nuts, and the
engine is fastened to it. The fuel tank is
installed through the wing opening. The
landing gear is attached with three 10-24
nylon screws. The radio goes in its
compartment to suit balancing needs.
Preflight: This is where you buy the
insurance for successful flight, so do it
carefully! Double check the following. Is
the engine shaft neutral to the thrustline? Is
the wing aligned? Is the tail aligned to the
wing? Is the balance within the range
indicated?
Balancing can be done with the wing
off. Arrange some risers for the fuselage
wing saddle (with the fuselage inverted).
Place a suitable stick on the risers, and find
a location where the fuselage will rock in
both directions, and there’s the balance
point!
Perform a radio-systems check. Do the
surface movements correspond to the
transmitter-stick movements? Is the
amount of surface travel sufficient? Don’t
forget the range check! This being a
cowled engine, the operation should be
checked with the cowl off; adjustments are
easier.
Make sure the engine operation is
correct. Are high and low speeds solid and
proper? Install the cowl and recheck
carefully; there should be no change. I
have been using an 11 x 7 propeller with
maximum rpm at 10,000. There’s more
than ample thrust!
Flying: Do you have a nice-flying model
with which you are familiar? Expect much
the same with Shoestring; it does not have
any vices. It grooves in normal flight, yet
responds smartly to control inputs. On
initial attempts, try maneuvers at a safe
altitude. There are no maneuvers that the
Shoestring is incapable of doing well.
Enjoy the applause you get when you
set the pretty model on the flightline, and
relish the way such good looks can
perform!
This project was done as a comparison to
determine if there is any advantage to the
inline arrangement versus a low wing.
Both versions have been flown
extensively, and for all practical purposes
no difference was apparent. Both
arrangements proved to be excellent
performers. Have fun! MA
Hal deBolt
2206 Greenwich Dr. Kings Pt.
Sun City Center FL 33573

DO YOU ADMIRE the clean lines and
beauty of a particular aircraft and tell
yourself, “Someday I must model that pretty
one!”? So it has been with me and the
Shoestring. However, it would be a complex
effort if it were built using normal
construction methods. In my ongoing
investigation, two desires came forward;
luckily they could be accomplished with the
Shoestring, and I would have the pretty
model and answers to my questions.
Years ago my good friend Art Schroeder
designed a Pattern model he called the “Eye
Ball.” The reasoning behind this different
layout had merit and proved so in flight.
The theory was that if all factors were on
the line of flight (thrust, wing lift, tail,
center of gravity, etc.), it would be like
balancing the model on a pivot.
If something disturbed the aircraft
stabilitywise with such an arrangement, the
stabilizing force to overcome the
disturbance would be minimal because
stabilizing effect is direct—not through
moment arms as with other configurations.
Conversely, when rolling and/or turning
maneuvers are desired, the control forces
would be applied direct and at a minimum
force and small movement. This would
reduce drag, adding to efficiency. The
Shoestring offered the opportunity to
evaluate this theory.
In regards to this model, I also noted the
current chosen styles for aerobatics. It
seems as though the sky is full of “inline”
types, such as the Lasers, Extras, and
July 2003 31
The Shoestring is elegant in flight. Its rounded wingtips and tail tips and its fluid cheek
cowls give this aircraft true charm.
Famous Goodyear racer lends aesthetics
to high-performance design
■ Hal deBolt
32 MODEL AVIATION
Even though the paint scheme is easy to apply, it is effective in
revealing the design’s shapes. The wheel pants add class!
Scale proportions have been stretched a bit but still retain classic racing looks of original. It’s an attention-getter!
Model Aviation Hall of Famer Hal deBolt taxis the “Shoe” out for
a flight. It’s capable of doing all Pattern maneuvers.
Sukhois. Wouldn’t it be nice to see
something different?
We have learned that “comparative
investigating” allows easy judgment of
superiority. Does an inline type have an
advantage compared to a low-wing model?
We wanted a 40-size aircraft, and we were
familiar with the Live Wire Cobra—an
excellent aerobatic design. Best of all, the
Cobra parameters would match the
Shoestring. All that was needed was to
move the wing from low to shoulder height
and change to Shoestring appearance.
The next objective was to find a simpler
way to obtain those flowing lines. In the
past we had extensively investigated the use
of composite, alternate materials for the
structure. For that we developed a 1⁄3-scale
RV-4 with a 90%-plastic structure. The
rounded fuselage was made from Styrofoam
with a fiberglass skin. The fuselage was
large, and the mass of foam that was
assembled was easy to carve and sand
before applying a skin shape.
The concept worked so neatly and
effortlessly that the idea was filed away to
try with a normal-size model when the
opportunity arose. As suspected, the
Shoestring offered that chance. As the
photos indicate, the technique produced the
pretty shape, and using this style structure
was no chore; one only has to try it to be
convinced of its worth. The Shoestring
proved an excellent vehicle for our
investigations, and a bonus was that we had
a likable, noteworthy aerobatic model to
enjoy.
The Shoestring is among the top in all
respects among others of its breed.
Weightwise it is a match for the balsa
Cobra. It is quick at full power, and it easily
performs all maneuvers at two-thirds power.
The most relaxing and enjoyable flights are
made at that setting.
CONSTRUCTION
Before commencing, it would be
convenient to have some items on hand that
The basic fuselage crutch assembly is fixtured as shown for
accuracy. Robust construction makes for a durable model.
Plywood plate mounted inside wheel pant allows use of nylon
antipivot screw. The wheel pants are removable.
The fuselage crutch is ready to accept the foam outer pieces,
which will be shaped with a knife and sanding block.
