as the downwash from each rotor hits the boom it generates a lump of harder air (sort of). Since there are 4 blades that means in one rev you get 4 impacts from this. The same applies to any other non uniformity that occurs once per revolution for each blade. All these forces add together vectorially (depending on their phase angles and magnitudes) but for very fundamental reasons they still result in a 4 per rev excitation. Higher frequencies are also generated but in the absence of anything strange they will always be at a  multiple of the number of blades. Blade to blade variation will cause vibartions at all multiples of one per rev.

wavelengths doesn't come into it.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.