Written and designed by Jim Ryan
Find the entire feature on page 28 in the December 2011 issue.
INTRODUCTION: In the late 1960s, the US Army contracted with Lockheed for the construction of a revolutionary attack helicopter called the AH-56 Cheyenne. In addition to the normal tail rotor, the Cheyenne had a variable-pitch pusher propeller. For hovering and low-speed flight, the propeller would spin at flat pitch, but for high-speed flight the propeller would gradually increase pitch, propelling the Cheyenne to higher speeds until its stubby wings were providing most of the lift.
At this point the main rotor was running nearly flat pitch, but it continued to provide pitch and roll control. This ingenious design gave the Cheyenne unprecedented speed and range.
In addition to its unique propulsion system, the AH-56 incorporated a number of other novel features that would eventually become standard on the next generation of high-performance attack helis. These included a rigid rotor head, terrain mapping navigation, and helmet-mounted sights.
Progress on the project was slowed by the ongoing war in Southeast Asia, and by the early 1970s the Army was becoming increasingly focused on Soviet armor. In the end, the Pentagon changed its mind and decided that, instead of a high-speed gunship, it really needed a tank killer that would use terrain as its main defense. The AH-64 Apache was the end result, and it has served capably for more than 30 years.
As a lifelong aviation nut, I was fascinated with the Cheyenne program when I was a kid. I was disappointed when it was canceled, so nearly 40 years later I decided to design my own AH-56, based on mechanics from the ubiquitous Align T-Rex 450.
The key to making this project feasible was Align’s release of a torque-tube tail retrofit kit, because this was the only practical way to drive the complex tail gearbox. With this important requirement checked off, I set out to build my own Cheyenne.
The completed Cheyenne made its debut at the 2011 IRCHA Jamboree. After working out some issues with controller programming, the heli flew beautifully, meeting all my hopes for the project. With the added lift capacity of the four-bladed head, the Cheyenne doesn’t handle at all like a heavy Scale model.
In subsequent flights the Cheyenne continues to improve. At flat pitch, forward flight is slow and predictable, and the pusher propeller can actually be a benefit since there’s little chance of the heli getting away from you. At full propeller pitch the Cheyenne accelerates briskly and really comes to life. It’s truly a delight to fly.
Designing and building the Cheyenne was one of the most challenging projects of my RC career, but also one of the most rewarding. Building the complex tail went smoothly, and the real challenges didn’t begin until I had to squeeze the mechanics into the slender fuselage. The good news is that all these puzzles have been solved, so your build should go easily. Good luck!