The cowl is made by first carving and sanding a foam form. Note
the spinner backplate bolted in proper position.
this structure requires beyond the normal scratch-building
necessities. You’ll need some 1⁄2-inch-thick Styrofoam sheet (not
the beaded type of foam) from a building-supply center,
cyanoacrylate (CyA) and aliphatic glues, 3-ounce fiberglass cloth,
and epoxy resin. Tools you will need include a sharp, long-blade
carving knife and a 2 x 15-inch sanding block with 40-grit paper.
With any aircraft it is convenient to have the wing and tail
available for fitting to the fuselage, so those will be assembled
first.
Wing: This wing configuration originated with the successful
Interceptor Pattern aircraft. The airfoil is progressive, with the
excellent NACA 65015 at the center and the extremely stable
65012 at the tips. True alignment is assured by assembling the
wing in “saddle fixtures,” as is done with full-scale airplanes. The
method is simple and quick. We could have used a foam-core
wing, but foam is considerably heavier than the balsa wing and we
needed to match the Cobra’s weight.
Assuming that you have produced the ribs and saddle fixtures,
assembly can commence. The wing is one piece, so mark lines on
the board at each rib station using the edge of your assembly board
and a trisquare. From the center rib front, scribe spanwise lines to
represent the leading-edge (LE) taper.
The saddle fixtures have an LE indicator. Aligning the indicator
with the LE line, erect saddle fixtures 1, 3, 5, 7, and 9, and lock
them in place with spots of CyA. The LE and trailing-edge (TE)
sheeting is tapered. Taper the eight sheets as called for on the
plans. Install the spanwise 3⁄16 square fixture strips, which support
the sheeting during assembly.
Now comes the easy part! Pin the LE and TE sheeting into the
fixtures, and align the sheeting edges with the fixture strips.
The spar is made from 3⁄32 firm sheet balsa, and it is in halves
for simplicity. They are tapered as indicated. The halves are
installed at the rear edge of the LE sheeting, and a 1⁄16 plywood
joiner is used at the center.
Install all ribs forward and rear. Forget the servo mounting in
the center rib for now. Add the LE and TE edge strips, and follow
with the bottom sheeting and the rib capstrips.
The basic assembly is completed, and the saddle fixtures ensure
positive alignment without constant attention. Remove the wing
from the fixtures.
The top of the wing is exposed. Add the needed fill-in sheet at
the tips and the top center-section sheeting, but not the bottom yet.
Contour the tip shape and blend to the sheeting. With a wide block,
sand the angle from the tip to rib 9, then install the lower tip
sheeting.
At the center-section, install the servo-compartment pieces to
fit your servo, and complete the sheeting. Attach the control horns
July 2003 33
to the strip ailerons, and fit the aileron hinges but do not cement
them. Adding the top capstrips completes the assembly.
To finish the wing and prepare for covering, shape the LE and
TE. With the ailerons in place, block-sand to perfection with 100-
grit paper. The LE attachment dowels must wait until the wing is
aligned on the fuselage.
Stabilizer and Elevator: For maximum efficiency, the tail surfaces
have airfoils. They are assembled in a similar manner as the wing.
This time the sheeting is glued together and the stabilizer outline is
sized. Using the TE, mark the rib stations on the sheeting. With the
saddle fixtures located, align the sheeting on them. The ribs can be
pressed down onto the sheeting and attached with CyA. Add the LE
and TE. Follow this with the remaining sheeting. Contour the tip
shape into the top sheeting. Block-sand from the contour to the
bottom of rib 5. Add the bottom tip sheeting.
For elevator construction, establish the outline on a balsa sheet.
Glue the LE in place on the sheeting, which is flat on the assembly
board. The ribs are simple triangles. As indicated, fabricate and
install them on the sheeting, then install the remaining sheeting.
Attach the elevator to the stabilizer with strip hinges, but do not
cement until covered. To complete, block-sand the surfaces to
perfection.
Rudder and Fin: As were the wing and horizontal tail, these are
assembled in saddle fixtures. The procedure is the same as for the
stabilizer/elevator, except that the tip contour is created on covering
sheets then drawn together when the last sheeting is installed.
Continue, and finish as with the horizontal tail.
The Shoestring’s finished cowling assembly is shown. The
fiberglass is easy to apply and to sand smooth.
Fuselage is fully shaped and ready to accept wing and tail
components. Don’t be afraid to try this unusual method.
The wing is assembled in a form-fitting fixture. The result is a
strong, warp-resistant structure. A deBolt trademark!
Type: Semiscale RC sport Pattern
Wingspan: 60 inches
Engine: .40-.46 two-stroke
Flying weight: 76 ounces
Construction: Balsa, plywood, foam
Covering/finish: Heat-shrink film, model
epoxy paint
34 MODEL AVIATION
Fuselage: With its futuristic structure, this
is where the difference is and perhaps some
new lessons will be learned. Understanding
the stress factor can assist one’s thinking.
There is a plywood rectangular “box,” and
foam is attached to it. All of the major
stresses the model encounters are absorbed
by this box. The engine, landing gear, wing,
and servos are connected to it. In effect, all
the foam does is transform the box into the
desired shape of the fuselage.
Use only aliphatic glue (Titebond,
Elmer’s, etc.) for fuselage assembly. There
will be joints in the foam sheeting. When
joining the foam, keep the glue away from
the outside or the inside edges of the joints
because glue seams are a detriment when
sanding foam. The joints will be further
secured when the covering is attached with
epoxy resin.
Produce the plywood parts for the
substructure box and assemble it. As there
would be with a normal structure, there are
two sides of foam. Scribe a long line on the
assembly board, and center the substructure
on it. Spot-glue with CyA to hold the
substructure in place. Cut two sides from
foam sheet and glue them to the
substructure using the line to assure
alignment. Install the two balsa bulkheads
between the sides. There is an angled wingfairing
joiner at the wing TE. The wing
fairing and fuselage have matching balsa
surfaces.
