On the Effect of Pitch/Mast-Bending Coupling on Whirl-Mode Stability

The influence of pitch/mast-bending coupling on whirl-mode instabilities is investigated using a simplified, linear model of a 4-bladed helicopter rotor and a nonrotating hub/mast support. An explanation of the proper procedure for modeling this coupling is given, and its rationale explained. Results using this model show that pitch/mast-bending coupling is usually destabilizing, and that the flap coupling component is more powerful than the pitch component. Since optimizing the pitch/mast-bending coupling may be difficult, or impractical, several methods for improving the stability characteristics are proposed.