To: Cyclic Hotline
The Hydraulic system may have downed the V22 it also got me fired from a consulting job at Boeing Helicopters in Philadelphia.
I continually brought the subject up of reduced reliability of the hydraulic system due to higher than anticipated cyclic operation. I did this several times and each time I did my boss told me it was a Bell problem. I would disagree and that ended the conversation. Each time I raised the subject he would get progressively more angry. Finally he had my contract cancelled.
Here is the problem. The hydraulic servos had two phases of operation. One phase was when the V22 was in the helicopter mode and the servos would be commanded to change the swashplate position just like on a helicopter. Flying in the helicopter mode represents only a minor fraction of the total duty cycle for the hydraulic system. When in the airplane mode the hydraulic servos are maintained in a rigid position holding the swashplate in the position necessary to place pitch in the proprotor for forward flight. Unless the pilot changes the collective pitch to change thrust levels on the prop the servos never move. The entire life cycle was predicated on this kind of scenario and this in turn established the reliability requirements for the system.
Now I’m going to say something and I don’t want British trained pilots to jump on me and tell me I’m wrong. When in the airplane mode the proprotor responds to the gyroscopic turning moments induced when the aircraft deviates from straightforward flight. The same thing is true for an aircraft propeller. In the case of the propeller the shaft is held rigid in the engine nose case or in smaller engines the crankcase. The propeller wants to precess but it can’t. The proprotor on the other hand is mounted in elastomeric elements that will permit the proprotor to precess.
Built into the proprotor are flapping sensors. If flapping (read precession) is detected the hydraulic servos would be commanded to restore the proprotor to the normal or flat or, originally commanded position. This precession also causes thrust divergence. Because the proprotors are counter rotating the thrust line will be different for the two proprotors making it difficult to control the aircraft. This divergence will be different for every maneuver. Because the V22 spends a lot of time maneuvering while on a mission and, especially when getting ready to discharge troops in a landing zone the hydraulic system can operate ten to a hundred times more than predicted in the reliability analysis.
Please don’t think that I am saying this was the cause for the failure. It is just something to consider.
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The Cat