The LEGENDARY SHOESTRING GOES PYLON RACING - 2011/04

April 2011 55
TheLEGENDARY SHOESTRING
Goes AMA PYLON RACING
by Joe Beshar
THE SHOESTRING has always been my
favorite RC model. I’ve worn out at least six
of them powered with four-stroke .91
engines, and I built a 41% version, powered
with a 3W-85EI gas engine, that weighed
almost 37 pounds.
Rodney Kreimendahl of Westfield,
Massachusetts, designed the full-scale
Shoestring racer. He was not a pilot, but an
avid modeler who loved airplanes and won
several modeling awards.
Influenced by Charles Lindbergh’s
transatlantic flight and unable to go to
college, Rodney became a draftsman for
Chance Vought. Then one summer he won a
scholarship to Northeastern University.
Before World War II, when Lockheed
won the Hudson contract from England,
Rodney was recruited as a structural
engineer to Burbank, California, to work on
the Lockheed YP-38 boom.
The Cleveland Air Races, which were
first held in 1929, were big events for
aviation enthusiasts. After the war a new
class of racers called “Goodyear” were
created. A design team at Lockheed
conceived of a racer that eventually became
known as the Cosmic Wind series.
The group had enough members on the
team who encouraged Rodney to start his
own design team, in which he did the design
work for his own racer. When he consulted
his wife about the name of the new airplane,
she came up with “Shoestring,” because the
project was being built on a shoestring
budget.
The Shoestring was first flown in 1949,
with an engine that race pilot Bob Downey
loaned to Rodney. The aircraft went on to
claim the most wins in aviation history.
It went up for sale after the 1952
Goodyear Air Races. In 1954 Rodney died
in an airplane crash while he was working on
a top-secret project whose name was never
released.
AMA event 421—Formula 1 Sport—was
conceived in 2002, based on then-available
ARF offerings such as the Great Planes
Shoestring 60, which has been discontinued.
It had a very light balsa-and-plywood frame
with iron-on covering.
Later Great Planes introduced two more
in the Cosmic Wind series: the Little Toni,
which is still available, and the Minnow,
which has been discontinued. If you can find
these models, they can be finished with a dry
weight of approximately 8 pounds.
Homemade models soon hit the
racecourse to compete with the ARFs. Using
conventional construction methods and
materials, these creations came out weighing
close to 9 pounds with standard radio
STATE
SPORT
o f t h e
Top Notch Products kit fits the
bill for sport and competition
04sig2.QXD_00MSTRPG.QXD 2/22/11 11:12 AM Page 55
56 MODEL AVIATION
Instructions included with the Top Notch kit are well detailed.
The elevator-surface parts are laid out roughly in order of
placement.
As is the fin, shown above, the horizontal stabilizer is assembled
with a flat core and half ribs. Full-span sheeting is applied to both
sides, clamped, and stored to hold flat.
Right: The rudder is constructed with
laminations of various balsa parts. The
vertical fin features a center core with half
ribs that are later sheeted with balsa.
The Top Notch “never fail”-guaranteed wing
spar is formed from laser-cut parts and
vertical-grain webbing. Tabs on the ribs
virtually eliminate the chance of producing a
crooked wing.
Alignment of the wing halves is assured
by the two 1/4-inch dowel pins. Clamping
the joint tightly removes excess epoxy
from the parts.
The Top Notch Shoestring requires much
laminating, as shown with the fuselage
sides and wing mounting block. Thin
materials are laser-cut more accurately.
Above: The Top Notch kit contains all
laser-cut balsa and plywood airframe
components. The cowling is fiberglass, as
are the wheel pants that attach to the
aluminum landing gear.
equipment—still under the 10-pound weight
limit. However, the lighter ARFs fared better
in the hands of skilled RC pilots.
Rather than drive the technology toward
exotic and expensive molded airframes and
ultralight radio gear, the proponents adjusted
the weight limit upward. In 2007 the adapted
rules included a minimum weight increase, to
8.75 pounds.
That way, conventional materials and
construction techniques could remain
competitive. That racing class is a “sport,”
after all, and is supposed to be fun—not
expensive, as we’ve seen Q-500 become.
In keeping with the “friendly” mission of
the race class, the Great Planes Formula 1 and
World Models Midget Mustang ARF 60-size
models are grandfathered in as legal for 421-
class competition. They need to make the
minimum weight, but adding ballast is much
easier than taking it out.
Thank you, Duane Gall, for keeping RC
Pylon Racing fun and interesting.
Top Notch Products sells a 28% kit of the
famous Shoestring Formula One racer, which
was designed from the beginning to be a
competition aircraft that complied fully with
the original AMA 421 rules. The aircraft
comes out lighter built stock.
The nearly 3/4 pound of weight that might
be needed to make the race minimum leaves
comfortable room to be built stronger or have
weight strategically added for better handling.
And in case repairs are necessary later, fixing
Photos by the author
04sig2.QXD_00MSTRPG.QXD 2/22/11 11:13 AM Page 56
April 2011 57
the model won’t make it less competitive.
Whether or not you plan to race, the Top
Notch model has proven to exhibit some of
the best flight characteristics, especially with
the throttle wide open.
The kit features 100% laser-cut balsa-andplywood
construction. The Top Notch IBeam
wing spar is guaranteed never to fail in
flight and assembles quickly. The kit also
contains a fiberglass cowl, aluminum landing
gear, a formed canopy, and complete building
instructions.
