POP-UP SPOILER LOAD ALLEVIATION PERFORMANCE IN UNSTEADY FLOW CONDITIONS

Abstract

The rapid growth of air travel and its significant environmental impact underscore the urgent need for innovative solutions in aviation. Reducing fuel consumption is key to achieving sustainability, with strategies focused on minimising aircraft weightmand optimising aerodynamic performance. Managing wing loads during turbulence,mgusts, and extreme manoeuvrers plays a critical role in reducing structural weight, enabling lighter and more efficient aircraft designs. This thesis investigates advanced load alleviation technologies, specifically part-span pop-up spoilers, to address these challenges and contribute to the future of sustainable aviation.

Through experimental investigations in a water tunnel, this research examines the aerodynamic performance of finite-span spoilers on both stationary and plunging wings, emphasising spoiler placement, geometry, and wing sweep. Lift and root bending moment measurements were collected, complemented by advanced flow visualisation techniques such as particle image velocimetry and volumetric velocimetry, to provide a comprehensive understanding of the resulting load changes.

For stationary wings, the effectiveness of spoilers was found to be strongly dependent on their spanwise and chordwise placement, as well as their height. Spoilers located further away from the wing-tip (inboard) were significantly more effective in alleviating loads compared to those placed further outboard. This performance improvement is attributed to the reduced influence of wing-tip vortices. Additionally, the chordwise position of the spoiler played a critical role to its effectiveness: trailing-edge spoilers were particularly efficient at lower angles of attack, where they contributed to altering the wing’s circulation. In contrast, leading-edge spoilers became more effective at higher angles of attack by inducing flow separation. However, trailing-edge spoilers lost their effectiveness at higher angles of attack as they became engulfed in separated
flow, while leading-edge spoilers were less effective at smaller angles and could even be detrimental by promoting the formation of separation bubbles.

In plunging conditions, key parameters such as reduced frequency of the wing’s plunging motion, plunge amplitude, and spoiler configuration were identified as critical factors influencing load alleviation. Each spoiler configuration exhibited a characteristic “cut-off frequency” of the wing’s plunging motion, beyond which its load alleviation capability significantly declined. In the pre-stall region, near the cut-off frequency, flow reattachment was identified as a risk for leading-edge spoilers, limiting their effectiveness. At high reduced frequencies, regardless of the angle of attack, the strength of the leading-edge vortex increased considerably, and spoilers were no longer able to influence its development. Additionally, trailing-edge spoilers were less effective as the angle of attack increases, due to leading-edge flow separation caused by the plunging motion.

Experiments on swept wings demonstrated a notable decrease in spoiler performance as the sweep angle increased. The leading-edge spoiler sheds a vortex from its inboard tip that is carried outboard due to the spanwise flow, leading to partial flow reattachment and a reduction in the separation area behind the spoiler. Conversely, the trailing-edge spoiler was unable to deflect the flow as effectively as it did on unswept wings, likely due to the inclined wake flow.

A comparative analysis between ailerons and spoilers placed at the trailing and leading-edges on unswept wings revealed that, under stationary conditions up to the stall angle, ailerons performed similarly to trailing-edge spoilers. In dynamic conditions, both ailerons and spoilers exhibited similar limitations, with their performance degrading beyond the identified cut-off frequency. The aileron outperformed the trailing-edge spoiler across all angles of attack, even providing modest load alleviation at stall and post-stall angles of attack. Despite the larger surface area of the ailerons, they exhibited similar performance limitations to spoilers, which were strongly dependent on the angle of attack and the cut-off frequency. The spoilers however, are significantly lighter, and allow for much faster actuation, making it particularly advantageous for high-frequency unsteady encounters, unlike conventional techniques such as the aileron.

Date of Award	23 Jul 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Zhijin Wang (Supervisor), Samuel Bull (Supervisor) & Ismet Gursul (Supervisor)