To: The Sultan
The elements that you alluded to being employed to dampen out what you call N-rev do exactly that but not for the reasons you gave. These systems are dynamic in nature and are installed on the rotor heads of some helicopters to dampen or eliminate the vibrations caused by the blades and their reaction to pitch and load changes. These cyclical loads cause a traveling wave that if not stopped would cause a cyclical load to be applied to the rotorhead and reflected in the airframe and felt by the pilots and crew/passengers.
Sikorsky uses the Bifilar system that has very heavy bob weights that are disposed between the blades. With centrifugal force the apparent weight of the bobweight is significantly increased. The traveling wave in order to get to the rotorhead must displace the weight from its’ radial position. To move the weight causes energy to be absorbed and the wave is cancelled. The MBB-105 and the BK-117, Bell 412 and some Hughes helicopters including the early version of the Apache have bob weights that are connected in most cases to the blades.
Some helicopters have two types of bob weights that are sensitive to different frequencies of the wave. Some Bell helicopters employ a static weight that is embedded in the blade at the nodal point for the wave in order to cancel it out by absorbing the energy of the wave. The Aerospatial helicopters have a heavy weight suspended on top of a stiff spring with the assembly being mounted on top of the rotorhead. Boeing Vertol has a mechanism mounted under the pilots’ seats to perform the same function.
It is my understanding that Bell developed an electromechanical system that worked in the same manner as a noise-canceling headset. The device sensed the vibration level and the cyclical frequency and generated an opposite sine wave and equal in magnitude which eliminated the vibration. This device was going to be installed in B-205s and HU1s but I don’t know if it was ever placed in wide use.
I do not believe that the incorporation of any of these energy absorption systems was incorporated to compensate for the loss of lift on a two blade or, a multi blade helicopter when the blade are disposed over the longitudinal axis of the helicopter. To eliminate the vibration problem Bell came up with the Nodamatic suspension system which isolates the dynamic system and the transmission from the airframe. When they went to a four-blade system on the 412 they incorporated the bob weights. Some Bell four blade heads do not incorporate the bob weights so I would assume that they went back to the embedded weights.
It is my opinion that the N-rev is in effect the reason these systems were developed with the N-rev being caused by the induced vibration resulting from the traveling wave..
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 March 2001).]