Install the top deck. As indicated, two
tapered lengths of foam extend from the
joiner to the stabilizer LE. The bottom edges
are angled so that they “lean in” a bit. Glue
them in place, and flatten their top edges
with a sanding block. Glue an appropriate
strip of foam to the top edges.
At the joiner position, create the angle
and glue the already shaped balsa surface to
the foam. On the top of the substructure at
the wing LE, position and glue the balsa
former in place.
On the substructure where the wing LE
will be attached, use some 1⁄8 balsa sheet to
cover the area and shape the top to suit the
former.
With the carving knife and long sanding
block, and using the joiner-plate shape as a
guide, carve and sand the top deck to shape.
Place the wing-joiner plate in position, and,
as with the rear top deck, fabricate two foam
side pieces as indicated. These should have
an angle on the lower edge. Glue these front
and back to the balsa facings. Block-sand
the edges flat, and glue an appropriate strip
of foam to them.
Fill the area on top of the substructure, in
front of the LE position and between the
formers, with foam. Glue the properly
shaped 1⁄4-inch balsa former onto the bottom
of the engine-mount bulkhead and another
to the front of the landing-gear mount. Fill
the area between with foam.
The bottom of the fuselage is a long
length of foam; leave this off until after the
equipment is installed.
Now comes the fun part: producing those
pretty flowing lines. There is a slight
curvature to the main sides. To begin this
shape, work in the lengthwise direction with
the long sanding block. Work carefully
because the foam comes off easily. With
that finished, create the rest of the shape,
carving and sanding so that the shapes flow
into the curved sides.
When the “rough” sanding is done, put
some sizable pieces of 100-grit sandpaper in
the palm of your hand and blend to the
various shapes, sanding them to perfection.
Remove the wing fairing. The lower
There is plenty of room for radio-system
components. Be sure to cushion the
receiver and battery pack.
The control linkage is a normal
configuration with nylon horns and wire
pushrods.
Removable fiberglass cowling allows
plenty of cooling air to get to the engine.
Note simulated scale lower intake.
The canopy section is attached to the
wing. Note single bolt hole to allow holddown.
Dowels secure the front.
Tank fits snug in forward fuselage. Be
sure to check fit during construction.
This is the finished cheek-cowl assembly
as seen from behind. Notice the cutouts
to fit around the engine.
July 2003 35
36 MODEL AVIATION
airfoil curvature is in the substructure.
Remove the side foam in the mount area
and prepare to install the wing. It will be
more convenient if the landing gear is
installed at this point, so attach it now.
There are two pieces of hardwood
attached to the substructure at the wing LE
to accept the wing-attachment dowels. Cut
the foam away in that area. Measure
carefully, and install the two birch dowels in
the wing. In slightly oversize hardwood
pieces, drill the needed holes for the dowels.
Place these hardwood pieces on the dowels
and center the wing in its saddle. Fit the
hardwood to the substructure and cement it
in place. Fill around it with foam scraps.
The rear wing attachment is a 10-24
nylon screw. Install a piece of 1⁄4 plywood in
the substructure level with the wing mount
at the TE. Drill a small pilot hole through
the wing, centered on the plywood mount.
Measuring from the fuselage rudder post
to each wingtip, make sure the wing is
aligned with the fuselage. Using the wing
hole, tap-drill the plywood mount.
Remove the wing and put the wing
fairing in place. Using the rib pattern
aligned with the fuselage wing saddle, mark
the fairing, and, with the wing back in place,
fit the fairing to the wing and fuselage.
Cement the fairing to the wing.
Enlarge the rear wing-attachment hole
through the fairing to 3⁄8 inch in diameter.
Center-drill a 1⁄2-inch length of 3⁄8-inchdiameter
dowel. Cement the dowel in the
hole so it rests against the wing. Tap the
plywood mount for a 10-24 thread. Attach
the wing with a nylon screw.
The stabilizer saddle is next. Notice that
the fuselage sides are parallel with the
thrustline and that the stabilizer is set so that
the lower side is 1⁄4 inch lower at the rear
edge, positive incidence. With the saddle
created, align the stabilizer horizontally by
adjusting the saddle so that it is parallel with
the wing.
Nothing has been done with the engine
or its cowl yet. Experience has shown that
cowl production is best done after the
fuselage has been fiberglassed.
It’s time to install the servos. A twoplus-
one tray fits nicely. Temporarily attach
the tail so that control-horn locations can be
ascertained. With a yardstick, mark the
pushrod path from the servo output to the
control horns on the fuselage. Locate places
on bulkheads 3 and 4 for pushrod guide
holes. At the rear the pushrods will exit
through the foam at shallow angles. The
passage holes can be created with 3⁄32-inchdiameter
music wire with a sharpened end.
Once all of the holes are made, insert the
rods and make sure they remain straight and
have no areas that bind or rub. Where the
rods pass through the foam, enlarge the
passages enough to accept plastic tubing.
Glue the tubing in place.
Center-drill a 1⁄4-inch-diameter dowel
with a 1⁄16-inch-diameter hole. Fabricate the
tail-wheel assembly from 1⁄16-inch-diameter
music wire and insert it into the dowel.
Glue the dowel to the fuselage end.
Full-Size Plans Available—see page 183
The fuselage is ready to be fiberglassed.
This process is easy to do, but it can be
messy; try to do it neatly. With foam you
must use epoxy resin; other types will melt
the foam. The fiberglass cloth I use is 3
ounces per square yard in weight. Cut a
piece of cloth large enough to wrap around
the entire fuselage. Arrange to have the
fuselage stable when upside-down.
Wet the entire bottom of the fuselage
with epoxy resin. Center the cloth laterally,
lay it centered on the bottom, and stretch it
tight lengthwise. With your fingers, press
the cloth into the resin, working from the
centerline outward until the cloth is attached
in all of the resined area.
