To: tacks
The Cheyenne was very stable in a hover with minimal impact due to the wings. In a hover the propeller was in flat pitch and the driving power was absorbed by the tail rotor. The pilot would introduce cyclic pitch and the helicopter moved forward in a conventional manner. Once the helicopter was at speed the pilot had a beta control and he increased pitch in the propeller and reduced tail rotor pitch and now the propeller was absorbing the power. The pilot would lower collective and at this point the wings were providing the lift. In this situation the helicopter was an autogyro with flight control being provided by cyclic input.
The pilot could also place the propeller in reverse pitch and this would counter the forward thrust of the rotor system. This allowed the pilot to place the helicopter in an inclined attitude relative to the horizon making the helicopter a very stable weapons platform.
At high speeds the helicopter rotor system became unstable. This could have been caused by one of two things.
1: Autogyros (certain types) use the rotor system as a means of control and the rotor is at flat pitch. With the introduction of forward cyclic input the blades can go to negative pitch causing the blades to flap down and make contact with the fuselage. This is the situation with the Cheyenne and on two occasions there was rotor incursion and in one case the rotor disintegrated and caused a great deal of damage to a wind tunnel. In the other case the pilot was killed.
2: The rotor blades on the Cheyenne were very stiff as compared to a conventional helicopter blade. The Cheyenne blades were also different from other helicopter blades of that time period. At each blade station the aerodynamic shape of the blade was different which introduced a degree of instability. The changing of the blade cross section coupled with the blade stiffness effected the phase angle of the blade and the pilot would never know where the disc would dip down with cyclic input. This unstable blade condition may have been the cause of the two incidents mentioned above.
It took two years of design development by Parker Bertea and they finally solved the problem of variable phase angles. Although the design changes gave the Cheyenne a very stable and vibration free ride the design was too complex and it contained a large number of failure points that were catastrophic in nature.
The Army cancelled the program and opened bids for the new AAH. This led to the Apache contract won by Hughes.
