Aeroelastic Stability Wind Tunnel Testing with Analytical Correlation of the Comanche Bearingless Main Rotor

A series of wind tunnel tests and analytical studies were performed on a 1/6-Froude-scale model of the RAH-66 Comanche bearingless main rotor system at the Boeing V/STOL Wind Tunnel. The primary focus was an investigation of the aeromechanical stability characteristics of the rotor-fuselage system with elastomeric and Fluidlastic® dampers. A secondary focus was correlation of analytical stability predictions with measured results, so that the methodologies could be used with confidence for the full-scale aircraft. An initial test with elastomeric dampers uncovered a limit cycle instability in hover at minimum flight weight. It was determined that the nonlinear stiffness and damping characteristics of the elastomeric damper, combined with the low roll inertia of the Comanche aircraft, was the fundamental cause of the instability. To solve this limit cycle problem, Fluidlastic® lead-lag dampers which provide increased damping and a more constant stiffness were incorporated in the Froude-scale model. A second test entry was used to verify that the Fluidlastic® lead-lag dampers eliminated the limit cycle instability and to clear the prototype aircraft for first flight. Overall, analytical correlation with the test data was very good. Analysis showed similar lag mode stability trends as that seen in the test data for both hover and forward flight.