Mitigation of Gust Loads Through Pitching
Wind gusts can cause large loads on aerial vehicles, affecting their controllability and hindering their performance. The negative impact is especially significant for small vehicles, which experience relatively stronger gusts due to their low speeds. This raises the necessity of a control approach for high-amplitude gusts. An effective control includes aerodynamic models that can predict the gust response in order to mitigate it preemptively. This study aims to develop an aerodynamic model that is able to anticipate the gust lift fluctuations and calculate a wing pitch motion that counteracts them. The model should be relatively simple to be useful in real-time applications, while predicting essential flow physics. Therefore, this work focuses on low-order models and uses quasi-steady and classical unsteady theory, following the work by Wagner and Kussner. However, these theories carry inherent flow assumptions that can produce inadequate results. Therefore, their limitations are carefully analysed.
The effectiveness of the models' pitch profile is studied experimentally in a towing tank. For this, a flat-plate wing is towed at Reynolds numbers between 25,000 and 50,000 as it encounters a gust. The analysis includes two simple gusts. The transverse gust points in the direction of positive or negative lift and is created with a previously designed gust generator. The streamwise gust points in the direction of positive drag and is created by accelerating the wing in the direction of motion. The flow around the wing is visualised using particle image velocimetry, and the loads on the wing are measured with a force balance. The study includes two main sets of experiments. The first set analyses the characteristics of the wing-gust encounter without mitigation. The data shows that the flow is characterised by a leading edge vortex and the advection of trailing edge vorticity. This flow produces a significant lift spike during the gust and, for high angles of attack, a secondary lift peak after exiting the gust.
The second set of experiments study the mitigation effectiveness of the pitch profiles calculated by the analytical models. The resulting force measurements demonstrate that the quasi-steady model achieves approximately 70% reduction of the gust lift peak while the unsteady version reduces this peak by 88%. The mitigation effectiveness for the latter only drops significantly for cases where the effective angle of attack (α) during an unmitigated case exceeds 60 degrees. In fact, the unsteady model success is linked to the small α maintained when mitigating the gust via pitching, which results in a small leading edge vortex that advects near the wing surface. The results demonstrate the value of classical unsteady theory mitigating gusts. Therefore, a new formulae of this theory is re-derived for gust mitigation applications.