Aerodynamic Loads Alleviation Using Rigid and Compliant Spoilers

: (Alternative Format Thesis)

Abstract

Aircraft wings experience their maximum aerodynamic loads when encountering gusts and turbulence. Mitigating these loads can facilitate reduced structural mass of the wings, thereby lowering the overall aircraft weight, enhancing fuel efficiency, and contributing to environmental sustainability goals. Traditionally, load alleviation is achieved by using large mechanical surfaces such as trailing-edge flaps, which are limited by their inability to operate at high frequencies. High-frequency actuation has been achieved through fluidic methods, such as jet blowing at the trailing edge; however, limitations related to the fluid supply and the diminishing effectiveness at higher flight velocities pose significant challenges to these techniques. Recently, the rigid mini tab spoilers have emerged as a promising alternative owing to their low-inertia properties. Previous studies have examined the aerodynamic performance of the rigid mini tab spoilers on aerofoils and on plunging wings in freestream conditions. This study investigates the performance of the part-span rigid mini tab spoilers on both the rigid and flexible wings in periodic travelling wave gusts, as well as the performance of even lighter compliant spoilers (surfaces) on the rigid wing in freestream conditions through wind tunnel experiments.

Both the rigid and flexible wings feature rectangular planforms with NACA 0012 aerofoil sections and semi-aspect ratio of 5. Tests were conducted at pre-stall geometric angles of attack ranging from 0◦to 10◦, corresponding to a chord-based Reynolds number of Re = 3.0 × 105. The performance of part-span (0.6 ≤ z/b ≤ 0.8) leading-edge and trailing-edge mini tab spoilers, representing separation control and circulation conitrol mechanisms respectively, was examined for alleviating the peak aerodynamic loads under periodic travelling wave gusts. Four compliant spoiler configurations (surfaces),namely both-ends-fixed compliant surface, inverted compliant surface, self-activated compliant spoilers, and rigid spoilers with compliant hinges, were also investigated on a rigid wing at the same spanwise locations under freestream conditions. The gusts were generated by bluff bodies (cylinders) positioned upstream of the wing. Lift, drag, and wing-root bending moment were measured, complemented by flow-field measurement using PIV and wing-surface deformation measurements using DIC on the flexible wing in this study.

Though there was no meaningful reduction in the maximum aerodynamic loads acting on the rigid wing at small angle of attacks (AOAs) with a leading-edge spoiler for separation control, the lift reduction increased with the AOA, and a reduction of∆ CL, max = −0.235 was achieved at 10◦ AOA. In contrast, circulation control by a trailing-edge spoiler resulted in nearly constant reductions of up to ∆CL, max = −0.17for all AOAs tested. For the flexible wing, the leading-edge spoiler led to reductions of up to 44% in CL, max, with little variation in percentage reduction with the AOA. While the trailing-edge spoiler resulted in larger reductions of up to 63% in the CL, max of the flexible wing at small AOAs, the percentage reduction in CL, max decreased swiftly when the AOA increased.

For the performance of the rigid minitab spoilers on both the rigid and flexible wings, the effectiveness of the spoilers reduces significantly with increasing gust amplitude due to intermittent flow separations and subsequent reattachments over the wings under the wake gusts. Under the same wake-gust amplitude, the leading-edge spoiler leads to reductions in the twist angle, and therefore the effective angle of attack, of the flexible wing. As a result, it exhibits more reductions in the maximum loads than the rigid wing. It appears that the performance of the spoilers was dictated by intermittent flow separations and subsequent reattachments over the wings under wake gusts.

Compliant spoilers (surfaces), consisting of flexible membranes, were attached to the suction surface of the wing to assess their ability in aerodynamic loads alleviation to ii the rigid wing in freestream conditions. The both-ends-fixed compliant surface on the suction surface near the leading edge forms a ‘hump’, which acted like mini tab spoilers for separation control and led to substantial lift reduction. Similar lift reduction performance is also observed with leading-edge inverted compliant surfaces that exhibits steady large deflection to induce massive flow separations. A maximum lift reduction of 44% at the angle of attack of 10◦ was recorded. The trailing-edge inverted compliant surfaces with both steady deflections and limit-cycle flapping motions demonstrated a nearly constant lift reduction of up to ∆CL ≈ −0.28 by deflecting their wake flows upwards and reducing the circulation of the wing. A steadily deflected inverted compliant surface with large height achieves a greater load alleviation compared to that undergoing limit-cycle flapping motion.

When the AOA is increased beyond a critical value, both the self-activated compliant spoilers and the rigid spoilers with compliant hinges flips over towards the upper surface of the wing, creating massive flow separations over the wing and leading to a maximum lift reduction of 32% at α = 10◦. However, hysteresis loops in lift and drag are seen when decreasing the angle of attack of the wing, as both self-activated compliant spoilers and rigid spoilers with compliant hinges remains on the upper surface of the wing. Future investigations, such as the stowing mechanism, are required for these types of spoiler.

Date of Award	18 Feb 2026
Original language	English
Awarding Institution	University of Bath
Supervisor	Zhijin Wang (Supervisor), Samuel Bull (Supervisor) & Ismet Gursul (Supervisor)