Lift Enhancement at Low Reynolds Numbers Using Self-Activated Movable Flaps

Abstract

I NRECENTyears, interest in micro air vehicles (MAVs) is rising. The reason for that is the availability of microand nanotechnology necessary for the design of these vehicles, but also the emergence of new threats, especially in form of terrorism. MAVs promise to be able to be deployed in closed environments, such as buildings, subway tunnels, and dense forests, allowing reconnaissance, surveillance, and search and rescue missions, for which the use of humans is dangerous or even impossible. To be able to operate in this environment, MAVs need to be small and capable to fly at low speeds, yet they need to be agile enough to move around corners and bends. Moreover, they have to be able to transport a payload, such as video surveillance equipment or sensors for chemical agent detection. So far, no operational MAVexists that fits within these operational limits. The reason for this is that conventional aircraft designs have difficulties in providing sufficient lift and aerodynamic efficiency at low Reynolds numbers [1]. The low lift coefficients cause problems in the design of MAVs, because the often prohibitively large wings are required. Hence, novel methods to increase lift at low Reynolds numbers need to be considered. A method to increase the lift coefficient, inspired by biological flows, is investigated here. Schatz et al. [2,3] reported that the use of a passive flap near the trailing edge results in a lift increase at high angles of attack. The use of a passive flap is inspired by the feather structure of a bird on the upper side of the bird’s wing [4,5]. At high angles of attack, it can be observed that the feathers start to pop up (Fig. 1). Schatz et al. [2,3] investigated the use of a passive flap to emulate this behavior. Their force measurements at Reynolds numbers above Re 1; 000; 000 showed an increase of lift of about 10%. Kernstine et al. performed experiments on this passive flap configuration at Reynolds numbers around Re 330; 000 [6], with similar results. Kernstine et al. focused especially on the optimal placement of the flaps and determined that the flap should be located closer to the midchord rather than the trailing edge to obtain optimal results. The Cl;max increased by a maximum of 15%. The role of the flap is to capture the trailing edge flow separation and to prevent it from creeping upstream. This allows the flow to be attached on a larger portion of the wing than would be the case without and, as such, sustain higher angles of attack without stalling. The advantage of this flap is that it is a very simple and robust device that does not require complex mechanisms, such as conventional slats and flaps. It is easy to install and even retrofit existing aircraft. Here, the possibility of applying this method to low Reynolds numbers at about Re 30; 000–40; 000, which is a more suitable range for MAV, is investigated. The aerodynamics of airfoils at this Reynolds number range are especially challenging due to the presence of laminar flow separation and the nonlinearities, such as hysteresis effects, associated with it. Awater tunnel is used to visualize the flow and to measure the effect of the flap at this Reynolds number range.