Lay the fuselage on one side, and coat
the exposed side with resin. Gently move
the cloth onto the resin-coated area; don’t
pull hard. Working with your fingers from
the bottom upward, press the cloth into the
resin. The objective is to have the cloth
attached halfway around the top. Trim any
excess cloth with scissors.
With that finished, coat the other side
with resin. Working from the bottom, press
the cloth into the resin with your fingers.
When you’re close to the top, trim the
excess so that there will not be a large
overlap.
Set the fuselage aside to allow the resin
to set; it should take approximately 24
hours.
Cowl: The cowl is complex in shape, but it
can be produced easily with this procedure.
It’s something you should remember for
future projects.
The Enya 46 four-stroke engine proved
to be an excellent match with the original
Shoestring. However, any engine roughly
that size should do well. Do not overpower
the model; it is a very low-drag design that
does not require excess power.
Install your engine and mount on the
firewall so that the crankshaft aligns with
the required thrustline. Plug all engine-inlet
holes with tissue paper to keep dirt and dust
out. Produce the 3⁄32 plywood cowl former.
Be sure that the outline is approximately 1⁄64
inch smaller than the fuselage firewall. Pin
the former securely to the firewall with a
piece of Saran Wrap between them.
Produce the cowl-spinner former from
1⁄16 plywood. This is secured against the
engine drive washer. To help you when
removing the finished cowl, make the
former hole 5⁄16 inch in diameter if your
engine has a 1⁄4-inch-diameter crankshaft.
Wrap masking tape around the shaft to fit
the former hole. When you are ready,
removing the tape will make it easier to
remove the cowl the first time.
Fill all around the engine with pieces of
foam. Use a bit of glue on each piece. The
object is to make the foam larger than the
finished cowl will be. Face off the front of
the cowl foam. Reduce the top foam until it
is close to flowing into the fuselage top
curvature. Mark a centerline on this surface
to use as reference, keeping the cowl
symmetrical. Produce the air-inlet holes.
This effort will teach you to be a
sculptor (if you are not one already). Begin
the sculpting by creating the cowl cheeks;
the inlet holes serve as guides. Don’t go too
far inward with them. Carving, flow the
bottom curvature into the spinner ring.
Blend that curvature into the cheeks. Do the
same for the upper curvature.
The cowl cheeks flow into the wing.
Install the wing. With the rib template,
create the required openings in oversize
pieces of foam. Fit those slots onto the
wing, and trim the foam close to the cheek
size. Cement the foam to the fuselage but
not to the wing.
Remove the wing, and shape the foam to
match the cowl cheeks. At the wing slots
the upper portion of the slot is cut loose at
the LE, and is then glued to the wing. The
lower portion remains on the fuselage and
unattached to the wing.
Cover the cowl foam with 3-ounce
fiberglass cloth. When that resin has set,
remove the cowl and sand it thoroughly.
Apply a layer of 3⁄4-ounce fiberglass cloth.
This layer will create a smoother surface.
Complete this process by fiberglassing the
wing fairing and the cheek fairings.
Sand all fiberglassed areas with 60-grit
sandpaper. Then apply a thin coat of
resin. When the resin has set and has been
sanded smooth, the Shoestring is ready
for covering and painting.
Covering and Finishing: Choosing a color
scheme is a highlight of a big project. The
Shoestring offers a variety of authentic
choices because of its full-scale racing
career. The Shoestring was top dog in
midget racing for a number of years, and
its ability was shown as it won with several
different owners and pilots. It seemed that
each ownership brought a new color and
scheme.
The first featured a yellow-green hue
that would be hard to duplicate. It’s widely
believed that the second time around the
trim scheme was kept, but the base color
was changed to blue. That’s the design I
chose for my Shoestring. A later Circus
Circus sponsorship inspired a wild scheme
of bright colors which would stand out on
any flightline!
The wing and tail are covered with ironon
film. I have found Coverite film easy to
use and durable. The tail is installed after
it’s covered. Before gluing the tail on,
check its alignment with the wing.
Paint the fiberglassed fuselage.
Preparation is essential. The easiest
procedure is to sand it out with 60-grit
paper until overlaps are blended, then add a
coat of thinned resin. (Acetone thins it
fine.) Make sure there are no
imperfections. Lightweight spackling
compounds in paint departments are
excellent for filling small dings and dents.
The same compound makes an easy primer
base coat. It fills well and sands
effortlessly. Put some compound in a cup,
and thin it with water to brushing
consistency.
There are numerous “hobby epoxy”
paints which may be fine for the color.
However, Rust-Oleum works fine and is
available in local hardware stores.
Installation: I would think that most
builders involved with this project have
been down this road a number of times.
There is nothing unusual about it. The
engine mount is held with T-nuts, and the
engine is fastened to it. The fuel tank is
installed through the wing opening. The
landing gear is attached with three 10-24
nylon screws. The radio goes in its
compartment to suit balancing needs.
Preflight: This is where you buy the
insurance for successful flight, so do it
carefully! Double check the following. Is
the engine shaft neutral to the thrustline? Is
the wing aligned? Is the tail aligned to the
wing? Is the balance within the range
indicated?
Balancing can be done with the wing
off. Arrange some risers for the fuselage
wing saddle (with the fuselage inverted).
Place a suitable stick on the risers, and find
a location where the fuselage will rock in
both directions, and there’s the balance
point!
Perform a radio-systems check. Do the
surface movements correspond to the
transmitter-stick movements? Is the
amount of surface travel sufficient? Don’t
forget the range check! This being a
cowled engine, the operation should be
checked with the cowl off; adjustments are
easier.
