Experimental investigations of this paper show that delta and reverse delta shaped small horizontal strakes (fineness ratio of 3%-5%) are capable of suppressing wing rock of slender delta wing above 35° angle of attack. Parametric study reveals that 45° sweep back and sweep forward angles are the most favourable combinations while most advantageous stream-wise location of strake is the beginning of the delta wing. The suppression capability is also evident during pitching-up, but no significant deterioration of lift to drag ratio is observed. The suppression is achieved by changes in the vortical flow field brought in by the strakes. During a roll, the strake (a) alters the 'away' normal and 'outward' spatial location of up-going semi-span's vortex to 'closer' normal and 'inward' spatial location, and (b) reduces the difference in normal distances of vortices of two semi-spans, both of which are the characteristics of no wing rock regime such as 15° angle of attack. Thus, the strake changes the 'asymmetric' flow to 'nearly symmetric'. Such change in vortical flow field is caused by the interaction of two vortices emanating from the strake with the vortex pair originating from delta wing.
Nomenclature= angle of attack, degree = maximum amplitude of oscillation, degree = span of delta wing, m = drag co-efficient = lift co-efficient = root chord length of delta wing, m = root chord of strake, m = roll angle, degree = mean roll angle, degree = fineness ratio = / / = lift to drag ratio = local semi-span, m ∞ = freestream velocity, m/sec = pitch rate, degree/sec = non-dimensional pitch rate = 2 ∞
The root cause of wing rock is investigated by examining two slender delta wings (700 and 850 sweep back angle) in wind tunnel using force measurement, pressure measurement and PIV techniques. The results show presence of asymmetric flow at 200 angle of attack and initiation of wing rock at the same point for 850 model while there is neither asymmetric flow nor wing rock for 700 model suggesting close relation of flow asymmetry with wing rock. Investigation with three apparently identical nose sections reveals that the asymmetry comes from the area very close to the wing tip. This asymmetric flow causes the vortices to interact in a complex way resulting in wing rock when the vortices are in close proximity (such as for 850 model), which is not the case when the vortices are ‘comparatively away’ (such as 700 model) from each other.
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