Abstract
Whirl flutter is an aeroelastic instability that affects propellers/rotors and the surrounding airframe structure on which they are mounted. Whirl flutter analysis gets progressively more complicated with the addition of nonlinear effects. This paper investigates the impact of nonlinear pylon stiffness on the whirl flutter stability of a basic rotor-nacelle model, compared to a baseline linear stiffness version. The use of suitable nonlinear analysis techniques to address such a nonlinear model is also demonstrated. Three types of nonlinearity were investigated in this paper: cubic softening, cubic hardening and a combined cubic softening—quintic hardening case. The investigation was conducted through a combination of eigenvalue and bifurcation analyses, supplemented by time simulations, in order to fully capture the effects of nonlinear stiffness on the dynamic behaviour of the rotor-nacelle system. The results illustrate the coexistence of stable and unstable limit cycles and equilibria for a range of parameter values in the nonlinear cases, which are not found in the linear baseline model. These branches are connected by a number of different bifurcation types: fold, pitchfork, Hopf, homoclinic and heteroclinic. The results also demonstrate the importance of nonlinear whirl flutter models and analysis methods. Of particular interest are cases where the dynamics of the nacelle are unstable despite linear analysis predicting stable behaviour. A more complete stability envelope for the combined model was generated to take account of this phenomenon.
Highlights
The aeroelastic instability known as whirl flutter is an important consideration in aircraft design
Two whirl flutter modes exist—forward whirl (FW) and backward whirl (BW)—identified by the sense of the whirl relative to the rotor’s rotation; forward denotes that the whirl and the rotor are spinning in the same direction
This article has demonstrated the use of continuation and bifurcation methods to provide nonlinear dynamic analysis of a basic rotor-nacelle system model
Summary
The aeroelastic instability known as whirl flutter is an important consideration in aircraft design. A review of the impact of various types of structural nonlinearity on system dynamics was provided by Breitbach [6], with further specific investigations conducted by Dowell and Ilgamov [7] In both cases, analytical frameworks and the effects of each nonlinearity on flutter predictions are suggested. In order to understand the effect of nonlinear model aspects on a system’s behaviour, suitable analysis methods must be used, namely continuation and bifurcation methods Such methods have so far been applied in only a small number of rotorcraft dynamical problems, flight mechanics [14], ground resonance [15] and rotor vortex ring state [16], though their inclusion in rotary-wing studies is steadily becoming more prevalent as they are powerful in solving problems such as the identification of instability scenarios of rotor blades [17].
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