Design and Testing of a Passively-Actuated Spoiler for Gust Load Alleviation

Abstract

Gust load alleviation has long been a goal in the design of aircraft wings. Reducing the peak stresses caused by extreme gust events allows for a lighter airframe with less drag and thus a more fuel-efficient aircraft. However, most existing gust load control systems are active, requiring external sensors and actuators to function, and this adds weight and complexity to an airframe. A natural response to these drawbacks is to instead utilise passively actuated load alleviation devices, which do not rely on external systems. Instead, the wing structure is designed such that the intelligence provided by active components is ‘built into’ the wing’s structural behaviour. In this thesis, a passively actuated spoiler is developed which deploys in response to increased wingbox strain during a gust encounter. Analytical, numerical and experimental methods are used to develop the spoiler design, which is shown to be capable of alleviating gust loads in an aircraft wing.

A passive strain actuated spoiler should exhibit a 'binary' structural response; it must remain fully stowed until a critical input strain is applied, whereupon it must deploy rapidly to alleviate load. By definition, a structure fulfilling these requirements will be highly non-linear, and will likely need to traverse a region of instability in order to achieve rapid deployment. However, this binary requirement is not fulfilled by canonical structural instabilities. A solution is to take a building-block approach by combining structural components with different fundamental instabilities into a single morphing structure. The output of one structure is connected to the input of a second, such that instability in the second structure cannot occur until instability has developed in the first structure. These structures are also designed such that the two instabilities interact with one another in order to achieve the desired response. Thus, this phenomenon is referred to as ‘sequential, interacting instabilities’.

Initially, simple analytical bar-and-spring models with limited degrees of freedom are explored in order to aid the conceptual design of such a structure. Using these simple models, it is demonstrated that structures which combine basic instabilities can be used to create ‘binary’ devices which meet the design requirements. These simple models are then used to generate initial designs for the spoiler which operate using the principle of combining instabilities. The physical understanding provided by the simple models allows for rapid iteration of the more complex spoiler designs via Finite Element modelling.

This building block approach is used to develop a spoiler design embodiment in which a morphing structure deploys a leading-edge tab into the airflow when a critical input wing strain is exceeded. A wind-tunnel test of the prototype spoiler is conducted in order to investigate its performance under the influence of aerodynamic load. In this test, the prototype is mounted within a wing, and is shown to be capable of deploying in response to an applied strain, significantly reducing the wing's lift coefficient in around 0.1s.

The addition of aerodynamic load is found to have very little impact on the nonlinear structural response of the spoiler below a critical airspeed and angle of attack. However, beyond this point, aerodynamic pressures prematurely destabilise the morphing structure, and spoiler deployment becomes increasingly erratic. This phenomenon is investigated using the bar-and-spring model that was originally used to develop the spoiler design. The model is modified to capture the destabilising effect of aerodynamic pressures, as well as a nonlinear contact condition between the spoiler control surface and the host wing structure. It is found that a critical level of aerodynamic load causes a step change in the structural response of the model, mirroring the behaviour of the physical spoiler during testing. The model demonstrates how this effect can be mitigated in the physical spoiler by carefully tuning key structural parameters.

Finally, aircraft-scale aeroelastic modelling including a strain-actuated spoiler is conducted. The airframe is modelled using simple beam elements, with additional spoiler loads being applied to the mesh in response to changing wing strain during a gust encounter. This modelling demonstrates that a strain actuated spoiler is capable of providing a reduction in wing root bending moment of 10-15% throughout a typical flight envelope. This model also permits the sensitivity of the load alleviation effect to spoiler design parameters to be investigated.

In summary, this thesis demonstrates experimentally and numerically that a strain actuated passive spoiler can be used to achieve a 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.

Date of Award	30 Sept 2025
Original language	English
Awarding Institution	University of Bristol
Sponsors	UKRI EPSRC
Supervisor	Mark Schenk (Supervisor), Alberto Pirrera (Supervisor) & Rainer Groh (Supervisor)