Lu asked:
Thanks for the detailed explanation. Now I want you to carry it one step further. If a 1/6 horsepower motor were strong enough to excite a 1000-pound dynamic system and cause it to bounce up and down I would like for you to define the fractional HP motor as it applies the excitation process. Remove the motor from the equation and substitute the 2 per rev. The motor is powered by electricity. What is the motivational force behind the two per rev and what causes it?
Nick answered:
Every rotor produces a natural force wave form at N per rev where N is the number of blades. Basically, there is an oscillation of the lift caused by the natural dynamics of the rotor. I mentioned in a previous post that the magnitude might get as large as 10% of the blade's lift. The frequency is driven by the number of blades, and the amplitude of the oscillation drops very quickly as you get more blades, which is why many-bladed helos are very much smoother than those with few blades.
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[This message has been edited by Nick Lappos (edited 29 June 2001).]