COLLECTIVE PITCH REQUIRED

The increment in collective pitch above that required to hover can be estimated from the same principles used for the power increment. The change in pitch can be based on the change in inflow velocity and the tangential flow at the three-quarter radius station.

A A

Д9= .75 (ПК)

For the example helicopter with a vertical rate of climb of 500 ft/min, the additional collective pitch is.4°.

ROTOR THRUST DAMPING

The momentum relationships can be used to derive an approximate equation for the change in rotor thrust coefficient as a result of a change in axial velocity at constant collective pitch. This allows the damping of main rotors and tail rotor to be calculated for use in studies of such maneuvers as jump takeoffs, power failures, and response to pedal inputs.

A hovering rotor with ideal twist was discussed in Chapter 1. If this rotor is in vertical flight, the equation for CT/a is:

The partial derivative evaluated at the initial value of CT/o is:

dCT/<5

dVr

This can be used for small rates of descent as well as climb and for inflow through the tail rotor due to turns or sideward flight where the rotor is not in the vortex ring state.