Aeroelastic Loads and Stability of Swept-Tip Hingeless Tiltrotors toward High-Speed Instability-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. Army results and validated with Boeing Model 222 test data from 1972. A 20° sweep back from 80%R increased instability speed to 405 kt, an improvement of more than 75 kt. The key mechanism is the aerodynamic center shift. The trade-off is the increase in control system and blade loads. Fundamental understanding of physics is provided. Proprotor air resonance emerged as the critical phenomenon, not whirl flutter. Predictions in powered mode are necessary. At least the 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% thick wings but systematic wind tunnel tests with modern equipment are necessary for further validation.