Controlling upwards and downwards gust loads on aerofoils by pitching

Abstract

A wing travelling at a constant angle of attack, α , experiences a large lift spike when subjected to a high-amplitude transverse gust. This study analyses the capability of a low-order model, which uses Wagner and Küssner theory, to calculate a pitch motion profile that mitigates such a lift increase. The model’s pitch profiles are tested experimentally by towing a wing at a constant α towards a top-hat transverse gust. Throughout the gust encounter, the wing is pitched with a profile predetermined by the model. Force and flow measurements are analysed for gust ratios of 0.5 and 1.0 and angles of attack of 0, 10, 20 and 45 degrees. In addition, two gust directions are analysed: upwards where the gust velocity is in the direction of positive lift, and downwards (in the opposite direction). The latter produces a large negative lift spike that can be especially dangerous for flying vehicles. Experimental results demonstrate strong mitigation, around 85%, of the gust loads up to an effective angle of attack of 60 degrees, independently of gust direction. The theories used achieve mitigation even at large perturbations and separated flows, where the assumptions used on their derivation are clearly invalid. The mitigation approach does, nevertheless, not mitigate the secondary lift peak that emerges after gust exit when the initial wing incidence exceeds 10 degrees. This secondary peak is a result of attached flow around the wing right after gust exit, which eventually develops into a LEV that sheds from the wing.