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Written by Donald R. Kling
The last of the Army Air Corps bombers
Construction article
Photos by the author
As seen in the October 2016 issue of
Model Aviation.

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Keystone I: 1/12 scale specifications

Wingspan: 74.75 inches
Length: 48.75 inches
Weight: 12 pounds
Motors: Two E-flite Power 32 brushless outrunners
Batteries: Two 4,000 mAh 4S LiPos
Controllers: Two E-flite 60-amp ESCs
Covering: Sig Coverall
Paint: Latex

Keystone II: 1/8 scale specifications

Wingspan: 112 inches
Wing chord: 12 inches
Length: 72.5 inches
Weight: 24 pounds
Motors: Two ElectriFly RimFire .80s
Batteries: Two Turnigy 5,000 mAh 5S LiPos
Dihedral: 3.25°
Sweep back: 4°
Receiver battery: Six-volt 2,000 mAh NiMH

About the Keystone Bomber

When my father passed away, he left behind a panoramic photograph taken in 1932 at Langley Field, Virginia, of the U.S. Army Air Corps (USAAC) 2nd Bombardment Group 59th Service Squadron, in which he served as a mechanic. Behind the group are three aircraft: a Keystone B-4, a Martin XB-907, and a Keystone B-6.

Close inspection with a magnifying glass revealed that the airplane on the left is a B-4 named “Columbia” (serial number 141). The Martin XB-907 was identifiable by its twin-engine monowing design, the engine cowlings, and its lack of a nose turret. The aircraft on the far right, the B-6, was easy to identify because it had “Pittsburgher” (serial number 167) painted on its nose.

The B-4 and B-6 differed only in their engine types. The fabric-covered aircraft had twin nine-cylinder radial engines, were built in Bristol, Pennsylvania, and were the last bi-wing bombers ordered by the USAAC. The aircraft were used for only six years until they were rendered obsolete by the Martin B-10 monowing.

In 2003, I acted on a latent interest in model airplanes, began flying, and the seeds of building were sown. The internet proved a valuable resource for information. I also found publications with Keystone articles that included a listing of all of the Keystones built, by model and serial number. This proved the data on the nose, where the aircraft were assigned, and their disposition (wrecked, surveyed, etc.).

There were also scale drawings of the side, top, and bottom views with engine and strut detail—enough to make one think about attempting to build the airplane. In the course of researching the aircraft, I found no evidence that there had been a flying model built, nor
were there any of the original aircraft
in existence.

At Barksdale Global Power Museum in Louisiana, I learned that there was an original section of a Keystone found in 1984 by airmen hiking in the woods at the base. They came across a metal framework that was later identified as the tail section of a Keystone Bomber, probably from the late 1930s when Keystones were flown there and used as bombing targets. The wreckage was donated to Shreveport, Louisiana, to help support a war reclamation effort; however, one tail section was missed and remained undiscovered for nearly 50 years.

The section now occupies a prominent diorama area in the museum—the last remnant anywhere in the world. The fabric is gone, but the framework remains in good condition. The aircraft’s serial number is known and has been promised to me. At the time of this writing, I had not yet received that information from the museum curator.

An excellent plastic model of a Keystone B-4, built by George Lee, resides in the U.S. Air Force Museum at Wright-Patterson Air Force Base, in Dayton, Ohio, in a glass case underneath the nose of a Martin B-10! The detail on George’s model is superb and served as a guide for my project.

I took diagrams from a magazine to a blueprint shop to have them enlarged to 1/12 scale. That would equate to a 74-inch wingspan, which would be ideal for an RC model. In measuring the magazine pages, I erroneously assigned them 1/96 scale. The enlarged prints were deemed inaccurate, too fuzzy, and unusable, so I began the task of hand drawing the plans.

It took several years to create usable drawings that I could build on because I was still in the workforce and often traveled. The time I spent on the project was limited. The first fuselage was constructed in left and right halves, but I found a flaw that I couldn’t correct or live with, so fuselage number two made its way to the board.

