Insights on the whirl-flutter phenomena of advanced turboprops and propfans

Abstract

The new aircraft configurations that have two pusher propellers or propfans located at the rear fuselage are susceptible to a whirl-flutter-related instability characterized by a complex dynamic coupling involving not only the propeller-nacelle whirl modes but also the natural modes of the supporting backup structure. It is shown that the single-engine classical whirl-flutter formulation does not correctly predict these particular situations of instability, which were called whirl induced flutter. The problem is treated in this paper in the form of a parametric study using a 15-degree-of-freedom model. The main stiffness properties of both the supporting backup structure and the engine mounting system are systematically changed and the mechanism of the aforementioned phenomenon is explained on simple physical grounds. a b d h hy,hz Nomenclature = distance between fuselage and propeller centerlines = engine-propeller center-of-mas s distance from the pylon elastic axis = propeller distance from the pylon elastic axis = pylon elastic axis flatwise displacement = aft fuselage displacements in the lateral and vertical directions, respectively = engine polar and transverse moments of inertia, respectively = propeller polar moment of inertia = aft fuselage polar moment of inertia = nacelle polar and transverse moments of inertia, respectively = V/QR - shock mount system lateral and vertical stiffnesses relative to the nacelle, respectively = aft fuselage lateral, vertical, and torsional stiffnesses, respectively