Collective 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 Rotorbreeze 2007:


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 transmis sion. These controls have no vertical movement unless commanded by the con-

trol 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 vibra-

tion or bounce in the aircraft can bequite severe and show a high lateral