Stall and Drag Divergence

Stall and drag divergence are of primary interest in forward flight, where they may limit the maximum speed and the maximum maneuvering capability. There can be cases, however, when either stall or drag divergence affects hover performance. The airfoil data of the NACA 0012 airfoil of Figure 1.10 shows the envelopes of the start of stall as a function of Mach number and also the envelope of the beginning of drag divergence as a function of angle of attack. For a rotor with this airfoil section, any local combination of angle of attack or Mach number above these boundaries produces stall and/or compressibility losses. If a computing procedure is used in which the airfoil lift and drag coefficients are evaluated at each blade element, these effects will automatically be accounted for; but if a, simpler method using an average lift coefficient and a corresponding average drag coefficient is used, the effects will be neglected. In this case, however, empirical corrections can be made using the following equation which has been based on a
comparison of the measured whirl tower results reported in reference 1.1 with calculations made with simple theory.

ACg/a = O. OOlAa + 0.05 AM

where Aa is the amount the average angle of attack (in degrees) exceeds the stall boundary at a Mach number corresponding to the 75% radius station, and AM is the amount the tip Mach number exceeds the drag divergence Mach number at the average angle of attack.