Relationship between Thrust and Pitch

The equation derived earlier for the thrust of a rotor with ideal twist can be written in nondimensional form:

the example helicopter and for solidities of half and twice this value are shown in Figure 1.9. For the example helicopter operating at a Cr/(J of.086, the required pitch at the tip is 6.7°. Note the relatively low slope of the curve near zero thrust. In this region, the induced velocity is important and a large amount of the blade pitch is used to compensate for it, with only a small proportion available to produce thrust. At higher values of thrust, the induced velocity is of less importance and more of the blade pitch is available to produce thrust.

The collective pitch of a blade with linear twist can be related to the tip pitch of a blade with ideal twist through the approximate relationship:

e„ = I e, -10,

Thus for the example helicopter with a linear twist of—10°, the collective pitch is approximately:

0 = I (6.7) – I (-10) = i7.6°