Exploration of Aeroelastic Stability Boundaries with a Soft-in-Plane Hingeless-Rotor Model

Abstract

T h e~a i r resonance aeroelastic stability boundaries of a. softin-plane, hingeless-rotor helicopter were explored using both an 1Rdegree-of-freedom analysis and aFroude-scaled,28in. diameter rotor model. Correlation between analytical results and test data was excellent in terms of stability boundaries, mode shapes, and frequencies, thus lending credibility to both. Some physical insights of the air resonance phenomenon were gained with both the analysis and the test. Sensitivity of the air resonance stability to some pertinent design considerations such as the precone of the feathering axis, control system flexibility, and blade in-plane damping was investigated. Again, the analytical trends agreed well with the test findings. Based on these sensitivities and the insight into the nature of the air resonance mode, some important design recommendations are made. NOTATION IN-PLANE DAMPING = 0.85 PERCENT CRITICAL ROTOR SPEED = 1,500 RPM COLLECTIYE PITCH AT lig THRUST -8.41 DEGREES COLLECTIVE PITCH -DEGREES FIGURE 2. Maneuver g effect on air resonance characteristics.