Design and Analysis of a Passively-Actuated Gust Load Alleviation Spoiler

Reducing the peak stresses caused by extreme gust events allows for the design of lighter and thus more fuel-efficient aircraft. Our work focuses on the design and analysis of a prototype passive gust load alleviation spoiler which deploys in response to a critical strain in the underlying wing structure. The spoiler comprises a non-linear morphing structure coupled to an aerodynamic control surface. The non-linear structure stores strain energy from the wing box during the initial phase of a gust, before releasing it dynamically via a structural instability (<i>i.e.</i> buckling event) when a critical level of input strain is exceeded. The released strain energy is used to rapidly deploy a leading edge tab, in order to stall the airflow over the wing and thereby reduce the maximum load the wing experiences during a gust event. The spoiler subsequently automatically retracts once the strain drops below a safe value, returning the wing to its clean configuration. The proposed passively actuated gust load alleviation spoiler does not rely on external sensors or actuators for its functionality; instead, the required sensing, control and actuation is directly embedded into the structural response of the morphing structure. This integrated approach significantly reduces system complexity, and provides an effective means to automatically protect a wing from experiencing excessive loads during an extreme gust event.<br/><br/>Previous work has proposed a concept for the strain-actuated spoiler, comprising a leading edge tab and a morphing structure which exhibits sequential structural instabilities. The load alleviation performance of this design was then investigated experimentally via wind tunnel measurements, demonstrating that the spoiler is capable of locally and rapidly reducing a wing's lift coefficient. In this work we focus on the load alleviation performance of the passive spoiler at the scale of a full airframe. We present aeroelastic modelling of an aircraft with wings equipped with strain-actuated spoilers, subjected to a range of one-minus-cosine gust loads. The results show that the strain-actuated spoiler is capable of providing a reduction in wing root bending moment (which is taken to size the wing structure) of 10-15% across a typical flight envelope for a commercial airliner. The sensitivity of the load alleviation effect to spoiler design parameters such as deployment time, triggering strain and the local reduction in lift coefficient were investigated. Our work to date demonstrates experimentally and numerically that a strain-actuated spoiler can achieve a gust load alleviation benefit on an aircraft wing. These results will inform future design iterations of the spoiler, paving the way for the technology to be scaled up to the wing of a commercial airliner.