Linearly Twisted Blades, Uniform Inflow

All helicopter rotor blades use some amount of spanwise twist in their shape, although in different forms and with different amounts. As will be shown, the use of blade twist provides the rotor with several important performance advantages. Many helicopter rotor blades are designed with a linear twist, so that 0 takes the form 6(r) — 6q + r0tw, where t9tw is the blade twist rate per radius of the rotor (i. e., in degrees per rotor radius or the equivalent in degrees per unit length of blade). Using this variation in 0(r), we get

Comparing this latter equation with Eq. 3.22 shows an interesting result, namely that a blade with linear twist has the same thrust coefficient as one of constant pitch when в is set to the pitch the twisted blade defined at the 3/4-radius [see also Gessow & Myers (1952) and Johnson (1980)].