Influences of Propeller Size and Spanwise Location on the Whirl Flutter of a Flexible Wing

Distributed electric propulsion has renewed interest in whirl flutter, since multiple propellers on highly flexible wings introduce aeroelastic characteristics that differ from those of the turboprop aircraft for which the classical theory was developed. With the total propeller disk area held constant, the individual propellers are resized and repositioned along the wing span to examine how their distribution influences the whirl flutter stability boundary. A four-propeller wing is modeled using a Galerkin discretization of the flexible wing, coupled with rigid propeller units mounted on flexible pylons. The propeller aerodynamic loads are determined using the Houbolt–Reed theory, with corrections accounting for compressibility and finite blade aspect ratio, and the coupled system is analyzed using both a frequency-domain and an augmented time-domain formulation. Three propeller layouts with the same total disk area and propeller mass are examined in detail, and the continuous range of configurations between them is then swept at each spanwise location. The results indicate that concentrating the total disk area into a single larger propeller has the most significant effect on the whirl flutter stability margin, while the underlying backward-whirl instability mechanism remains unchanged. A moderate concentration of the disk area toward the inboard region of the wing provides a small improvement of 3.6%. Spanwise placement has a secondary influence and becomes less significant as the disk area becomes increasingly concentrated.