Dynamic Twist

A rotating blade may be subjected to torsional moments that can modify the twist distribution significantly from the nonrotating, built-in twist. Reference 1.8 reports measured "dynamic twist” of up to 5° on a full-scale whirl tower rotor. Several sources of torsional moments can be identified. One is the airfoil’s aerodynamic pitching moment about the quarter chord, which is a function of blade camber and the local combination of angle of attack and Mach number. Figure 1.23 shows the measured pitching moment characteristics of a symmetrical and of a cambered airfoil from reference 1.9. The plotted parameter is the product of the pitching moment coefficient and the Mach number squared, which is proportional to the actual pitching moment. Even for the symmetrical airfoil, th^ pitching moments are not small except at combinations of low angles of attack and low Mach numbers. Another significant source of a torsional moment is th^ position of the airfoil aerodynamic center with respect to the blade flexual axis. Ary

Source: Stivers, “Effects of Subsonic Mach Number on the Forces and Pressure Distributions on Four NACA 64A-Series Airfoil Sections at Angles of Attack as High as 28°,” NACA TN 3162, 1954.

aerodynamic center position forward of the flexual axis will result in nose-up twisting moments as lift is increased. Some designers use swept-back tips to counteract this effect.

The last effect is a centrifugal flattening moment sometimes known as the tennis racket effect, named for the tendency of a tennis racket to try to align its plane with the plane of rotation as it is swung in an arc. The forces and moments due to centrifugal forces acting on a blade with positive pitch are shown in Figure 1.24. It may be seen that the forces acting on the mass elements at the leading and trailing edges produce a torsional moment that tends to twist the blade toward flat pitch. This moment not only twists the blade but also produces a control moment that must be counterbalanced to hold the blades at a positive pitch. This moment may be reduced by the use of balancing weights located perpendicular to the blade

FIGURE 1.24 Twisting Moments Due to Centrifugal Forces

chord, as shown in Figure 1.24. These are usually called Chinese weights, for reasons perhaps better left unexplained.

Dynamic twist has proved to be a problem in correlating measured thrust with measured collective pitch, but since the thrust and the power are both affected to about the same degree, small amounts of dynamic twist have little effect on the power-to-thrust relationships.