I would agree with all of the comments above related to mechanical origins for these vibes, but some vibes outside of operating RPM are always going to be there as the rotor system is tuned for 100%. Certainly, all control rod bearing rod ends should be inspected, and the skid gear attachments, but much of this is the nature of the beast. If it's smooth in flight regimes, then it's doing its job.
There is a Bell Engineering explanation to this as well, and it ties into a couple of comments above, with the same solution, pull a small amount of collective to smooth is out.
https://www.uh1ops.com/amp/2015/06/10/collective-bounceCollective Bounce
Pilots have asked me to please do something about aircraft that have a bounce with flat pitch running on the ground. I tell them to pull up slightly on the collective until the bounce is reduced. I could adjust the pitch change links to make the aircraft smoother, the problem is I have just changed the auto rotation RPM. I try to get them to understand auto rotation RPM is much more important than their short term comfort. The following article from Rotor Breeze explains why this happens. Peter Frinchaboy
From Bell Helicopters Rotorbreeze 2007:
LEADING EDGE PITCH LINK MAIN ROTOR TO MAST COUPLING PHENOMENON
Many Bell Helicopter models have leading edge pitch link installations on the main rotor hub. Each of these is subject to the vibration or ground bounce caused by the Leading Edge Pitch Link Main Rotor to Mast Coupling Phenomenon. This has often been mistakenly identified as being caused by Negative Pitch in the blades. This article explains why leading edge pitch link rotors can have this bounce.
1. The main rotor mast bearing of all Bell aircraft has axial play by design. This play is usually only observable when the mast does not have any weight from the rotating controls or main rotor hub attached to it. This allows the mast to move up and down axially on the mast bearing. It also allows some angular/side-to-side movement at the top of the mast. For example, the model 407 has maximum allowable axial play (up & down) of 0.065 inch.
2. The swashplate and support are mounted fixed to the top of the transmission. These controls have no vertical movement unless commanded by the control inputs from the pilot.
3. When the main rotor attains sufficient rpm and blade pitch, it lifts the mast and rotating controls up the distance of the axial play in the mast bearing from the static position (at the down position of the axial play) to the operating position (at the top of the axial play). The point at which this happens is variable due to rpm and the basic minimum pitch angle (Auto-Rotation Speed Adjustment) set in the blades.
4. If the basic minimum pitch angle set in the blades is such that lift occurs as the main rotor reaches operating rpm, the main rotor lifts. The leading edge pitch links are fixed and as the rotor lifts, the pitch links decrease the blade pitch angle and the rotor loses lift and moves back down the amount of the axial play.
When the rotor moves down, the pitch links now increase the blade pitch and the rotor lifts back up and then the whole process keeps repeating. The process repeats at a frequency of the rotor rpm times the number of blades. This vibration or bounce in the aircraft can be quite severe and show a high lateral vibration or a high vertical vibration on test equipment on the ground.
5. Usually a slight increase in collective pitch will cause the rotor to sustain its lift and the vibration will go away. In many cases this indicates that the auto- repeats at a frequency of the rotor rpm times the number of blades. This vibration or bounce in the aircraft can be quite severe and show a high lateral vibration or a high vertical vibration on test equipment on the ground. rotation rpm is set on the high side of the adjustment range. Normally, the auto-rotation rpm should be decreased, which will increase the basic minimum pitch angle set in the blades. This will cause the rotor to sustain lift at flat pitch and at operational rpm (100%). Alternatively, the pilot must input a slight amount of collective pitch at operational rpm (100%) to prevent the bounce.
Either method will stop the vibration and bounce.
The following Bell Helicopter aircraft have leading-edge pitch links. The table shows the normal maximum axial play you will see in their respective mast bearings as a guide to how much vertical movement the mast has.
Model Max Mast Axial Play Model Max Mast Axial Play
206A/B/B-3 0.037 inch 206L/L-1/L- 3 0.037/0.039 inch.
206L-3/L-4 0.058 inch OH-58A/C 0.037 inch
UH-1 (204 rotor) 0.038/0.040 inch 204/B 0.038/0.040 inch
205/A/A-1/B 0.038/0.040 inch 212 0.038/0.040 inch
210 0.038 inch 407 0.065 inch
412 0.040 inch
The 427 mast bearing has 0.085 inch axial play but is limited by design in its movement.
Note: the models UH-1(540 style rotor), all AH-1’s, 222/B/UT, 230, 214A/B/C, 214 ST, and 430 have trailing edge pitch links and are not subject
These numbers are for reference only and not for use as an inspection requirement for the bearings. Note that some models have alternate bearings with different normal maximum axial play. Always consult the appropriate maintenance manual for current allowables.
If you have questions on this subject, feel free to contact me.
David C. Burch
Senior Customer Support Representative
Bell Helicopter Textron, Inc.
P.O. Box 460640
Aurora, CO 80046-0640
Phone: +1-(720)-870-7414
Facsimile: +1-(817)-278-0096
[email protected]