Make sure the engine operation is
correct. Are high and low speeds solid and
proper? Install the cowl and recheck
carefully; there should be no change. I
have been using an 11 x 7 propeller with
maximum rpm at 10,000. There’s more
than ample thrust!
Flying: Do you have a nice-flying model
with which you are familiar? Expect much
the same with Shoestring; it does not have
any vices. It grooves in normal flight, yet
responds smartly to control inputs. On
initial attempts, try maneuvers at a safe
altitude. There are no maneuvers that the
Shoestring is incapable of doing well.
Enjoy the applause you get when you
set the pretty model on the flightline, and
relish the way such good looks can
perform!
This project was done as a comparison to
determine if there is any advantage to the
inline arrangement versus a low wing.
Both versions have been flown
extensively, and for all practical purposes
no difference was apparent. Both
arrangements proved to be excellent
performers. Have fun! MA
Hal deBolt
2206 Greenwich Dr. Kings Pt.
Sun City Center FL 33573

DO YOU ADMIRE the clean lines and
beauty of a particular aircraft and tell
yourself, “Someday I must model that pretty
one!”? So it has been with me and the
Shoestring. However, it would be a complex
effort if it were built using normal
construction methods. In my ongoing
investigation, two desires came forward;
luckily they could be accomplished with the
Shoestring, and I would have the pretty
model and answers to my questions.
Years ago my good friend Art Schroeder
designed a Pattern model he called the “Eye
Ball.” The reasoning behind this different
layout had merit and proved so in flight.
The theory was that if all factors were on
the line of flight (thrust, wing lift, tail,
center of gravity, etc.), it would be like
balancing the model on a pivot.
If something disturbed the aircraft
stabilitywise with such an arrangement, the
stabilizing force to overcome the
disturbance would be minimal because
stabilizing effect is direct—not through
moment arms as with other configurations.
Conversely, when rolling and/or turning
maneuvers are desired, the control forces
would be applied direct and at a minimum
force and small movement. This would
reduce drag, adding to efficiency. The
Shoestring offered the opportunity to
evaluate this theory.
In regards to this model, I also noted the
current chosen styles for aerobatics. It
seems as though the sky is full of “inline”
types, such as the Lasers, Extras, and
July 2003 31
The Shoestring is elegant in flight. Its rounded wingtips and tail tips and its fluid cheek
cowls give this aircraft true charm.
Famous Goodyear racer lends aesthetics
to high-performance design
■ Hal deBolt
32 MODEL AVIATION
Even though the paint scheme is easy to apply, it is effective in
revealing the design’s shapes. The wheel pants add class!
Scale proportions have been stretched a bit but still retain classic racing looks of original. It’s an attention-getter!
Model Aviation Hall of Famer Hal deBolt taxis the “Shoe” out for
a flight. It’s capable of doing all Pattern maneuvers.
Sukhois. Wouldn’t it be nice to see
something different?
We have learned that “comparative
investigating” allows easy judgment of
superiority. Does an inline type have an
advantage compared to a low-wing model?
We wanted a 40-size aircraft, and we were
familiar with the Live Wire Cobra—an
excellent aerobatic design. Best of all, the
Cobra parameters would match the
Shoestring. All that was needed was to
move the wing from low to shoulder height
and change to Shoestring appearance.
The next objective was to find a simpler
way to obtain those flowing lines. In the
past we had extensively investigated the use
of composite, alternate materials for the
structure. For that we developed a 1⁄3-scale
RV-4 with a 90%-plastic structure. The
rounded fuselage was made from Styrofoam
with a fiberglass skin. The fuselage was
large, and the mass of foam that was
assembled was easy to carve and sand
before applying a skin shape.
The concept worked so neatly and
effortlessly that the idea was filed away to
try with a normal-size model when the
opportunity arose. As suspected, the
Shoestring offered that chance. As the
photos indicate, the technique produced the
pretty shape, and using this style structure
was no chore; one only has to try it to be
convinced of its worth. The Shoestring
proved an excellent vehicle for our
investigations, and a bonus was that we had
a likable, noteworthy aerobatic model to
enjoy.
The Shoestring is among the top in all
respects among others of its breed.
Weightwise it is a match for the balsa
Cobra. It is quick at full power, and it easily
performs all maneuvers at two-thirds power.
The most relaxing and enjoyable flights are
made at that setting.
CONSTRUCTION
Before commencing, it would be
convenient to have some items on hand that
The basic fuselage crutch assembly is fixtured as shown for
accuracy. Robust construction makes for a durable model.
Plywood plate mounted inside wheel pant allows use of nylon
antipivot screw. The wheel pants are removable.
The fuselage crutch is ready to accept the foam outer pieces,
which will be shaped with a knife and sanding block.
The cowl is made by first carving and sanding a foam form. Note
the spinner backplate bolted in proper position.
this structure requires beyond the normal scratch-building
necessities. You’ll need some 1⁄2-inch-thick Styrofoam sheet (not
the beaded type of foam) from a building-supply center,
cyanoacrylate (CyA) and aliphatic glues, 3-ounce fiberglass cloth,
and epoxy resin. Tools you will need include a sharp, long-blade
carving knife and a 2 x 15-inch sanding block with 40-grit paper.
With any aircraft it is convenient to have the wing and tail
available for fitting to the fuselage, so those will be assembled
first.
Wing: This wing configuration originated with the successful
Interceptor Pattern aircraft. The airfoil is progressive, with the
excellent NACA 65015 at the center and the extremely stable
65012 at the tips. True alignment is assured by assembling the
wing in “saddle fixtures,” as is done with full-scale airplanes. The
method is simple and quick. We could have used a foam-core
wing, but foam is considerably heavier than the balsa wing and we
needed to match the Cobra’s weight.
