Abstract
Tiltrotor aircraft are growing in prevalence due to the usefulness of their unique flight envelope. However, aeroelastic stability—particularly whirl flutter stability—is a major design influence that demands accurate prediction. Several nonlinearities that may be present in tiltrotor systems, such as freeplay, are often neglected for simplicity, either in the modelling or the stability analysis. However, the effects of such nonlinearities can be significant, sometimes even invalidating the stability predictions from linear analysis methods. Freeplay is a nonlinearity that may arise in tiltrotor nacelle rotation actuators due to the tension–compression loading cycles they undergo. This paper investigates the effect of a freeplay structural nonlinearity in the nacelle pitch degree of freedom. Two rotor-nacelle models of contrasting complexity are studied: one represents classical whirl flutter (propellers) and the other captures the main effects of tiltrotor aeroelasticity (proprotors). The manifestation of the freeplay in the systems’ dynamical behaviour is mapped out using Continuation and Bifurcation Methods, and consequently the change in the stability boundary is quantified. Furthermore, the effects on freeplay behaviour of (a) model complexity and (b) deadband edge sharpness are studied. Ultimately, the freeplay nonlinearity is shown to have a complex effect on the dynamics of both systems, even creating the possibility of whirl flutter in parameter ranges that linear analysis methods predict to be stable. While the size of this additional whirl flutter region is finite and bounded for the basic model, it is unbounded for the higher complexity model.
Highlights
Tiltrotor aircraft such as the XV-15 shown in Fig. 1 aim to combine the speed and range of turboprop aircraft with the VTOL capabilities of helicopters
This relatively large flight envelope makes them highly versatile and they are attractive to both civil and military operators. It is in increasing their maximum cruising speed that the aeroelastic instability known as whirl flutter is encountered
It is caused by the interaction of the wing’s elasticity, gyroscopic moments acting on the rotor as a whole and aerodynamic forces and moments acting on the rotor disc
Summary
Tiltrotor aircraft such as the XV-15 shown in Fig. 1 aim to combine the speed and range of turboprop aircraft with the VTOL capabilities of helicopters This relatively large flight envelope makes them highly versatile and they are attractive to both civil and military operators. It is in increasing their maximum cruising speed that the aeroelastic instability known as whirl flutter is encountered. The physical origin is typically the coupling between the wing torsional motion and rotor in-plane forces [3]. These in-plane forces may destabilise the whole aircraft’s short period flight modes [4]. From a pilot’s perspective, it is encountered at or beyond a certain onset speed, when the damping ratio of one or more whirl flutter modes becomes negative
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