Frequency Response of Aerodynamic Load Control through Mini-tabs

Abstract

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 6.6x105 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 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.