Hovering in the Wings asks (denoted by >> <img src="wink.gif" border="0"> :
>>1. Is the theory you have described:......
that of Sikorsky's (I assume Jerry Abbey's) or your own??
Nick sez: It is what Dynamicists (like Jerry!) tell me and is the current accepted theory within the industry.
>>2. Can you elaborate further your explaination of a 1 per being a function of the blade flapping?? I was always under the impression that a 1 per was a function of an out-of-blance condition.
Nick sez: I was referring to the blades flapping motion as an intrinsic 1/rev resonance. This is not a reference to any vibration, but to the natural flapping motions of the blade. Similarly, the n/rev vibrations are intrinsic outputs of the dynamics of the rotor, and are not the result of something broken that must be fixed or adjusted.
>>3. By "root Shear" (luv that term), I assume you mean a transmission of energy from the blade to the mast (via the root obviously)???
>>4. Is this "shearing" a vertical or a lateral vibration??
Nick says: exactly so, if you picture the blade as a kind of mechanical shaker attached to the hub arm. The shear forces are in both lateral (really lead-lag) and vertical directions.
>>5. In your experience (or Sikorskys view) what flight regime are n pers most noticeable???
Nick sez: Mostly in transition and at high speed.
>>7. Finally, what CAUSES the vertical component and lateral component of the N per rev vibe (not N+1, N-1 vibe)??
Are you referring to the Bifilar as the absorber of in-plane N+1 & N-1 per rev vibes - I assume so - not the n per rev absorbers typically fitted to the airframe??
Nick sez: The blade produces varying forces in all axies as it swings about the mast, due to the assymetrical conditions of upwind and downwind flight, the natural dynamic vibrations of the blade as it leads and flaps and also due to disturbances from other blades. The n/rev we feel is the sum of the forces from all the blades as felt in the stationary system.
The root shears passed to the head are in all directions, and produce responses in the airframe that can actually amplify (if the airframe has resonances at those frequencies). A absorber can be very effective even if not aligned with the root shear. Picture the rotor hub arm being shaken by a blade in the vertical direction. If a bifilar is on that rotor head, it will feel the resulting motion of the head as both a vertical and a lateral (sort of a rolling motion), and so will be effective even so. The bifilar is tuned to either n-2 per rev or n+1 per rev, usually n-1 since this lower frequency is more disturbing to the airframe. We have flown double bifilars to absorb both frequencies.
For Dave Jackson, who said: ....I have the audacity to think that the intermeshing configuration might be better than the coaxial. The intermeshing rotors have their centers of rotation laterally offset from the centerline of the craft and this means that the 75%R position on the advancing blades is closer to the craft's longitudinal centerline.
Dave, the intermeshing could be worse, because the blades directly fly in each other's wakes. Let me know how it works, OK?
[ 15 January 2002: Message edited by: Nick Lappos ]</p>