AbstractAircraft encounter increased aerodynamic loads when exposed to gusts, turbulence andmanoeuvres. Currently, these loads are mitigated through the use of ailerons and spoilers to reduce lift, in turn reducing the loads passed to the aircraft structure. However, these actuators are limited in their frequency response and cannot control loads produced by higher frequency events. Therefore, an actuator which can mitigate high frequency oscillatory loads is required, with a deployment reduced frequency, k of up to 1. One such promising load control actuator is the minitab, consisting of a small span-wise strips, similar to the Gurney flap, deployed normal to the airfoil upper surface. Key to the actuator’s high frequency response is its low inertia, meaning that a small energy input can achieve a significant effect. To investigate the efficacy of the minitab on load alleviation a series of steady state, periodic and transient measurements were conducted at a Reynolds number of 6.6 x 105. These experiments aimed to fully evaluate the effect of chordwise location, mini-tab height and angle of attack on steady state load control. The dynamic response was categorised, in terms of magnitude, phase and time delay by the periodic and transient measurements.Mini-tabs of height h/c = 0.02 and 0.04 were employed in a steady state configurationacross a range of chordwise locations to investigate the effects of mini-tab height and chordwise position. Overall, the mini-tab was found to have a lift reducing effect which increased with height. It was found that the effect of the chordwise location was highly dependent on the angle of attack. Placement close to the trailing edge induced a large effect at α = 0°, creating an effective change in camber comparable to conventional Gurney flap use. Peak suction over the lower surface increased resulting in a reduction of ΔCL = -0.48. Approaching stall, effectiveness decreased as the mini-tab became immersed in the separated flow. Placement at xf/c = 0.60 produced an almost constant lift reduction between α = 0° and 5° of ΔCL ≈ -0.60, with a gradual reduction to stall. A mini-tab positioned close to the leading edge (xf/c = 0.08) was found to separate the flow effectively at low incidences but with no noticeable change in lift observed. It was found that the flow separation produced by the mini-tab effectively eliminated the suction peak on the upper surface. However, placement close to the leading edge has increasing effectiveness towards stall, as the shear layer induced by the separation was displaced further from airfoil surface. Peak lift reduction at stall was found to be ΔCL ≈ -0.67. The optimum chordwise location for peak lift reduction is dependent on the airfoil angle of attack: the position of the mini-tab for maximum lift reduction moves towards the leading edge as the angle of attack increases.IThe second stage utilised a deployable mini-tab up to reduced frequencies, k = 0.79, placedat xf/c = 0.85, to assess the mini-tab’s frequency response. The force measurements indicate that the mini-tab has a decreasing effect on lift reduction with increasing actuation frequency. This trend is comparable to Theodorsen’s function, based on the change in circulation. For α = 0°, the normalised peak-to-peak lift reduction decreased from 1 for steady state deployment to around 0.6 at k = 0.79. In addition, a phase lag exists between the mini-tab deployment and the aerodynamic response which increased with actuation reduced frequency, k. However, the measured phase lag is substantially larger than Theodorsen’s prediction. Increasing the angle of attack, α reduced the mini-tab’s effect on lift while increasing the phase angle when comparing equal k values. Particle Image Velocimetry measurements indicate that the delay and reduction in effectiveness of periodic deployment is due to the presence and growth of the separated region behind the mini-tab. Overall, the mini-tab was found to be an effective, dynamic lift reduction device with the separated region behind the mini-tab key to the amplitude and phase delay of lift response.Finally, the aerodynamic response of the mini-tab was investigated during a transientdeployment. The delay in aerodynamic response to mini-tab actuation was consistent with literature. The normalised deployment period, τdeploy did not provide a significant alteration in the aerodynamic response for deployment periods below τdeploy = 3, with the aerodynamic response reaching the steady state value around τ = 6-8. The aerodynamic response of the mini-tab was approximated using a simple, 1st order system response to a ramp-step input of gradient 1/τdeploy, indicating that the aerodynamic response of the mini-tab is further delayed for higher angles of attack, due to the presence of separated flow in the vicinity of the mini-tab. PIV measurements were utilised to analyse the effect of transient mini-tab deployment, indicating a delay in the development of the separation region created by the mini-tab, producing a corresponding delay in aerodynamic response. In addition, outward deployment was found to have a slower aerodynamic response than inward deployment, as the flow was found to take to detach slower than to reattach.
|Date of Award||29 Jun 2017|
|Supervisor||David Cleaver (Supervisor) & Ismet Gursul (Supervisor)|
Aerodynamic Loads Alleviation Using Mini-tabs
Heathcote, D. (Author). 29 Jun 2017
Student thesis: Doctoral Thesis › PhD