Twist

High blade twist produces good hovering performance and delays retreating blade stall at high forward speeds, but it also produces high vibration in forward flight as it causes the blades to bend as they go from the advancing to the retreating side. The effects on performance in hover and forward flight can be deduced from the discussions in Chapters 1 and 3, but the question of vibration is not easily quantified. At this writing, designers are generally selecting values of main rotor blade twist in the —8° to —14° range. If the dynamicists can find ways of reducing the vibration felt in the aircraft, the aerodynamicists will undoubtedly recommend higher values of twist.

Variations on the conventional twist distribution should be considered for special reasons. High twist that is good for hovering out of ground effect was shown to be too high for efficient hover in ground effect in reference 10.3. In addition, twist that is beneficial in powered flight is detrimental in autorotation. Thus the decision about what twist to use on a new design may depend on its projected use. In Chapter 1 there is a discussion of the tip vortex interference problem in hover. At least one helicopter design has attempted to deal with this by using a special nonlinear twist at the tip, as reported in reference 10.4. This twist distribution is shown in Figure 10.5. Yet another variation has been used to reduce the negative angle of attack in the reverse-flow region at high speed. This is shown in Figure 10.6 and is from reference 10.5.