Sleek, scale military heli built using modified Align 450 parts
To order the featured plans, see page 128, or visit www.modelaircraft.org/shop/plans.
In the late 1960s, the U.S. Army contracted with Lockheed to construct the AH-56 Cheyenne, an attack helicopter that had a variable-pitch pusher propeller in addition to a tail rotor. The propeller would spin at flat pitch during hovering and low-speed flight, but gradually increase pitch for high-speed flight until the wings could provide most of the lift. When the main rotor was running at nearly flat pitch, it would provide pitch and roll control.
As a child, Jim Ryan was fascinated by the Cheyenne and disappointed when the program was canceled. Nearly 40 years later, he designed his own AH-56, based on the mechanics of an Align T-Rex 450 heli. Align had released a torque-tube tail retrofit kit, which was the only practical way to drive the tail gearbox on the Cheyenne. Replicating the tail was central to the project.
Jim wanted to make the heli as true to scale as possible, but because the fuselage was long and slender, it posed some challenges. A standard 150-tooth main gear was too large, so Jim modified the mechanics to use a 132T gear. The low fuselage profile did not provide enough height for a standard 450-size motor, so a clearance hole was cut, providing air to cool the motor.
This project was not for a beginner builder. It required machining and fabrication skills beyond those that are used with a traditional helicopter build, although Jim did offer alternatives and solutions to help simplify the build.
The tail unit was fabricated using parts from the Align torque-tube retrofit kit. A spare tail unit was purchased because an extra gearbox side plate was needed. Standoffs for the extended tail rotor mount were fabricated from 4mm aluminum knitting needles that were cut to 2½ inches long. The extended tail rotor shaft was a 120mm piece of 3mm stainless drill rod. An offset bellcrank was fabricated for tail rotor pitch control, with a pivot arm to keep the pitch-control slider from flopping around. The pusher propeller shaft was a spare tail-rotor shaft that was press fitted into the bevel gear.
The main frame was primarily the upper side plates of a T-Rex 450 V2. The rear cyclic servo had to be relocated to fit inside of the Cheyenne. Jim modified the motor mount to allow the motor to slide far enough to mesh with the smaller gear, and had to grind down the base of the pinion so it wouldn’t rub on the autorotation gear. The stock gear train could be used if the builder did not mind cutting clearance slots in the body.
The mechanics were tested and installed into the body of the Cheyenne. Jim used a removable canopy for battery access and made the hatch between the cockpit and main shaft as small as possible. Stub wings were carved from soft balsa and mounted to the body using carbon-fiber joiner tubes from The Composites Store.
The retractable landing gear folded backward into the sponsons. Jim actuated it with a simple gear train that was built with RC car pinions and used a metal gear microservo to actuate the gear.
Jim opted for the standard Army markings that are on the AH-56 exhibited at the Army Aviation Museum at Ft. Rucker, Alabama. He generated CAD drawings for the markings and ordered paint stencils from Callie Graphics. The color scheme was done in Testors Model Master enamel paints, applied with an airbrush, then clear coated with matte lacquer.
According to Jim, the Cheyenne flew similarly to the stock T-Rex, “except that it’s slower because of the propeller disk drag.” Top speed was impressive, and there was very little pitch change and no adverse handling issues. For a more scalelike appearance, Jim switched to a Black Angel four-blade head with weighted SmartModel scale blades. “At full propeller pitch, the Cheyenne accelerates briskly and really comes to life. It’s truly a delight to fly,” he stated.
The completed AH-56 Cheyenne debuted at the 2011 International Radio Controlled Helicopter Association (IRCHA) Jamboree. It also placed first in the Helicopter class in the R/C Model Competition at the 2011 Toledo Show: R/C Model Expo.
The AH-56 Cheyenne was featured in the December 2011 issue of Model Aviation as AMA Plans Service number 1059, and is available for $10, plus shipping and handling. AMA members can access the MA Digital Library on the magazine’s website to read more about this helicopter and its construction. See page 128 or go to www.modelaircraft.org/plans.aspx to order.
