Collective and Cyclic Pitch Ranges

The main rotor pitch travels must be adequate to trim the helicopter throughout its flight envelope while leaving suitable margins for maneuvering. The methods of Chapters 3 and 8 can be used to calculate the trim values at the extreme flight conditions. Then these should be increased by 10 to 20 percent. The configuration lists of Appendix В show what designers of some existing helicopters have chosen.

The minimum collective setting must be that which will correspond to 100 percent rotor speed in autorotation at the most extreme combination of gross weight and altitude in the expected flight envelope. This is usually at the lowest gross weight and at sea level, since this is where the CT/a is the lowest.

As a general rule, the maximum collective pitch is required when making a flare from autorotation to allow all the available rotor energy to be used as the rotor slows down.

The cyclic pitch requirement for trim is unsymmetrical. For example, the forward longitudinal cyclic pitch required at the maximum forward speed is much higher than the aft cyclic pitch required at the maximum allowable rearward speed. By recognizing this fact, the designer can save control system weight and space compared to designing for as much aft travel as forward. Similarly, the total lateral cyclic travel an be minimized by biasing toward the high-speed trim value.