Nonlinear Analysis and Bifurcation Characteristics of Whirl Flutter in Unmanned Aerial Systems

Aerial drones have improved significantly over the recent decades with stronger and smaller motors, more powerful propellers, and overall optimization of systems. These improvements have consequently increased top speeds and improved a variety of performance aspects, along with introducing new structural challenges, such as whirl flutter. Whirl flutter is an aeroelastic instability that can be affected by structural or aerodynamic nonlinearities. This instability may affect the prediction of potentially dangerous behaviors. In this work, a nonlinear reduced-order model for a nacelle-rotor system, considering quasi-steady aerodynamics, is implemented. First, a parametric study for the linear system is performed to determine the main aerodynamic and structural characteristics that affect the onset of instability. Multiple polynomial nonlinearities in the two degrees of freedom nacelle-rotor model are tested to simulate possible structural nonlinear effects including symmetric cubic hardening nonlinearities for the pitch and yaw degrees of freedom; purely yaw nonlinearity; purely pitch nonlinearity; and a combination of quadratic, cubic, and fifth-order nonlinearities for both degrees of freedom. Results show that the presence of hardening structural nonlinearities introduces limit cycle oscillations to the system in the post-flutter regime. Moreover, it is demonstrated that the inclusion of quadratic nonlinearity introduces asymmetric oscillations and subcritical behavior, where large and potentially dangerous deformations can be reached before the predicted linear flutter speed.

Parametric whirl flutter study using different modelling approaches

This paper demonstrates the importance of assessing the whirl flutter stability of propeller configurations with a detailed aeroelastic model instead of local pylon models. Especially with the growing use of electric motors for propulsion in air taxis and commuter aircraft whirl flutter becomes an important mode of instability. These configurations often include propeller which are powered by lightweight electric motors and located at remote locations, e.g. the wing tip. This gives rise to an aeroelastic instability called whirl flutter, involving the gyroscopic whirl modes of the engine. The driving parameters for this instability are the dynamics of the mounting structure. Using a generic whirl flutter model of a propeller at the tip of a lifting surface, parameter studies on the flutter stability are carried out. The aeroelastic model consists of a dynamic MSC.Nastran beam model coupled with the unsteady ZAERO ZONA6 aerodynamic model and strip theory for the propeller aerodynamics. The parameter studies focus on the influence of different substructures (ranging from local engine mount stiffness to global aircraft dynamics) on the aeroelastic stability of the propeller. The results show a strong influence of the level of detail of the aeroelastic model on the flutter behaviour. The coupling with the lifting surface is of major importance, as it can stabilise the whirl flutter mode. Including wing unsteady aerodynamics into the analysis can also change the whirl flutter behaviour. This stresses the importance of including whirl flutter in the aeroelastic stability analysis on aircraft level.

Stability analysis of whirl flutter in rotor-nacelle systems with freeplay nonlinearity

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.