Stability Analysis of Whirl Flutter in Rotor-Nacelle Systems with Structural Nonlinearities

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Tiltrotor aircraft are steadily proliferating as the understanding of their design matures. Their large flight envelope is a combination of that of helicopters and that of turboprop aircraft, and as a result tiltrotors are highly attractive to both civilian and military operators. Ongoing improvements to their design include increasing their payload capacity and raising their cruising speed. However, addressing the latter is where whirl flutter is encountered. Whirl flutter is a destructive aeroelastic instability that becomes active above a certain airspeed. Occurrences have shown that it is able to destroy aircraft structures rapidly. In making tiltrotors go faster, tackling
whirl flutter is unavoidable.

A substantial amount of research into whirl flutter has been conducted, using mathematical models sometimes validated by wind tunnel testing. However in deriving these models, some of the necessary simplifying assumptions might be faulty, preventing prediction of important
results. Particular examples of such simplifications are using linear expressions in parts of the model where nonlinear expressions would be more accurate, or predicting the whirl flutter onset using stability analyses that are incompatible with the nonlinearities or their effects. It is these
two examples, and their resulting impacts on whirl flutter, on which this work focuses.

This work uses two whirl flutter models to investigate the effects of two structural nonlinearities on the models’ whirl flutter stability, a novel piece of work within the tiltrotor aeroelasticity field. The models are contrasting in complexity, covering (1) classical whirl flutter and (2) tiltrotor
aeroelasticity, the latter being more complex. The two structural nonlinearities reflect features of real-world systems that might otherwise be overlooked. They are (1) a smooth, low-order polynomial stiffness representation, and (2) a quasi-nonsmooth freeplay nonlinearity. In this way,
the effects of both model complexity and nonlinearity type may be understood. Continuation and Bifurcation Methods (CBM) are used to detect and quantify the new behaviours caused by the nonlinearities. Stability boundaries are used to summarise the changes compared to the linear
versions of the models.

Both nonlinearities have a significant impact on the whirl flutter characteristics of both systems, leading to the creation of several whirl flutter solution branches. Some of these whirl flutter solution branches expand the parameter regions over which whirl flutter is possible,
causing whirl flutter to be possible at higher structural stiffness values and at lower airspeeds than the predictions given by linear analysis for each model. In the more complex tiltrotor-specific model, some especially rich dynamics are predicted, including quasi-periodic and even chaotic
behaviours.
Date of Award11 May 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorDjamel Rezgui (Supervisor) & Branislav Titurus (Supervisor)

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