Propeller-wing whirl flutter

Abstract

Electrification of aviation is driving new aircraft configurations consisting in slender, lighter wings with several propellers spread across the wing span, including the wing tips. Thinner wings with high aspect ratios tend to be more flexible, and thus, more susceptible to wing flutter, a dynamic aeroelastic instability characterised by diverging oscillatory motions of the wing. As propellers are flexibly attached to the wing, another aeroelastic instability, called precession or whirl flutter, can occur. The nature of this phenomena lies in the additional forces and moments created by rotating blades as the propeller hub describes a “whirling” motion about the static thrust axis due to gyroscopic effects. These dynamic instabilities can happen independently or coupled together causing severe structural damage on the aircraft and when occurred during flight, lead to fatal accidents. Hence, it becomes essential to analyse any new propeller-driven aircraft configuration regarding wing flutter and whirl flutter. A mathematical model has been derived to analyse the dynamic behaviour of a flexible cantilever wing with flexibly attached tractor propeller/s: the wing structural model is a linear dynamic Euler-Bernoulli beam model, the wing aerodynamic model is represented by a combination of strip theory and two-dimensional Theodorsen's unsteady aerodynamic theory, and the propeller aerodynamic model is defined by Houbolt-Reed propeller aerodynamic derivatives. The aeroelastic behaviour of a wing with flexibly mounted propeller/s may be very different from that of a wing with rigidly mounted propeller/s. Reasons lie in the interaction between the additional whirl modes and the wing modes due to gyroscopic coupling. Whereas propeller and wing aerodynamics may drive the (coupled) backward whirl mode unstable, wing aerodynamics alone may drive the (coupled) forward whirl mode unstable.