Resonance avoidance in variable speed rotorcraft using an applied compressive load

Abstract

One of the concepts considered by the rotorcraft industry to improve aircraft performance whilst reducing emissions is the use of a variable speed rotor. This concept varies the rotor speed, in addition to collective pitch, to achieve trim and minimise fuel burn. However, a rotor blade’s natural frequencies and rotor harmonics (both functions of rotor speed) may intersect due to the change in speed which will induce unsatisfactory vibrations in the aircraft. The present research considers the use of an applied compressive load to the blade to alter its natural frequencies and avoid resonance. Initially, an in-vacuo model of an untwisted blade subject to an applied compressive load is developed and validated using a Finite Element model and an experiment. Subsequently, it is shown that compressive loading is able to sufficiently alter the in-plane and out-of-plane natural frequencies of the blade such that a desired separation between the natural frequencies and rotor harmonics would likely be achievable. The load required did not exceed the buckling load of the blade, nor would it have required large powerful actuation. The model fidelity was increased to capture torsional deformation, pretwist and noncoincident mass and elastic axes. It was then shown that, for a set of three test aircraft, the range of rotor speeds that would be operable increased significantly through the use of an applied compressive load. Finally, the fidelity was again increased to include geometric nonlinearities and unsteady aerodynamic loads in hover. The investigation that followed showed that rotor speed and the applied compressive load had a marked impact on the damping ratios of the natural frequencies of the blade. It is possible to exploit these impacts to change damping ratios, as well as natural frequencies, to expand the operable range of rotor speeds of an aircraft.

Date of Award	24 Mar 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Branislav Titurus (Supervisor) & Ben K S Woods (Supervisor)