Wind gusts can be highly detrimental to the performance of fixed wing aircraft. At low flight speeds, gusts transverse to the lifting surfaces can cause massive changes to the angle of incidence, fluctuations in lift and drag, and result in flight instability. This may be catastrophic for small drones, and hazardous for larger aircraft during take-off and landing. Onboard gust sensing and counter-control is a promising solution, but requires a model of the wing-gust interaction. The problem is that only simple linear models such as Küssner’s theory exist, and the physics of large amplitude wing-gust encounters is unknown. This work is a fundamental study into the physics of such interaction. The aims are to uncover the phenomenon which contribute toward the force response of the wing. In particular, the role of free vortices and added mass are investigated and compared with the conditions modelled within Küssner’s transverse gust theory.

Two sets of experiments were conducted using flat plate wing models in a water towing tank. Data was acquired using a combination of Particle Image Velocimetry (PIV), dye flow visualisation, and force measurements. A novel methodology was also developed to isolate added mass effects from standard resolution PIV. The first experiment involved accelerating the plate in translation and angular directions to validate the added mass extraction methodology, and investigate viscous effects on added mass. The experiments successfully demonstrated both the technique, and that the potential flow added mass solution is valid even for viscous and separated flows.

For the second experiment, equipment was constructed to facilitate the generation of a ‘sharp edged’ top-hat shaped gust velocity profile in the towing tank. The wing models were towed through this, thereby replicating a wing-gust encounter. Test cases with gust ratios of 0.2, 0.5 and 1.0, as well as Reynolds numbers from 5,000 to 40,000 were conducted. The results showed that Küssner’s model predicted the force response for each encounter surprisingly well, albeit discrepancies emerged at the higher gust ratios. This was attributed to significant leading edge separation as well as deflection and subsequent roll-up of the gust shear layers. For wing-gust encounters it was shown that the force component attributed to added mass in Küssner’s model is not equivalent to that of an accelerating body, rather it can be attributed to the relative advection of gust shear layer vortices. We call this a ‘non-circulatory vortex force’. A second non-circulatory vortex force was additionally proposed, attributed to the generation of free vortices. This was shown to be responsible for the buoyancy and added mass like force, for cases where a flow field is accelerated past a stationary body.