Cheyenne problems.
As Nick had indicated the Cheyenne suffered from sever rotor instability. This was caused by the inherent stiffness of the blade and the fact that the blades had a different camber at different blade stations, which contributed to the instability. On past posts the subject of phase angle shift was discussed. The problem with the Cheyenne was that the shift was not predictable and because the blades would behave differently under different conditions resulting in the helicopter flying in an uncommanded direction. The shift and the aerodynamics overcoming the blade stiffness under certain aerodynamic situations caused the blade to dip down and hit the fuselage.
I believe there was only one crash caused by this inherent instability and it killed the pilot. This particular Cheyenne had a downward firing ejection seat but it is problematic if the pilot could have ejected being so close to the water surface. Besides, he was not sitting in the ejection seat. The second incident of blade instability took place inside the large wind tunnel at Ames Labs in San Francisco, California. The breakup was so sever the wind tunnel was out of commission for quite some time.
Lockheed worked on the problem by trying different positions of the swashplate in relation to control input and subsequent control output. It didn’t work and it was like a dog chasing its’ tail never quite succeeding. The problem was turned over to Parker Bertea the builders of the servo system and the rest of the hydraulics system. They worked the problem for over two years and finally came up with a solution. It worked something like a SCAS system in that cyclic input would be monitored and the subsequent movement of the blade was also monitored. If the blade did not move in the commanded direction a modifying signal was sent to the servo to correct for the unwanted movement. Pilots that flew the Cheyenne with this system installed stated that it was absolutely the smoothest helicopter they had ever flown. However the blade movement monitoring system was part electronic and part mechanical and it was full of potential single point failures as to make the helicopter extremely dangerous to fly. At this point the program which was on hold was cancelled.
I do not believe that the control gyro action contributed to the blade divergence. Lockheed developed the Cheyenne based on previous designs, which were highly successful. They believed that they could scale the design up and it almost worked. The Army however was constantly adding systems and mission capability to the point that the rotor system could not perform according to specs. Lockheed requested that they be allowed to increase the rotor diameter and the Army refused. Since they could not increase the rotor diameter they changed the design of the blades to increase lift and that is when the problems started.