To: HeliEng
My statement about the Bifiler system was based on an article in one of the major aviation journals describing the system when it was first developed. In that article it indicated that the weights were capable of moving in plane which would support your statement about damping out lateral vibration. It also indicated that the weights were free to move in the vertical direction (up and down) which would act like the pendular dampers on other helicopters. If the article was wrong then I am wrong.
When the Apache was first designed the pendular dampers were similar to those on the model 500 which had two pendular dampers suspended on a common axis. The two dampers were of different sizes and weights which were tuned to specific frequencies of the standing wave on the blades in flight. I believe they may have removed one or both when the Apache went into service at least they were talking about it while I was still on the program.
As a matter of interest the MIL Mi-8 HIP has a system that looks just like the Sikorsky Bifiler damping system.
The Bifiler system and the pendular weight systems work in the same way. Both systems employ (Please don’t jump on me) centripetal and centrifugal force. The centrifugal force acting on the weights increases their resistance to movement and the weights are aligned with the blade. If any vibratory force such as the traveling wave moves down the blade from the tip to the root it will have to displace the weight of the pendular damper or the Bifiler damper. This weight will absorb the movement of the blade and damp out the frequency before it can enter the rotorhead or at least it will reduce the intensity of the force. On the Bifiler system the weight is free to move in plane so that any inplane vibration that feeds back to the rotorhead will be absorbed by the damping element.
I now stand open to criticism.
[ 15 January 2002: Message edited by: Lu Zuckerman ]</p>