Session: SYMP S-1: Integrated Systems

Paper Number: 135650

135650 - A Passively Actuated Spoiler Using Sequential, Interacting Instabilities 

Extreme gust events and their resulting air frame loads often drive the structural design of aircraft. Reducing the stresses induced by such events allows for a reduction in air frame mass, hence numerous gust load alleviation systems have been proposed. However, a key drawback of most systems is that they are `active’, meaning they rely on sensors to detect gusts, and on actuators to move control surfaces in order to reduce lift and thus wing loading. These ancillary systems add not only complexity and cost (at the point of manufacture as well as over the aircraft lifetime), but also add weight to the air frame.

To address this challenge, we are developing a `fully passive' load alleviating spoiler, which deploys automatically in response to the deformation of the wing during gust loading. The strain in the skin of the deformed wing is the only stimulus for the deployment of this spoiler, which therefore requires no sensors, actuators or feedback controllers. Instead, the intelligence imparted by such active devices is embedded into the inherent mechanics of the structure.

In operation, the spoiler remains fully stowed until a triggering strain level is reached. It then deploys as rapidly as possible. In our earlier work [1], we show that such a response may be achieved by deliberately incorporating benign elastic instabilities into the design of the spoiler. In particular, we utilise the concept of sequential instabilities, in which multiple branching point or limit point bifurcations are combined in a building-block approach. In other words, a critical strain in the wing skin results in the elastic buckling of a component, which in turn triggers the deployment of the spoiler via a second structural instability. Such sequential instabilities allow for a much richer range of structural responses than would be possible if these bifurcations were utilised independently.

The development of this building-block approach in turn led to the development of a new concept for a passively actuated spoiler. The current design embodiment deploys a leading-edge tab into the airflow when a critical input strain is exceeded. The initial instability is an Euler column that buckles due to the wing deformation, while the second is incorporated into the kinematics of the tab’s deployment. The structural behaviour of this design is tuned using Finite Element (FE) modelling, heavily informed by an understanding of the fundamental mechanics underpinning sequential, interacting bifurcations explored in a previous publication [2]. 

We demonstrate the functionality of this spoiler design concept by developing a physical table-top demonstrator. This demonstrator is shown to exhibit the same mechanical behaviour predicted by FE modelling. Clearly the response to aerodynamic load must also be considered, and to this end we conduct a wind-tunnel test of the prototype. In this test, the prototype is mounted on a NACA 0012 aerofoil, which is in turn mounted on a 6-axis balance capable of sampling aerodynamic loads at rates of up to 1kHz. This allows the spoiler’s transient effect on aerodynamic load to be investigated during deployment, thus verifying the suitability of the device in a wing-mounted scenario.

 

References:

[1] Wheatcroft, E. D., Shen, J., Groh, R. M. J., Pirrera, A. and Schenk, M., 2023. “Structural function from sequential, interacting elastic instabilities”. Proc. R. Soc. A., 479: 20220861

[2] Wheatcroft, E, Shen, J, Groh, R, Pirrera, A, & Schenk, M. "Conceptual Design of a Shape-Adaptive Structure With Tailored Structural Instability." Proceedings of the ASME 2022 Conference on Smart Materials, Adaptive Structures and Intelligent Systems. Dearborn, Michigan, USA. September 12–14, 2022. V001T03A002.

Presenting Author: Ed Wheatcroft University of Bristol

Presenting Author Biography: Ed is an Aerospace Engineering PhD candidate at the University of Bristol, UK. He studies nonlinear structures, with a particular focus on using instabilities to achieve novel functionality. Prior to his PhD he worked as a Wing Structures Engineer at Airbus UK

Authors:

Ed Wheatcroft
Jiajia Shen
Rainer Groh
Alberto Pirrera
Mark Schenk