Author: Joe Beshar
Edition: Model Aviation - 2011/04
Page Numbers: 55,56,57,58
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The LEGENDARY 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, first held in 1929, were big events for aviation enthusiasts. After the war a new class of racers called “Goodyear” was created. A design team at Lockheed conceived of a racer that eventually became known as the Cosmic Wind series.

The group had enough members who encouraged Rodney to start his own design team; 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 since 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 weighed close to 9 pounds with standard radio 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 rules. 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 trying to remove weight.

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 comply 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 build stronger structure or to add weight strategically for better handling. And in case repairs are necessary later, fixing 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-and-plywood construction. The Top Notch I-beam 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 and join them with the formers.

Make the laminated wing bolt plate from one 1/4" light plywood and one 1/16" aircraft-plywood part. Press in the 1/4-20 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" 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, 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"-diameter dowels at the front and a 6-32 x 1" bolt at the aft end. Install the engine, place the cowl on the firewall, and screw it in place.

Wing assembly

  • On the wing, use a straightedge to align the false leading edge (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 until it aligns, and tack-glue 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; tabs should interface with W11.
  • Install the 1/16" wing trailing edge (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 requires 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" 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.

Control surfaces and tail

  • 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.
  • 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 used for the stabilizer. Lay the core sheet flat and install all the split ribs on one side. Install the TE with a straightedge. Trim the corners off the LE and install each rib aligned to the LE, then install the LE.
  • Trim and sand off all irregularities of every structure. Finish with temperature-sensitive adhesive-backed covering and apply the decals.

As 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.

Specifications

  • 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 (dry)
  • Wing loading: 18–21 ounces/square foot
  • Engine: .60–.91 two-stroke or .91 four-stroke
  • 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

The rules

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 to create something of your own. 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 great-flying model to a pylon course and find out who’s the fastest—for the sport of it.

Joe Beshar [email protected]

Sources

Transcribed from original scans by AI. Minor OCR errors may remain.