Increased aerodynamic loads during gusts, turbulence and maneuvers define the outer envelope of aircraft structural design. Mini-tabs, small spanwise strips that protrude normal to the airfoil's upper surface, have been studied to alleviate this requirement. To investigate the mini-tab's steady state effects, force and Particle Image Velocimetry measurements were conducted at Re = 6.6 x 10 5 on a NACA0012 airfoil. Mini-tabs of height, h/c = 0.02 and 0.04 were placed at a wide range of chordwise locations. In general, the optimum location for peak lift reduction moves towards the leading edge as the angle of attack increases, with significant effect on the lift curve gradient. Trailing edge placement was effective at small angles. Placement close to the mid-chord provided a constant effect across 0° ≤ α ≤ 5°. For both locations, the baseline flow separation progresses ahead of the mini-tab with increasing α, which reduced effectiveness at stall. In comparison, placement close to the leading edge, xf/c = 0.08, was ineffective for small α. At high α, a large flow separation reduced lift by up to Δcl ≈ -0.67, but increased the unsteady forces.

Aircraft and wind turbines are exposed to increased loads during gusts and turbulence, necessitating a stronger and stiffer structure. The field of aerodynamic load control aims to reduce this need, mitigating the extreme loads at the fluid structure interface. Force, Particle Image Velocimetry and pressure measurements were conducted on a NACA0012 airfoil equipped with mini-tabs, small span-wise tabs that were to the airfoil's upper surface, at a Reynolds number of 6.61 x 10 5 . Mini-tabs of height h/c = 0.02 and 0.04 were employed across a range of chord-wise locations to investigate the effects of mini-tab height and chord-wise 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 chord-wise location was highly dependent on the angle of attack. Placement close to the trailing edge induced a large effect at zero degrees. 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 minitab 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 chord-wise 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.

Load control is the reduction of extreme aerodynamic forces produced by gusts, maneuvers and turbulence, to enable lighter, more efficient aircraft. To design an effective control system the actuator's response, in terms of amplitude and phase lag, must be known. Current load control technologies are limited to low frequency disturbances due to their large inertia. This paper evaluates a potential high frequency alternative: the mini-tab using periodic and transient deployments on a NACA0012 airfoil in wind tunnel experiments. Periodic deployment for reduced frequencies, k ≤ 0.79 exhibits a normalized lift response amplitude which decays with increasing k comparable to Theodorsen's circulation function, but with substantially higher lag. Transient deployment, at rates as low as τdeploy = U∞tdeploy/c = 1, illustrated a delay in aerodynamic response. The delay is larger for outward mini-tab motion than inward, τ ≈ 6 and 4 respectively for α = 0° and increases with α. The flowfields show that the delay in response and reduction in effectiveness for dynamic mini-tab deployment is due to delayed growth of the separated region behind the mini-tab. The aerodynamic response due to mini-tab deployment was approximated as the response of a first order system, pertinent to control system design. This simple characterization for amplitude reduction and delay in response makes it well suited to loads control.

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Load control is the reduction of extreme aerodynamic forces to enable lighter, more efficient aircraft. Current load control technologies are limited to low frequency disturbances. In this paper the mini-tab, a small, span-wise tab placed on the airfoil upper surface, is investigated as a high frequency alternative through periodic oscillations to identify its unsteady aerodynamic transfer function. Force measurements were conducted on a NACA0012 airfoil at a Reynolds number of 6.6x10 5 with a deployable mini-tab located at xf/c = 0.85, with actuation performed at reduced frequencies, k ≤ 0.79. 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 normalized 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 minitab 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.