An Experimental Investigation of the Vortex Merging over a Cranked-Delta Wing at Subsonic Speed

Abstract

A series of experiments was carried out to investigate the aerodynamic characteristics of a cranked-delta wing model. It has been noted by various researches that in the vicinity of the take-off angles of attack, an instability in the longitudinal stability of aircraft equipped with cranked delta wings occurs where its origin is not well understood yet. To further study this phenomenon, a semi-span cranked delta wing model was designed and built. Surface pressure data for various angles of attack at low subsonic speeds, to better simulate take-off and landing conditions, were measured. The only test limitations of concern were inaccessibility to an accurate balance to measure forces and moments and to correlate them with the surface pressure data. However, analysis of the surface reassure data showed formations of two distinct strong vortices over the wing surface with strong suction peaks at their cores. The interaction between the two vortices increased with angle-of-attack, and as a result, the outer vortex moved inward while the inner vortex moved outward. At a certain angle-of-attack, these vortices merge with each other and at higher angles of attack the vortex breakdown moved onto the wing surface and as a result, the suction peak collapsed and spread in the spanwise direction. Surface pressure data clearly shows that the angle of attack where the vortex burst moves onto the wing surface correlates well with the previous studies regarding the mentioned longitudinal instability. These findings could help designers to optimize the crank angles to avoid such an undesired phenomenon.