Assuming that you have produced the ribs and saddle fixtures,
assembly can commence. The wing is one piece, so mark lines on
the board at each rib station using the edge of your assembly board
and a trisquare. From the center rib front, scribe spanwise lines to
represent the leading-edge (LE) taper.
The saddle fixtures have an LE indicator. Aligning the indicator
with the LE line, erect saddle fixtures 1, 3, 5, 7, and 9, and lock
them in place with spots of CyA. The LE and trailing-edge (TE)
sheeting is tapered. Taper the eight sheets as called for on the
plans. Install the spanwise 3⁄16 square fixture strips, which support
the sheeting during assembly.
Now comes the easy part! Pin the LE and TE sheeting into the
fixtures, and align the sheeting edges with the fixture strips.
The spar is made from 3⁄32 firm sheet balsa, and it is in halves
for simplicity. They are tapered as indicated. The halves are
installed at the rear edge of the LE sheeting, and a 1⁄16 plywood
joiner is used at the center.
Install all ribs forward and rear. Forget the servo mounting in
the center rib for now. Add the LE and TE edge strips, and follow
with the bottom sheeting and the rib capstrips.
The basic assembly is completed, and the saddle fixtures ensure
positive alignment without constant attention. Remove the wing
from the fixtures.
The top of the wing is exposed. Add the needed fill-in sheet at
the tips and the top center-section sheeting, but not the bottom yet.
Contour the tip shape and blend to the sheeting. With a wide block,
sand the angle from the tip to rib 9, then install the lower tip
sheeting.
At the center-section, install the servo-compartment pieces to
fit your servo, and complete the sheeting. Attach the control horns
July 2003 33
to the strip ailerons, and fit the aileron hinges but do not cement
them. Adding the top capstrips completes the assembly.
To finish the wing and prepare for covering, shape the LE and
TE. With the ailerons in place, block-sand to perfection with 100-
grit paper. The LE attachment dowels must wait until the wing is
aligned on the fuselage.
Stabilizer and Elevator: For maximum efficiency, the tail surfaces
have airfoils. They are assembled in a similar manner as the wing.
This time the sheeting is glued together and the stabilizer outline is
sized. Using the TE, mark the rib stations on the sheeting. With the
saddle fixtures located, align the sheeting on them. The ribs can be
pressed down onto the sheeting and attached with CyA. Add the LE
and TE. Follow this with the remaining sheeting. Contour the tip
shape into the top sheeting. Block-sand from the contour to the
bottom of rib 5. Add the bottom tip sheeting.
For elevator construction, establish the outline on a balsa sheet.
Glue the LE in place on the sheeting, which is flat on the assembly
board. The ribs are simple triangles. As indicated, fabricate and
install them on the sheeting, then install the remaining sheeting.
Attach the elevator to the stabilizer with strip hinges, but do not
cement until covered. To complete, block-sand the surfaces to
perfection.
Rudder and Fin: As were the wing and horizontal tail, these are
assembled in saddle fixtures. The procedure is the same as for the
stabilizer/elevator, except that the tip contour is created on covering
sheets then drawn together when the last sheeting is installed.
Continue, and finish as with the horizontal tail.
The Shoestring’s finished cowling assembly is shown. The
fiberglass is easy to apply and to sand smooth.
Fuselage is fully shaped and ready to accept wing and tail
components. Don’t be afraid to try this unusual method.
The wing is assembled in a form-fitting fixture. The result is a
strong, warp-resistant structure. A deBolt trademark!
Type: Semiscale RC sport Pattern
Wingspan: 60 inches
Engine: .40-.46 two-stroke
Flying weight: 76 ounces
Construction: Balsa, plywood, foam
Covering/finish: Heat-shrink film, model
epoxy paint
34 MODEL AVIATION
Fuselage: With its futuristic structure, this
is where the difference is and perhaps some
new lessons will be learned. Understanding
the stress factor can assist one’s thinking.
There is a plywood rectangular “box,” and
foam is attached to it. All of the major
stresses the model encounters are absorbed
by this box. The engine, landing gear, wing,
and servos are connected to it. In effect, all
the foam does is transform the box into the
desired shape of the fuselage.
Use only aliphatic glue (Titebond,
Elmer’s, etc.) for fuselage assembly. There
will be joints in the foam sheeting. When
joining the foam, keep the glue away from
the outside or the inside edges of the joints
because glue seams are a detriment when
sanding foam. The joints will be further
secured when the covering is attached with
epoxy resin.
Produce the plywood parts for the
substructure box and assemble it. As there
would be with a normal structure, there are
two sides of foam. Scribe a long line on the
assembly board, and center the substructure
on it. Spot-glue with CyA to hold the
substructure in place. Cut two sides from
foam sheet and glue them to the
substructure using the line to assure
alignment. Install the two balsa bulkheads
between the sides. There is an angled wingfairing
joiner at the wing TE. The wing
fairing and fuselage have matching balsa
surfaces.
Install the top deck. As indicated, two
tapered lengths of foam extend from the
joiner to the stabilizer LE. The bottom edges
are angled so that they “lean in” a bit. Glue
them in place, and flatten their top edges
with a sanding block. Glue an appropriate
strip of foam to the top edges.
At the joiner position, create the angle
and glue the already shaped balsa surface to
the foam. On the top of the substructure at
the wing LE, position and glue the balsa
former in place.
On the substructure where the wing LE
will be attached, use some 1⁄8 balsa sheet to
cover the area and shape the top to suit the
former.
With the carving knife and long sanding
block, and using the joiner-plate shape as a
guide, carve and sand the top deck to shape.
Place the wing-joiner plate in position, and,
as with the rear top deck, fabricate two foam
side pieces as indicated. These should have
an angle on the lower edge. Glue these front
and back to the balsa facings. Block-sand
the edges flat, and glue an appropriate strip
of foam to them.
