To: Dave Jackson
At the risk of being publicly chastised for my offering an opinion on this matter I offer the following:
A vertical vibration may be caused by an out of track blade but I believe that it is most likely caused by the undamped sinusoidal wave that travels down the blade from the tip to the root.
Some helicopters have pendular weights on the
blades or the blade roots as energy absorbers while on the Sikorsky rotor heads they use a Bifiler* System (SP) that absorbs this energy. Some Bell blades have a weight inside the blade with the weight being placed at the nodal point of the wave. Some French helicopters have a weight supported by a coil spring with this weight being installed on the top of the rotorhead while other helicopters have a system that works like a noise canceling head set which sends out a sine wave that is equal to but opposite in polarity. This type of device can be mechanical or electronic. This sinusoidal wave was not too much of a problem on earlier designs that used the NACA (NASA) 0012 airfoil. When the helicopters started using unsymmetrical airfoils the problem started to manifest itself.
*Bifiler if I spelled it correctly means two threads and I can’t determine why they would chose this name for the system.
I shall now assume the position.
[ 14 January 2002: Message edited by: Lu Zuckerman ]</p>