Deciding how to power the airplane and engineer the wing arrangement became a priority, because it directly influenced the fuselage construction. I decided to make the wings in three sections, with a permanent center and detachable outer sections for ease of transport. The wings had a slight sweepback and dihedral that began at the same point—perfect for the outer section attachment point.

At 1/12 scale, the flat-bottom airfoil was 1 inch. The engine nacelles were suspended between the upper and lower wings, so I decided that glow power was out of the question. There wasn’t room for fuel tanks in the wings, and I felt that the fuel lines from the fuselage would be too long, so electric power was my best option.

The outer wing sections attach to the center section using mating brass tubing. They are secured with a spring-loaded plywood hook that snaps into a slot on the mating wing section. A paper clip through a tiny hole in the center section end rib depresses the hook and releases the wing for disassembly.

The author detailed the dummy engines to give the model a more finished look.

The outer wing sections attach to the center section using mating brass tubing that is secured with a spring-loaded plywood hook that snaps into a slot on the mating wing section. A paper clip inserted through a tiny hole in the center section end rib depresses the hook and releases the wing
for disassembly.

I mounted hardwood blocks with 2-56 or 4-40 blind nuts in areas where the airfoil-shaped aluminum struts and the cabane struts attached to the fuselage. Fourteen-gauge wire located between the ESCs and the motors is encased by struts. Three wires were required. Two fit in the main strut to the engine nacelle and the third in one of the thinner struts to the lower part of the fuselage at the wing’s trailing edge.

The fit is tight because the struts are reinforced with flat carbon-fiber stock. The nose section, also a gun turret, is removable—exposing slide rails for battery tray installation and removal.

This model flew on Memorial Day 2012 at the Arizona Model Aviators’ field in Mesa, Arizona, and was piloted by Phil Todd. Although it flew and landed without incident, it was slightly underpowered.

The Power 32 E-flite motors and Zippy 5,000 mAh 4S LiPo batteries performed well, but were outmatched by the additional weight needed to balance the airplane—roughly 12 pounds. The construction of
the nacelles didn’t allow for larger motors, so I retired what would become known as Keystone I.

1/8-Scale Keystone B-4

Approximately 18 months later, I completed the 1/8-scale Keystone B-4 (Keystone II) with several internal modifications. The construction techniques were generally the same and that saved time. I reinforced the spars with carbon-fiber tape and made the flying wires functional, giving greater strength to the wings that are joined and supported by a 3/4-inch dowel-filled PVC pipe.

Larger ElectriFly RimFire .80 motors, Turnigy 5,000 mAh 5S LiPo batteries, and three-blade 15 x 7 Master Airscrew propellers for each motor provided sufficient power for the 24-pound airplane. It drew roughly 50 amps at full throttle.

Pull-pull control linkages were used for the elevator and rudder, and the aileron servos are mounted in the wings. All servos are JR analog with the exception of the rudder servo, which is a Hitec HS-5485HB digital.

The larger of the two Keystone models under construction.

The author with the 1/8- and 1/12-scale versions of the Keystone.

The 1/8-scale model made its maiden flight on December 30, 2014, at the flying field in Mesa. This was my first experience with a biplane and multiengine. My thanks go to Charles Nelson, from Massachusetts, for lending his expertise in locating flight-threatening imperfections and his invaluable advice on biplane flight characteristics.

Both airplanes are entirely scratch-built, although I did have 120 ribs laser cut for the larger model. The aircraft are covered with Sig Koverall, primed, painted with ordinary latex house paint, and finished with a clear coat.

I doubted that there would be any color photos of the original airplane, but I was able to locate a color drawing that served as a second guide for my model. The Keystone’s original olive drab and yellow colors are documented, so they were easy to reproduce. Surprisingly, in the 13 years since I began the project, I have never heard of another flight-capable Keystone model.

Both models were painted with ordinary latex house paint and finished in clear coat. This is the 74.75-inch wingspan Keystone.

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