Aeroelastic Loads and Stability of Swept-Tip Hingeless Tiltrotors Toward 400 knots Flutter-Free Cruise

Abstract

A hingeless hub tiltrotor with swept-tip blades was examined comprehensively with a new rotorcraft aeromechanics solver developed at the University of Maryland. The solver was verified with hypothetical U.S. government results and validated with Boeing M222 test data from 1972. A 20◦ sweep back from 80%R increased instability speed to 395 knots, an improvement of 70 knots. The key mechanism is the aerodynamic center shift. The trade-off is the increase in control system loads. Fundamental understanding of the physics is provided. Air resonance emerged as the critical phenomenon, not whirl flutter. Predictions in powered mode is necessary. At least first rotor flap, lag, and torsion modes need to be included. Rotor aerodynamics should use airfoil tables; wing aerodynamics is not important for air resonance. Analysis shows high speed flight is achievable with 13.5% wings but systematic wind tunnel tests with modern equipment is necessary for further validation.