Abstract Summary
Propeller aircraft engines, and more generally engines with a large rotating part (turboprops, high bypass ratio turbojets, etc.) are widely used in the industry and are subject to numerous developments in order to reduce their fuel consumption. In this context, unconventional architectures such as open rotors or distributed propulsion appear, and it is necessary to consider the influence of these systems on the aircraft's stability in flight. Indeed, the tendency to lengthen the blades and wings on which these propulsion devices are fixed increases their flexibility and promotes the whirl flutter risk. This phenomenon of aeroelastic instability is due to the precession movement of of the propeller rotation axis, which changes the attack angle of the flow on the blades and creates unsteady aerodynamic forces and moments that can amplify the motion and make it unstable. The whirl flutter instability can ultimately lead to the engine destruction. We note the existence of a critical speed of the incident flow. If the flow velocity is lower than this value, the motion is damped and the system is stable, whereas beyond this value, the flow provides energy to the system (negative damping) and the motion becomes unstable. A reference model of whirl flutter is based on the work of Houbolt & Reed who proposed an analytical expression of the aerodynamic load on a rigid blade propeller whose axis orientation is subject to small perturbations. Their work considered a propeller having four degrees of freedom (forward translation and roll neglected), a flow undisturbed by the blades and a propeller not generating any thrust in the absence of precession. The unsteady aerodynamic forces were then obtained using the thin airfoil theory and the strip theory. In the present study, a general movement of the propeller is considered (six degrees of freedom). The acceleration and rotation of the flow by the propeller are modeled using a Blade Element Momentum Theory (BEMT) approach, and the thrust is considered by the choice of an arbitrary blade pitch angle. The aerodynamic load is obtained using Theodorsen’s theory, a more complete method than thin airfoil theory, which models the wake vorticity and takes into account the phase delay of the aerodynamic load with respect to the propeller motion. Due to the frequency dependency of the lift and moment, a reduced order model of the aerodynamic load is constructed in order to perform linear stability analysis. This step, which was not necessary in the work of Houbolt & Reed, leads to the apparition of new “hidden” variables modeling the dynamic of the flow. This model of the aerodynamic load on the propeller is then coupled with two structural models to study the stability of a full propeller/wing system - a first one modeling a propeller in pitch and yaw and a second more complex one using beams to represent the wing and the pylon.
