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Hi all,

Can someone explain to me briefly as to what 4/rev hub loads mean exactly ?? I know that I will see 4 wavelengths in the response if it is 4/rev and similarly for other kinds of response. But, I don't understand the physical significance behind this. Why am I seeing 4 wavelengths in the case of a 4-bladed rotor's hub loads and for that matter even higher harmonics hidden  ? Can someone throw more light on this ?



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.


Greg Locock

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