To: Graviman
What Nick stated about electronic stability control is correct. However what he stated about the gyro becoming unstable is not. It was the design of the blades that caused the problems. As a matter of history the US Army kept adding equipment to the AH-56 to the point that Lockheed wanted to extend the blades to increase lift. The Army would not allow any change to the planform of the AH-56 because it would impact the support and transportation of the helicopter.
Lockheed aerodynamics engineers (read Ray Prouty) decided to redesign the blades to increase the lift without changing blade length. They accomplished this by adapting a radical design approach where at each blade station the blade had a different shape and camber. Unlike a conventional design the aerodynamic centers and centers of pressure were different at each station making the blade unstable. This instability manifested itself in blade divergence. This blade divergence was unpredictable and varied with airspeed, disc loading, and other factors.
On two occasions the divergence manifested itself in such a way that the blade struck the cockpit while the helicopter was in flight killing the pilot. This particular helicopter was equipped with a downward firing ejection seat taken from a B-47 however it was mounted in the gunner’s position and the pilot was sitting in the rear seat. It is problematic if the pilot could have ejected as the divergence was so fast and the helicopter was flying close to the waters of the Pacific. The second case of blade divergence took place in a large wind tunnel at Ames Research Center causing severe damage to both the helicopter and the wind tunnel.
The problem of divergence was turned over to Parker Bertea the makers of the AH-56 hydraulic system. I do not know if they investigated the use of electronics but the decided to go with an Electro-mechanical feedback loop. The system consisted of flap detectors and the resultant signal was transmitted via mechanical linkage the ran through the mast and the resultant signal was fed to a black box that monitored blade movement in relation to the signal being supplied to the servo. If there was a variance between the detected signal and the servo input the black box would alter the servo input to compensate for the divergence. A system very similar to this design is used on the Lynx, which has a 15-degree rigging offset. When the pilot pushes cyclic the electronic detectors measure any difference between pilot input and blade response and a signal is sent to the servos to counter the effect of the offset.
The Parker Bertea design worked perfectly according to Lockheed and Army test pilots but it was rife with single point catastrophic failures. By this time the program was cancelled.
