Covert-inspired flaps for lift enhancement and stall mitigation
Chengfang Duan, Aimy Wissa
Abstract
, where small scale UAVs operate. Detailed experiments and simulations are used to investigate how the covert-inspired flap affects lift and drag on an airfoil that exhibits sharp or sudden stall (i.e. the NACA 2414 airfoil) and one that exhibits soft or gradual stall (i.e. an E387(A) airfoil). The effects of the flap chord-wise locations and deflection angles on lift and drag is investigated, through wind tunnel experiments, for two types of flaps namely, a freely-moving flap and a static flap. Results show that the static covert-inspired flap can delay stall by up to 5° and improve post-stall lift by up to 23%. However, the post-stall lift improvement characteristics and sensitivities are highly affected by the airfoil choice. For the soft stall airfoil (i.e. E387(A)), the stall onset delay is insensitive to changing the flap deflection angle, and the flap becomes ineffective when the flap location is changed. In contrast, for the sharp stall airfoil (i.e. NACA 2414), the post-stall lift improvements can be tuned using the flap deflection angle, and the flap remains effective over a wide range of chord-wise locations. Numerical studies reveal that the lift improvements are attributed to a step in the pressure distribution over the airfoil, which allows for lower pressures on the suction side upstream of the flap. The distinctions between the flap-induced lift enhancements on the soft and sharp stall airfoils suggest that the flap can be used as a tunable flow control device for the sharp stall airfoil, while for the soft stall airfoil, it can solely be used as a stall mitigation device that is either on or off.