To: Dave Jackson
When the Cheyenne Helicopter was first built it flew quite well. I don’t know if you are familiar with the rotor system on the Cheyenne or not. The Cheyenne had a rigid rotor system that was free to feather but was rigid in plane and flapped by flexing the blade and the arms of the rotorhead. In order to introduce pitch change into the blades the pilot would move the cyclic and caused a servo to move. This servo was attached to a horseshoe shaped spring, which was compressed. The spring compression was transmitted via control linkage to a gyro that was mounted above the rotorhead. This gyro which consisted of four arms disposed at 90-degrees to each other and each arm had a very heavy weight at the end. This produced a gyroscope rotor that was capable of imparting a great deal of force. The individual arms were attached to the pitch horns on the blades. When the pilot input cyclic movement and the gyro would displace 90-degrees later inputting a pitch change into the blades and the rotor system like a gyro would displace 90-degrees in the direction of rotation.
As the design progressed the Army kept adding weight to the Helicopter. Lockheed indicated that the helicopter could no longer meet its’ performance guarantees and Lockheed requested a change in the rotor diameter to regain the necessary lift. The Army said no so Lockheed went back to the drawing board. What they came up with was a bastard design that not only had negative twist like other helicopters they also included a camber which added a degree of instability. Then they did something that was quite novel and they painted themselves into a corner in the process. Not only was the blade cambered it had a different camber at different blade stations so from the root to the tip the blade had constantly changing in its’ cross section.
This design in concert with the blades inherent stiffness caused major problems as the blade would not fly or precess to the position commanded by the pilots’ cyclic input. The first time this problem manifested itself a pilot was lost, as was the helicopter. The next time it happened it took out the wind tunnel at Ames labs in California.
The problem was eventually solved but the solution was so complex that it became sensitive to a single point failure that would cause loss of the helicopter. The solution although complex worked similar to the electronic system on the Lynx helicopter that corrects for a 15-degree coupling in the cyclic control.
Now, way back then when I learned how the system worked Ray Prouty indicated that there were two gyroscopic elements. The control gyro which precessed and the gyroscopic precession of the rotor. It seems that that assumption is no longer valid.