Whirl Flutter Analysis with Propeller Aerodynamic Derivatives Computed by Unsteady Vortex Lattice Method

Abstract

Whirl flutter analysis plays an important role for the design of tilt-rotors and propairplanes. A new module called ZWHIRL is developed in ZAERO to facilitate whirl flutter analysis in the frequency domain. ZWHIRL has a couple of benefits over others such that, ZWHIRL employs the g-method and yields the final results right after the mode tracking, unlike Nastran with which users have to further post-process Nastran outputs to find out flutter conditions. In light of Nastran’s propeller effect matrix preparation program’s drawbacks of being lack of blade twist angle consideration and the fixed usage of one particular set of units, an alternative propeller matrix preparation program, ZUV, is also developed in this work. ZUV utilizes the unsteady vortex lattice method to compute the propeller aerodynamic derivatives first and then generates the propeller effect matrices towards the system stiffness and damping matrices along with the gyroscopic effects. Numerical example of the BAH wing/nacelle/propeller configuration is selected to demonstrate the capability of ZWHIRL and ZUV. Since the twist angle distribution for the BAH propeller blades is not available, a numerical parametric study with various twist assumptions are performed, and their impact towards the whirl flutter results are investigated thereafter.