Fill the area on top of the substructure, in
front of the LE position and between the
formers, with foam. Glue the properly
shaped 1⁄4-inch balsa former onto the bottom
of the engine-mount bulkhead and another
to the front of the landing-gear mount. Fill
the area between with foam.
The bottom of the fuselage is a long
length of foam; leave this off until after the
equipment is installed.
Now comes the fun part: producing those
pretty flowing lines. There is a slight
curvature to the main sides. To begin this
shape, work in the lengthwise direction with
the long sanding block. Work carefully
because the foam comes off easily. With
that finished, create the rest of the shape,
carving and sanding so that the shapes flow
into the curved sides.
When the “rough” sanding is done, put
some sizable pieces of 100-grit sandpaper in
the palm of your hand and blend to the
various shapes, sanding them to perfection.
Remove the wing fairing. The lower
There is plenty of room for radio-system
components. Be sure to cushion the
receiver and battery pack.
The control linkage is a normal
configuration with nylon horns and wire
pushrods.
Removable fiberglass cowling allows
plenty of cooling air to get to the engine.
Note simulated scale lower intake.
The canopy section is attached to the
wing. Note single bolt hole to allow holddown.
Dowels secure the front.
Tank fits snug in forward fuselage. Be
sure to check fit during construction.
This is the finished cheek-cowl assembly
as seen from behind. Notice the cutouts
to fit around the engine.
July 2003 35
36 MODEL AVIATION
airfoil curvature is in the substructure.
Remove the side foam in the mount area
and prepare to install the wing. It will be
more convenient if the landing gear is
installed at this point, so attach it now.
There are two pieces of hardwood
attached to the substructure at the wing LE
to accept the wing-attachment dowels. Cut
the foam away in that area. Measure
carefully, and install the two birch dowels in
the wing. In slightly oversize hardwood
pieces, drill the needed holes for the dowels.
Place these hardwood pieces on the dowels
and center the wing in its saddle. Fit the
hardwood to the substructure and cement it
in place. Fill around it with foam scraps.
The rear wing attachment is a 10-24
nylon screw. Install a piece of 1⁄4 plywood in
the substructure level with the wing mount
at the TE. Drill a small pilot hole through
the wing, centered on the plywood mount.
Measuring from the fuselage rudder post
to each wingtip, make sure the wing is
aligned with the fuselage. Using the wing
hole, tap-drill the plywood mount.
Remove the wing and put the wing
fairing in place. Using the rib pattern
aligned with the fuselage wing saddle, mark
the fairing, and, with the wing back in place,
fit the fairing to the wing and fuselage.
Cement the fairing to the wing.
Enlarge the rear wing-attachment hole
through the fairing to 3⁄8 inch in diameter.
Center-drill a 1⁄2-inch length of 3⁄8-inchdiameter
dowel. Cement the dowel in the
hole so it rests against the wing. Tap the
plywood mount for a 10-24 thread. Attach
the wing with a nylon screw.
The stabilizer saddle is next. Notice that
the fuselage sides are parallel with the
thrustline and that the stabilizer is set so that
the lower side is 1⁄4 inch lower at the rear
edge, positive incidence. With the saddle
created, align the stabilizer horizontally by
adjusting the saddle so that it is parallel with
the wing.
Nothing has been done with the engine
or its cowl yet. Experience has shown that
cowl production is best done after the
fuselage has been fiberglassed.
It’s time to install the servos. A twoplus-
one tray fits nicely. Temporarily attach
the tail so that control-horn locations can be
ascertained. With a yardstick, mark the
pushrod path from the servo output to the
control horns on the fuselage. Locate places
on bulkheads 3 and 4 for pushrod guide
holes. At the rear the pushrods will exit
through the foam at shallow angles. The
passage holes can be created with 3⁄32-inchdiameter
music wire with a sharpened end.
Once all of the holes are made, insert the
rods and make sure they remain straight and
have no areas that bind or rub. Where the
rods pass through the foam, enlarge the
passages enough to accept plastic tubing.
Glue the tubing in place.
Center-drill a 1⁄4-inch-diameter dowel
with a 1⁄16-inch-diameter hole. Fabricate the
tail-wheel assembly from 1⁄16-inch-diameter
music wire and insert it into the dowel.
Glue the dowel to the fuselage end.
Full-Size Plans Available—see page 183
The fuselage is ready to be fiberglassed.
This process is easy to do, but it can be
messy; try to do it neatly. With foam you
must use epoxy resin; other types will melt
the foam. The fiberglass cloth I use is 3
ounces per square yard in weight. Cut a
piece of cloth large enough to wrap around
the entire fuselage. Arrange to have the
fuselage stable when upside-down.
Wet the entire bottom of the fuselage
with epoxy resin. Center the cloth laterally,
lay it centered on the bottom, and stretch it
tight lengthwise. With your fingers, press
the cloth into the resin, working from the
centerline outward until the cloth is attached
in all of the resined area.
Lay the fuselage on one side, and coat
the exposed side with resin. Gently move
the cloth onto the resin-coated area; don’t
pull hard. Working with your fingers from
the bottom upward, press the cloth into the
resin. The objective is to have the cloth
attached halfway around the top. Trim any
excess cloth with scissors.
With that finished, coat the other side
with resin. Working from the bottom, press
the cloth into the resin with your fingers.
When you’re close to the top, trim the
excess so that there will not be a large
overlap.
Set the fuselage aside to allow the resin
to set; it should take approximately 24
hours.
Cowl: The cowl is complex in shape, but it
can be produced easily with this procedure.
It’s something you should remember for
future projects.
The Enya 46 four-stroke engine proved
to be an excellent match with the original
Shoestring. However, any engine roughly
that size should do well. Do not overpower
the model; it is a very low-drag design that
does not require excess power.
