Tilt Nacelle Aerodynamics and Its Application to Fan Blade Stresses

A scale model of a V/STOL tilt nacelle with a 0.508 m single-stage fan was tested in the NASA Lewis 9 x 15 Low Speed Wind Tunnel to ascertain inlet aerodynamic and fan aeromechanical performance over the low-speed flight envelope. Fan blade stress maxima occurred at discrete rotational speeds corresponding to integral engine order vibrations of the first flatwise bending mode. Increased fan blade stress levels coincided with the occurrence of internal boundary-layer separation, but became severe only when the separation location had progressed to the entry lip region of the inlet. The inlet/fan system could operate within the low-speed flight envelope without incurring fan blade stress limits, although boundary-layer separation did occur for certain operating conditions.