Aeroelastic deformation effects on the stopped-rotor dynamics of an X-wing aircraft

A new design concept in the development of vertical takeoff and landing aircraft with high forward flight speed capability is that of the X-wing. The X-wing is a stiff, bearingless helicopter rotor system that can be stopped in flight and the blades used as two forward-swept wings and two aft-swept wings. Because of the unusual configuration in the fixed-wing mode, there is a high potential for aeroelastic divergence or flutter and coupling of blade vibration modes with rigid-body modes. An aeroelastic stability analysis of an X-wing configuration aircraft was undertaken to determine whether these problems were likely. This paper reports on the results of dynamic stability analysis in the lateral and longitudinal directions, including the vehicle rigid-body and flexible modes. A static aeroelastic analysis using the normal vibration mode equations of motion was performed to determine the cause of a loss of longitudinal static margin with increasing airspeed. This loss of static margin was found to be due to aeroelastic wash in of the forward-swept blades and wash out of the aft-swept blades moving the aircraft aerodynamic center forward of the center of gravity. This phenomenon is likely to be generic to Xwing aircraft.