To: Chuckolamofola
First of all, I would like to congratulate you for knowing the technical names of the dynamic and electrical elements used to decrease or eliminate unwanted vibrations on helicopters. Some of these units were created after I left the respective programs and I have never worked on some of the programs so my information comes from Aviation Leak and Space Technology, Rotor & Wind and other trade journals. My responses are keyed to the numbered paragraphs in your post (Below)
1) Not all of the N per rev. systems are dynamic. On the 206B the isolation mount reduces the lateral 2 per rev generated by the 2 bladed m/r system. The isolation mount is nothing more than a rubber shock absorber.
2) As the Sultan pointed out 2 bladed rotors have a 2 per rev and 3 bladed 3 per etc. I don't know if the reason is due to the traveling wave or not but these same phenomena can be measured in ground based fan systems. This is also known as the "Blade Passage Frequency"
3) In addition to the BOB weights as you called them the (Bell calls then Pendulum Absorbers), the 222 employs a Nodal Beam System to reduce the vertical 2 per rev's and a nose Frahm damper to reduce the lateral 2 per rev's. All three are dynamic but only one is mounted on the rotor. Which one reduces the traveling wave?
4) In addition to the SPAs (Simple Pendulum Absorber) you mentioned on the Bell 412 they also employ a nose Frahm damper for lateral 4 per rev. reduction. The 412's you saw without Spa’s were probably Agusta made as they have some customers that would rather live with the payload gain than having a smoother ride and the associated payload loss when the SPA's are installed. Weights haven't been added to m/r. The 407 doesn't have SPA's but does have the Frahm damper installed on top of the rotorhead.
Sikorsky uses the Bifilar on the 76 to reduce the 3 and 5 per rev's and a mass absorber similar to the Frahm to reduce cabin 4 per revs' on later models. On the S76A model they use the VTA (Vertical Trim Amp.)to reduce the vertical 4 per rev. This unit mounted in the nose uses a combination of hydraulics and electronics to perform the magic feat.
The Aerospatial (ECF) AS350/355 employ the spring mass on top of the head to reduce lateral 3 per rev's and then hammers under the pilot's seats to reduce the vertical 3 per rev. The hammers are just a weight mass mounted on a flat spring and tuned to reduce the vertical N per rev. of the m/r.
5) I don't know if I agree with your statement regarding loss of lift as the blade passes over the longitudinal axis. If this happens as you say then on a two bladed rotor, one would think you would feel a 1 per rev. and on a 4 bladed rotor you would also experience a 1 per rev. In flight a 1 per rev. that is not a result of mass imbalance is usually caused by the unequal lift of one m/r blade.
Based on your theory of the traveling wave, what would cause a 2 per rev. imbalance on a Bell 412?
6) With respect to your 2 per rev in engine to transmission driveshaft, most any driveshaft that is misaligned will show a 2 per rev of shaft speed. I doubt the 2 per rev caused the grease loss but more likely was caused by burping the grease from the seals due to misalignment.
General response:
Sikorsky first discovered the traveling wave phenomenon when they installed a movie camera on top of an S-51 rotorhead that was mounted on a whirl stand. Sikorsky was looking for indications of leading and lagging and they found two things that they had not anticipated. Up until that time it was thought that the advancing blade lagged due to increased airloads and the retreating blade led due to decreased air resistance. The opposite was found to be true. What they also discovered was that the blades as viewed spanwise looked like a sinusoidal wave used in describing AC electrical flow. It is my understanding that when this movie was shown to helicopter pilots some of them stopped flying.
Responses:
1) The N per rev I was addressing on the Bell has to do with the alignment of the blades along the longitudinal axis and the loss of lift. This causes a two-per-rev vertical beat and has nothing to do with a lateral beat. I would think that if you have a lateral beat it relates to an imbalance condition.
2) I agree with what The Sultan stated about the vibrations on helicopter blades in general. As I indicated in (1) above the vibrations can be traced to one of several phenomena. On two bladed rotor system the vertical beat can be traced to the alignment of the blade as on a Bell. The other vibrations can be traced to an imbalance in the rotor system or, the presence of an uncompensated for traveling wave.
3) If the Bell philosophy carried through from other designs none of the described elements counters the traveling wave. As I had indicated in my post, Bell embedded a weight in the blade at the nodal point of the traveling wave and it is this weight that stops the wave from reaching the rotorhead.
4) I had indicated that the 412 rotor system had the pendular weights but that other Bell four blade systems did not. I also alluded to the fact that they may have reverted to the embedded nodal weights in the blades. It was my understanding that the Bifilar system was developed to counter the traveling wave generated vibrations. Whether they were three per rev or five per rev I can’t say. The system you described by Sikorsky that employs hydraulics and electronics was I believe developed by Hughes and was never used by them. The system has sensors that detect the vertical beat and sends a signal to the hydraulic servos to extend or retract to counter the vertical beat by introducing a countering pitch change. On all of the other elements we agree. I just didn’t know the correct names.
5) On a two-blade system you feel a two per rev vertical beat because the blades comes into alignment with the longitudinal centerline twice in a 360-degree rotation. The one per rev you described on a four-blade system could relate to an out of track on one of the blades. Using my theory to explain why you don’t get the vertical beat on a four-blade system due to the alignment of two blades with the centerline of the helicopter is that you still have two blades generating lift. I don’t know if this is true or not but, you don’t get this type of a beat on a four blade system unless something is amiss.
6) As described in my post this grease pumping phenomenon was first discovered on the 214 because of the movement of the transmission relative to the fixed engine. Assuming a rotational speed of 250 RPM on the rotor, the engine to transmission misalignment occurred 500 time per minute. The misalignment could be several inches depending on the maneuvering of the helicopter. This is what I stated in my post and you agreed with it. The two per rev caused the problem and the Nodamatic suspension compensated for it.
Here is an after thought:
The traveling wave has always been present on helicopter blades from the very beginning but the resultant vibrations were determined to be acceptable to pilots and passengers. When the problems began to surface was when helicopter manufacturers began developing and installing high lift blades which unlike previous designs, which were symmetrical, the new high lift blades were unsymmetrical. The symmetrical designs had highly predictable aerodynamic characteristics where the unsymmetrical blades did not. Unsymmetrical blades want to climb and dive and as such must be restrained by fixed points on the rotorhead and in the flight control systems. This locks the forces in the blades and external devices were incorporated to eliminate or at least minimize the resultant fuselage vibration.
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 March 2001).]