Free-to-Roll Oscillations of Low Aspect Ratio Wings

Abstract

This Thesis aims to enhance the aerodynamic knowledge of the freetoroll behaviour of low aspect ratio wings by investigating newly found freetoroll oscillations. Efforts were initially made on freetoroll nonslender delta wings and roll oscillations were found with a low Strouhal number, which will affect wings with this planform in flight manoeuvres. It has also been shown that the nonzero roll trim angles are not due to the round leadingedge of the wing being tested as previously thought. Phaseaveraged velocity measurements revealed the hysteresis effects during the self induced roll oscillations.

Oscillations were observed for a nonslender delta wing of sweep angle of 50o with fixed separation points and this phenomenon was seen for delta wings up to 60o sweep (i.e. slender) with less reattached flow. Similar experiments done on cropped delta wings showed that the cropped portion reduces the nonzero roll trim angles, therefore the wing was stable. Different oscillations were found and investigated in a second region at higher angles of attack for delta wings of 57.5o and 60o sweep angle. Work was then performed on pitching freetoroll delta wings, which were pitched up or down at a constant rate.

Low aspect ratio (AR ≤ 4) rectangular wings showed that oscillations and even autorotation existed at high angles of attack. It is suggested that the tip vortices are responsible for the oscillations. At low angles of attack, the separation bubble was asymmetric for the round leadingedge, which explains the nonzero roll trim angle for the freetoroll wing. This separation bubble spread across the extent of the wingspan with increasing angle of attack and the oscillations were seen to begin once the separation bubble had left the trailingedge and the sideedge vortices had increased in strength sufficiently. The vortices are seen to be well separated, and thus the effect of the sideedge vortices on one another is expected to be negligible, unlike for delta wing rock. Finally, other planforms were tested including a Zimmerman and an elliptical wing. Oscillations were found too, though smaller in magnitude.

Date of Award	1 Oct 2010
Original language	English
Awarding Institution	University of Bath
Supervisor	Ismet Gursul (Supervisor) & Zhan Wang (Supervisor)