To: Grisoni
Your comment about low rotor rpm being the cause of rotor divergence is because Robinson said so and, you believed it.
In none of the accident reports did it specifically state low rotor rpm was the cause. It may have implied that (Please read the newspaper article above)but in most cases it said it was unknown. Until I wrote my report about the problem with the rotorhead design and the effect of that design relative to pilotage of the helicopter. The NTSB has indicated that they plan on revisiting the reports of accidents to see if the design of the rotorhead played a part in the accidents.
Low rotor rpm will not cause rotor divergance.
When the rotor rpm drops below the centrifugal force necessary to keep the blades in their respective radial positions the blades fold up and do not contact the airframe. Read the POH.
Fuselage contact is caused by retreating blade stall and or the incorrect control input to counter the condition and or significantly high flapping angles and not low rotor rpm as you have indicated.
These high flapping angles can be caused by many control inputs and two of these inputs are mentioned in the POH. Side slip and, flying out of trim. Don't argue with me by saying that you can perform these maneuvers as I will agree with you. But, the POH and the respective AD says you can't do them or, you will cause high flapping angles and strike the fuselage.
To: Helo Teacher
Go back and reread my post. I said that it is like a mild case of retreating blade stall. I did not say it was retreating blade stall. I stated that it was dysymmetry of lift and gyroscopic precession that caused the so called flap back. If the pilot counter the condition by pushing forward cyclic the condition will stop.
I also stated that if the pilot does not counter it by pushing forward and he allows the blades to come back then the disc will no longer be parallel to the swashplate and the delta hinge effect coupled with with gyroscopic precession will cause the helicopter to roll left (assuming I got it right as I had to visualize the entire action in my mind)
As far as your comment to Seismicpilot about Bell mast bumping and subsequent rotor loss you are totally correct. At my last account there were somewhere around 53 206 and UH-1 rotor loss accidents due to incorrect recovery from zero G. As far as loss of tail rotor effectiveness it can happen to any helicopter if the conditions are right.
To: Grisoni
Nowhere in my postings have I said that the Robinson helicopters are unreliable. There have been several fatal accidents caused by the failure of control elements but these have been fixed which restores the demonstrated reliability that you indicated.
My posts have been directed at the design of the rotorhead and the rigging procedure as the cause om many of the major accidents and not the reliability of the elements involved in my report.
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The Cat
[This message has been edited by Lu Zuckerman (edited 29 November 2000).]