There are some very knowledgeable people on this thread, so with trepidation, and for the fun of it, the following is humbly submited.
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Lu, could it be that Nick is correct in that the thrust of the rotor disk is quite consistent, irrespective of the location of the blades?
Could it be that in forward flight with a 2-blade rotor the downwash from the blades is striking the fuselage at 2/rev. During hover, this downwash is coming from center of the rotor disk where it is minimal. As the helicopter's forward speed increases, the downwash striking the fuselage will become stronger, since it is now coming from closer to the leading edge of the rotor disk. This will give the fuselage a 2/rev vertical vibration. This segment of the downwash is initiated when a blade is pointing forward but it reaches the fuselage when the blades are out to the sides.
[i.e. The rotor thrust is constant and the parasitic drag oscillates at 2P.]
A continuation of the oscillations, particularly if there is harmonic excitement, will cause both the mast and the fuselage to oscillate. The rotors oscillations will be the greater by far since it has the least mass, but, the rotor's oscillations will be restricted to the mast, hub and inner portion of the disk. The tips will oscillate very little and the thrust will vary very little.
Just a guess.
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A test for pylon rocking is described on the following web page:
http://www.adtdl.army.mil/cgi-bin/at.../1-211/Ch7.htm
[This message has been edited by Dave Jackson (edited 30 June 2001).]
[This message has been edited by Dave Jackson (edited 02 July 2001).]