Hovering in the Wings
Thanks for the leads. Unfortunately, Prouty's "Practical Helicopter Aerodynamic" is out of print. I will look into the paper by Stephen Jacklin.
The first couple of paragraphs of Nick's posting, combined with other information that he has previously provided, go a long way toward describing some of the sources of nP vibration.
It appears that the primary source is due to the asymmetrical airflow during forward flight and this
might (nothing is impossible <img src="eek.gif" border="0"> ) be an intrinsic feature. The angle of attack of the blades' elements will be different at different locations on the disk. Increasing a blade's angle of attack causes the coefficient of lift and the coefficient of drag to increase as well. Since any increases in these two coefficients are not quite equal, both the lift and the drag can not be made equal on the advancing side and the retreating side. If the lift is equalized between the advancing and retreating sides, then the difference in drag will result in a nP vibration.
The web page link, which Nick gave, lists some secondary sources such as retreating blade stall, advancing tip compression and vortices from previous blades. I suspect that these are temporal and can be eliminated or greatly minimized.
The fuselage, stabilizers, and the tail rotor (for those who must put up with such a handicap <img src="smile.gif" border="0"> ) are, no doubt, additional sources.
Perhaps an electronic, or a mechanical means beyond the swashplate, will or are advancing the control of the rotor above 1P.
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The above may have many errors and is certainly incomplete. Corrections and expansion, please.
[ 17 January 2002: Message edited by: Dave Jackson ]</p>