Influence of Airfoil Type on Pitching Moments
During the autogiro era of rotary wing flight, some frightening moments resulted from extreme blade twisting and high control loads with cambered blade airfoil
FIGURE 6.35 Measured Lift and Drag Dynamic Characteristics
Source: Philippe & Sagner, “Aerodynamic Forces Computation and Measurement on an Oscillating Aerofoil Profile with and without Stall," AGARD CP 111, 1972.
sections that had high aerodynamic pitching moments. In one case, an autogiro with airfoil sections that had nose-down pitching moments entered an unscheduled high-speed dive as the aerodynamic moments twisted the advancing blade nose down and produced effective forward cyclic pitch, even though the pilot had his stick all the way back! Experiences like this led to a period of almost exclusive use of low-moment, symmetrical airfoils. The development of stiffer blades and control systems has alleviated the problem somewhat, so that cambered airfoils with some inherent pitching moment can again be considered. It is noteworthy, however, that even symmetrical airfoils can produce substantial moments at high angles of attack and high Mach numbers. This is illustrated in Figure 6.36, from
FIGURE 6.36 Comparison off Pitching Moment Functions
Source: Dadone, "Helicopter Design Datcom,” Vol. I. “Airfoils," USAAMRDL CR 76-2, 1976.
reference 6.57, which shows the product of the pitching-moment coefficient and Mach number squared for the symmetrical NACA 0012 airfoil within the Mach number range encountered by rotors. (This presentation has been chosen since the product is directly proportional to the actual moment generated at the blade element.) Also shown are two cambered airfoils, one with forward camber and one with aft camber. It may be seen that forward camber has only a small effect on the pitching moment, whereas aft camber has a large effect.
The amount of aerodynamic pitching moment that can be tolerated in a given helicopter rotor depends on the structural and dynamic characteristics of the blades, hub, and control system. Thus the experience of various helicopter manufacturers has been different. Vertol has found that for their rotors, the value of cm— the pitching moment coefficient at zero angle of attack and low Mach number—should be slightly positive (about 0.01) to maintain a satisfactory control system oscillatory load level. On the other hand, the use of a highly cambered airfoil with а г of about —0.07 on a Hughes tail rotor, reported in reference 6.58, produced only a small increase in oscillatory pitch link loads compared to the symmetrical airfoil it replaced, since the loads due to the aerodynamic pitching moments were out of phase with the existing pitch link loads. Tests of a model rotor using both a NACA 0012 airfoil and the same airfoil with the aft 20% deflected down 5° are reported in reference 6.59. It was found that for cases in which the retreating blade was stalled, the control loads were lower for the blade with the modified trailing edge than for the blade with the standard NACA 0012 airfoil, although below stall the opposite was true. It appears that the component of control and blade loads produced by the various modes of blade deflection can be as high as those due to aerodynamic pitching moments on the airfoil, and that the two can either add or subtract depending on the particular flight condition and the dynamic characteristics of the blade.
At the time of this writing, the accepted limit for cm for main rotors is:
It can be expected that more experience with cambered airfoils on a variety of rotors will lead to the establishment of more rational limits.