Analytical modelling and preliminary design of a small-scale experimental whirl flutter demonstrator Wind tunnel laboratory (Université de Liège)

Abstract

The whirl flutter phenomenon is one of the main concerns when designing aeroelastic structures such as tiltrotor or pylon-propeller systems. First identified by Taylor and Browne in 1938, whirl flutter is a self-excited dynamic aeroelastic instability. It is driven by the coupling between the gyroscopic effects induced by the rotating propeller and the aerodynamic forces acting on the propeller blades. Due to the gyroscopic coupling, two modes appear, namely the forward and backward whirling modes. Depending on the configuration, their stability behaviour can differ. This work aims to design a small-scale whirl flutter demonstrator in order to support the validation of a finite element-based numerical method for whirl flutter prediction, currently under development at the University of Liège. In particular, the present work investigates the whirl flutter behaviour of two configurations that differ by their source of flexibility. The first one considers a flexible pylon, while the second one considers a rigid pylon with linear traction springs. The results are obtained using an analytical method, which relies on several assumptions and uses a quasi-steady aerodynamic approach. For both configurations, the results are obtained for the APC 15.75x13-3 propeller, with a rotational speed of 2000 rpm and free-stream velocities ranging from 7 m/s to 13 m/s. They show that only the backward whirling mode leads to whirl flutter, while the forward mode remains stable. Both configurations are compared, and the spring-based one appears to be more convenient from an experimental point of view, since the flexible pylon exhibits significant static tip deflection and rotation. Finally, experimental procedures for the flexible pylon configuration are presented, and all the components are described in detail. The motor-pylon connection is designed, while the motor propeller link is also discussed. Additionally, the propeller rotational speed control is described through the use of a pulse-width modulation device.