Nonlinear Stability Analysis of Floating Wingtips with Control Surface Freeplay

Flared folding wingtip (FFWT) devices have been shown to enable higher aspect ratios in future aircraft designs while meeting airport width restrictions and reducing the loads experienced by an airframe during gust encounters and manoeuvres. Studies have highlighted that including an additional control surface on the wingtip can augment the performance of such devices. This paper aims to explore the effect of freeplay on this additional control surface and, in particular, will explore how freeplay affects the stability of this nonlinear system. As such, this study presents the results of an experimental study with a semi-span wing incorporating an FFWT with an additional control surface. The results show that freeplay leads to the onset of small limit cycle oscillations (LCOs) well below the linear flutter speed of the original system. The introduction of freeplay is also shown to re-stabilise the model at higher velocities, and these effects are similar across a wide range of freeplay region sizes and flare angles.