Active Control of Rotorcraft Instabilities

Abstract

Tiltrotors designs have emerged at the forefront of hybrid rotorcraft in the last 100 years. Their characteristic, rotating nacelle-rotor assemblies facilitate performance comparable to helicopters in take-off and fixed wing aircraft in cruise, the combination of which are exceedingly valuable for transportation. Existing designs are seeking to improve on their cruise speeds which are currently limited by whirl flutter instability, whereby self-sustaining aerodynamic forces generated at the rotor become coupled to the elastic wing structure. The result is a characteristic circular vibration pattern, which if unmitigated, results in significant and sometimes catastrophic damage to the rotorcraft. Successful attempts at flutter suppression have thus far centred on passive changes to tiltrotor designs, which are not easily carried over to the larger designs and load carrying capabilities envisioned for the next generation of tiltrotors.

This research covers a series of investigations into the whirl flutter phenomenon and its influence on tiltrotor dynamics in airplane mode flight. Active mitigation strategies that eliminate flutter by introducing counter-loading in real time are considered extensively as a means of bypassing the negative effects of structural redesign. While previous forays into active mitigation have highlighted promising control of existing flight mechanisms like the wing flaperon and swashplate, it is problematic to have a single mechanism responsible for flight control as well as flutter mitigation since their objectives may differ during operation. A novel active approach, acting separately from the flight controls - the mini-tab – is therefore proposed and assessed. A simplified propeller whirl flutter model is used to build understanding of the phenomenon, before transitioning to a more complex tiltrotor flutter mode for the appraisal of the mini-tab.

The mini-tab is shown to be comparable to the wing flaperon in its ability to mitigate flutter, producing a 44 𝑚𝑠−1 extension in the XV-15 flutter boundary relative to the uncontrolled case. A linear quadratic regulator (LQR) control strategy designed using linear approximations to nonlinear aerodynamic mini-tab loading is found to be an effective alternative to more complex, and computationally expensive nonlinear control techniques. Stability boundary comparisons, between linear and nonlinear mini-tab models highlight important effects of the control approach, and constraints, including control gains induced into the nonlinear mini-tab model by the linear controller, and nonlinear effects associated with tab saturation.

Date of Award	29 Mar 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Samuel Bull (Supervisor), Andrew Hillis (Supervisor), Nathan Sell (Supervisor), Jonathan Du Bois (Advisor) & David Cleaver (Advisor)