Install your engine and mount on the
firewall so that the crankshaft aligns with
the required thrustline. Plug all engine-inlet
holes with tissue paper to keep dirt and dust
out. Produce the 3⁄32 plywood cowl former.
Be sure that the outline is approximately 1⁄64
inch smaller than the fuselage firewall. Pin
the former securely to the firewall with a
piece of Saran Wrap between them.
Produce the cowl-spinner former from
1⁄16 plywood. This is secured against the
engine drive washer. To help you when
removing the finished cowl, make the
former hole 5⁄16 inch in diameter if your
engine has a 1⁄4-inch-diameter crankshaft.
Wrap masking tape around the shaft to fit
the former hole. When you are ready,
removing the tape will make it easier to
remove the cowl the first time.
Fill all around the engine with pieces of
foam. Use a bit of glue on each piece. The
object is to make the foam larger than the
finished cowl will be. Face off the front of
the cowl foam. Reduce the top foam until it
is close to flowing into the fuselage top
curvature. Mark a centerline on this surface
to use as reference, keeping the cowl
symmetrical. Produce the air-inlet holes.
This effort will teach you to be a
sculptor (if you are not one already). Begin
the sculpting by creating the cowl cheeks;
the inlet holes serve as guides. Don’t go too
far inward with them. Carving, flow the
bottom curvature into the spinner ring.
Blend that curvature into the cheeks. Do the
same for the upper curvature.
The cowl cheeks flow into the wing.
Install the wing. With the rib template,
create the required openings in oversize
pieces of foam. Fit those slots onto the
wing, and trim the foam close to the cheek
size. Cement the foam to the fuselage but
not to the wing.
Remove the wing, and shape the foam to
match the cowl cheeks. At the wing slots
the upper portion of the slot is cut loose at
the LE, and is then glued to the wing. The
lower portion remains on the fuselage and
unattached to the wing.
Cover the cowl foam with 3-ounce
fiberglass cloth. When that resin has set,
remove the cowl and sand it thoroughly.
Apply a layer of 3⁄4-ounce fiberglass cloth.
This layer will create a smoother surface.
Complete this process by fiberglassing the
wing fairing and the cheek fairings.
Sand all fiberglassed areas with 60-grit
sandpaper. Then apply a thin coat of
resin. When the resin has set and has been
sanded smooth, the Shoestring is ready
for covering and painting.
Covering and Finishing: Choosing a color
scheme is a highlight of a big project. The
Shoestring offers a variety of authentic
choices because of its full-scale racing
career. The Shoestring was top dog in
midget racing for a number of years, and
its ability was shown as it won with several
different owners and pilots. It seemed that
each ownership brought a new color and
scheme.
The first featured a yellow-green hue
that would be hard to duplicate. It’s widely
believed that the second time around the
trim scheme was kept, but the base color
was changed to blue. That’s the design I
chose for my Shoestring. A later Circus
Circus sponsorship inspired a wild scheme
of bright colors which would stand out on
any flightline!
The wing and tail are covered with ironon
film. I have found Coverite film easy to
use and durable. The tail is installed after
it’s covered. Before gluing the tail on,
check its alignment with the wing.
Paint the fiberglassed fuselage.
Preparation is essential. The easiest
procedure is to sand it out with 60-grit
paper until overlaps are blended, then add a
coat of thinned resin. (Acetone thins it
fine.) Make sure there are no
imperfections. Lightweight spackling
compounds in paint departments are
excellent for filling small dings and dents.
The same compound makes an easy primer
base coat. It fills well and sands
effortlessly. Put some compound in a cup,
and thin it with water to brushing
consistency.
There are numerous “hobby epoxy”
paints which may be fine for the color.
However, Rust-Oleum works fine and is
available in local hardware stores.
Installation: I would think that most
builders involved with this project have
been down this road a number of times.
There is nothing unusual about it. The
engine mount is held with T-nuts, and the
engine is fastened to it. The fuel tank is
installed through the wing opening. The
landing gear is attached with three 10-24
nylon screws. The radio goes in its
compartment to suit balancing needs.
Preflight: This is where you buy the
insurance for successful flight, so do it
carefully! Double check the following. Is
the engine shaft neutral to the thrustline? Is
the wing aligned? Is the tail aligned to the
wing? Is the balance within the range
indicated?
Balancing can be done with the wing
off. Arrange some risers for the fuselage
wing saddle (with the fuselage inverted).
Place a suitable stick on the risers, and find
a location where the fuselage will rock in
both directions, and there’s the balance
point!
Perform a radio-systems check. Do the
surface movements correspond to the
transmitter-stick movements? Is the
amount of surface travel sufficient? Don’t
forget the range check! This being a
cowled engine, the operation should be
checked with the cowl off; adjustments are
easier.
Make sure the engine operation is
correct. Are high and low speeds solid and
proper? Install the cowl and recheck
carefully; there should be no change. I
have been using an 11 x 7 propeller with
maximum rpm at 10,000. There’s more
than ample thrust!
Flying: Do you have a nice-flying model
with which you are familiar? Expect much
the same with Shoestring; it does not have
any vices. It grooves in normal flight, yet
responds smartly to control inputs. On
initial attempts, try maneuvers at a safe
altitude. There are no maneuvers that the
Shoestring is incapable of doing well.
Enjoy the applause you get when you
set the pretty model on the flightline, and
relish the way such good looks can
perform!
This project was done as a comparison to
determine if there is any advantage to the
inline arrangement versus a low wing.
Both versions have been flown
extensively, and for all practical purposes
no difference was apparent. Both
arrangements proved to be excellent
performers. Have fun! MA
Hal deBolt
2206 Greenwich Dr. Kings Pt.
Sun City Center FL 33573