Whirl of an Aircraft Power-plant Installation and its Interaction with the Flutter Motion of a Flexible Wing

Abstract

Results of propeller whirl analyses in four degrees of freedom are presented for a typical turbo-prop aircraft power-plant installation consisting of a rigid engine supported with elastically restrained freedoms in pitch and yaw at the end of a flexible nacelle cantilevered from a rigid support structure. Calculations were made for a range of structural and aerodynamic parameters. Only one of the four modes was found to become unstable within the airspeed range covered, the critical whirl airspeed being very sensitive to engine-mount damping and to the nacelle and engine-mount stiffnesses. The interaction between power-plant whirl and wing flutter was investigated by extending the calculations to include twelve degrees of freedom, four normal modes of vibration of the half wing plus four modes for each of two power-plant installations on the half wing. The results suggest that the stability of the power-plant whirl modes is not adversely affected by motion of the wing, but that in certain circumstances the critical airspeed for wing flutter may be reduced slightly by a coupling with an unstable power-plant whirl mode of comparable frequency.