To: Bladestrike
“Any of you learned fellows know at which RPM gyroscopic procession starts to become an issue? I'm thinking about blade sailing on startup/shutdown and where the moment applied by the wind will have its affect”.
Most helicopters, other than the Robinson, that have the capability to flap or for the blade to move in relation to the rotorhead have some type of centrifugally operated locking device that will not unlock until the blade has achieved sufficient rotational velocity and centrifugal forces to sustain the input of cyclic pitch with the resultant movement of the blade. Sikorsky helicopters at least those that I am familiar with have similarly operated locks to restrict flapping and drooping. Some Hughes helicopters do not have locks that restrict flapping but they do have a device that restricts drooping movement. This device supports the static weight of the rotor blades and distributes that weight of the blades evenly. If one blade flaps and stalls out or aerodynamically, is forced to droop, the kinetic energy is translated to the ring support and is absorbed by the resistance of the other three or four blades to move upward. This device is not normally in use once the blades cone up. If in the process of maneuvering one blade hits this stop it will simply shift the ring position in relation to the mast centerline.
I would suspect that those helicopters that are equipped with the centrifugally operated devices are capable of cyclic input and be susceptible to gyroscopic precession once the devices are activated. It is not to say that at speeds lower than the speed necessary to release the locking devices the gyroscopic precession is not present however, the blades are not capable to sustain lift and they could stall out contacting personnel or the fuselage.
On our B-47 (HTL-1) we had no such devices and unless the blades were restrained they would be free to move all over the place both from wind passing over the flight deck or, because of the ship pitching and rolling. This led to a one-time experience. The ship was rolling in heavy seas and the blades were moving all over the place. Prior to our going aboard the icebreaker we removed the springs from the centrifugal clutch. This meant that just as soon as the engine started the blades were moving at engine speed (via the transmission). This was the first time we started the helicopter since coming aboard. The blades were dipped down relative to the local horizon and when the engine started the blades immediately aligned with the local horizon. The forces were so great that the helicopter almost flipped over. Following that experience I would hold the blade tip in the neutral flap position and when the engine started I would pull my hand back. I also learned something from that experience and that was to move my hand back a bit faster as I got hit with the following blade. This model of Bell had restraining cables (Sprague Cables) in the rotorhead which limited the amount of teeter both in the non driven condition and the flight condition.
Now I will take cover.
[ 18 November 2001: Message edited by: Lu Zuckerman ]