Leading Edge Pitch Link Main Rotor to Mast Coupling Phenomenon

Thought this article is a good item to share. Published in Rotorbreeze 2006 by the now retired David Burch Bell Helicopter Senior Customer Service Representative.

December 7, 2006

To: All Bell Helicopter Operators.

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 document explains why leading edge pitch link rotors can have this bounce.

The main rotor mast bearing of all Bell aircraft have 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 which often causes some concern but is not important. For example: the model 407 has maximum allowable axial play (up & down) of 0.065 inch.
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.
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.
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.
Usually a slight increase in collective pitch will cause the rotor to sustain it’s lift and the vibration will go away. In many cases this indicates that the auto 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. I show the normal maximum axial play you will see in their respective mast bearings as a guide to how much vertical movement the mast has. 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.

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

210/212 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 to this problem as the lift of the rotor inputs a pitch increase into the rotor blades.

David C. Burch

Senior Customer Support Representative

Bell Helicopter Textron, Inc.