This Shoestring is a fun build; following
is a review of the experience.
Construction: Build the three laminations
that will comprise the firewall. Assemble the
two fuselage sides with the former notches in
the sides. Put together the two fuselage sides
with the former notches in the sides.
Make the laminated wing bolt plate from one 1/4 light-plywood and
one 1/16 aircraft-plywood part. Press in the 1/4 x 20 blind nuts.
Adhere the wing bolt plate assembly and servo tray with epoxy.
Install F-4 and F-3 with epoxy. Install the firewall assembly and tank
floor with epoxy.
Assemble and install the hatch hold-down block to F5A. Add some
1/4 and 3/8 triangular stock at the firewall and landing gear block, for
extra strength.
Install the 3/16 stabilizer doublers. Mount the five 1/4-inch square
stringers, top and bottom, between the firewall and F3. Install outer
formers and then the wing saddle. Check the outer sides of the
fuselage for smoothness and obstructions, and then sand and assemble
the upper skins.
The hatch is constructed on the fuselage with the wing in position;
it is retained with two 1/8-inch-diameter dowels at the front and a 6 x
32 x 1-inch bolt at the aft end. Install the engine, place the cowl on the
firewall, and screw it in place.
SHOESTRING
Model type: All-wood RC kit
Skill level: Intermediate builder and pilot
Wingspan: 64 inches
Wing area: 765 square inches
Length: 54 inches
Weight: 6-7 pounds
Wing loading: 18-21 ounces/square foot
Engine: .60-.91 two-stroke or .91 four-stroke
The author with his
seventh Shoestring
model to date. He
admits that the Top
Notch kit was enough
fun to recommend to
his friends.
The hatch is constructed on the fuselage
with the wing in position and is retained by
two 1/8-inch-diameter dowels at the front and
a 6 x 32 x 1-inch bolt at the aft end.
An O.S. .91 Surpass engine is an AMA
event 421-legal power system for this
Shoestring. Side mounting makes muffler
installation easier.
Radio: Four channels minimum, five
standard servos
Construction: Laser-cut balsa and
plywood, aluminum landing gear,
fiberglass cowl and wheel pants
Covering/finish: Heat-shrink film,
fuel-matching fuelproof paint
Price: $229
04sig2.QXD_00MSTRPG.QXD 2/22/11 11:16 AM Page 57
58 MODEL AVIATION
On the wing, use a straightedge to align
the false LE with the plans. Assemble the twin
sets of W7, W7-A, W8, and W8-A opposite
one another.
Place the bottom shear web on the bench
with the slots up. Slide each rib into its
appropriate slot. Install the top shear web
onto the assembly until it aligns, and tackglue
with a small amount of CA. Do the
same at the tip of the shear web.
Slide each rib tab into the appropriate
notch in the false LE. Install the plywood
wingtip in the slots of the LE and the shear
web tip and tabs to interface with W11.
Install the 1/16 wing TE in the slot in W7,
and slide it forward into the slots at the TEs
of ribs W2-W6. Tabs will engage in rib W1.
Install the aileron servo-bay TE. Install
and glue the top spar flange. Plane the false
LE to contour with the wing ribs and apply
the top sheeting. Each wing skin will require
four 4 x 36 sheets of 1/16 balsa. Install the
two servo-mount rails.
Assembling the wing halves is a two-step
process. Install two 1/4 x 1-inch dowels in
the holes provided in the 1/4 light-plywood
wing-joiner rib. Assemble the wing halves
with the wing-joiner rib. Prop up one
wingtip 4 inches at W12 for the correct
dihedral. Glue on the wing-bolt plate.
The aileron is laminated from three
pieces. The bottom piece is 3/16 balsa, the
center-section is assembled from 3/32
plywood, and the top is 1/4 balsa.
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Assemble the stabilizer from two
parts, with the core sheet flat. Install all
split ribs and the TE. Turn the part over
and install all the ribs on the opposite
side. Assemble both stabilizer skins, sand
out all irregularities, and apply the balsa
skin.
Assemble the elevator sections, fin,
and rudder using the same technique you
did with the stabilizer. Lay the core sheet
flat on the bench and install all the split
ribs on one side. Install the TE with a
straightedge. Trim the corners off of the
LE and install each rib aligned to the LE,
and install the LE.
Trim and sand off all irregularities of
every structure. Finish with temperaturesensitive
adhesive-backed covering and
apply the decals.
As I mentioned, the lightly loaded
aircraft is a pleasure to fly as a sport
model. Before you know it, you will be
ready for AMA Pylon Racing.
The rules we have today for the AMA
Sport Formula 1 class give us more
options, even if the ARFs aren’t
available. Today it’s easier than ever to
build your own racer, such as the Top
Notch Shoestring, or maybe you have
something in mind that you want to create
yourself. There are many Formula 1
designs from which to choose.
As before, the idea is for it to be easy
to bring a great-looking and -flying
model to a pylon course and find out
who’s the fastest—for the sport of it. MA
Joe Beshar
[email protected]
Sources:
AMA 2011-2012 RC Pylon Racing rules:
http://bit.ly/fMtcIO
Top Notch Products
(615) 866-4327
www.topnotchkits.com
National Miniature Pylon Racing
Association
www.nmpra.org
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