Pitch Range

The pitch range designed into the tail rotor must provide for both maximum antitorque capability and adequate control in all flight regimes. On the negative side, the critical condition is directional control in autorotation. Experience indicates that a value of —10° to —15° at the 75% radius station is adequate for this. The tabulation of Appendix В lists values used on contemporary heli­copters.

On the positive side, the collective pitch must be at least high enough to develop the maximum value of CT/oT used earlier when calculating the required tail rotor solidity. A check should then be made for right sideward flight conditions to determine if this pitch is high enough to account for the increased inflow while still providing a thrust adequate for both antitorque and maneu­vering. Choosing a higher value than necessary will cause a problem for the designer of the tail rotor drive system. During a fast hover turn with torque, the tail rotor pitch is very low. If the pilot quickly stops the turn by using maximum pitch, the angles of attack on the tail rotor blades will initially increase by that change before the final inflow pattern an establish itself. This angle may be enough to stall the tail rotor transiently, with a resulting high torque spike in its drive system. Designing for a torque that is caused by a maximum pitch higher than absolutely required for normal operation will increase the helicopter’s weight and cost. A discussion of this problem faced during the development of the Hughes AH-64 will be found in reference 10.8.

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