Session: 01-07-01: Nonlinear Problems in Aerospace Structures

Paper Number: 160827

160827 - Wind Tunnel Testing and Reduced Order Modelling of a Passively-Actuated Gust Load Alleviation Spoiler. 

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. As a result, many load control systems have been proposed or implemented on aircraft wings. However, most existing 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.

Our work focuses on a prototype passive load alleviation spoiler which deploys in response to strains in a connected wing structure. The spoiler comprises a morphing nonlinear structure coupled to an aerodynamic control surface. The nonlinear structure stores strain energy from the wing box during the initial phase of a gust, before releasing it dynamically via a structural instability (i.e. buckling) when a critical level of input strain is exceeded. This energy is used to rapidly deploy a leading-edge tab, stalling the flow over the wing and thus reducing the overall load the wing experiences during the gust. The spoiler is then automatically retracted once the strain drops below a safe value, returning the wing to its clean configuration. The passively actuated gust load alleviation spoiler prototype has recently been validated using wind tunnel experiments. The prototype deploys rapidly, significantly reducing the wing’s lift coefficient, and achieves this in response to input strains on the order of 0.1%.

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 a simple, reduced degree of freedom model analysed with generalised path-following techniques. The model is able 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.

The work presented here demonstrates experimentally that a strain actuated passive spoiler can be used to achieve a load alleviation benefit on a wing. These results of both the experiment and modelling 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.

Presenting Author: Alberto Pirrera University of Bristol

Presenting Author Biography: Alberto Pirrera is a Professor of Nonlinear Structural Mechanics at the Department of Aerospace Engineering of the University of Bristol, where he has been a faculty member since 2013, holding an EPSRC Early Career Research Fellowship (2015-2020), and where he completed his PhD in 2011. Before that, Alberto obtained his Master’s in Aerospace Engineering from Università degli Studi di Palermo, in Italy. His academic home is the Bristol Composites Institute (ACCIS), where he is a co-director of the EPSRC Centre for Doctoral Training in Composites Science, Engineering and Manufacturing. A modeller and a theoretician specialising in engineering science, Alberto’s research interests lie in the area of structural analysis, design and optimisation. In recent years, he has focused on well-behaved nonlinear structures, on morphing, adaptive and shape changing devices and on wind turbine blades.