Design of efficient high-lift configurations with Coanda flaps

Active flow control applied to high-lift systems is a promising solution to improve low-speed flight capabilities and reduce noise emissions of commercial aircraft. However, too high power requirements in relation to the achieved lift gains have prevented active high-lift systems from being largely employed in the aeronautical industry. In this context, this work develops technologies to enhance the aerodynamic efficiency of an active high-lift system by means of RANS numerical simulations. The transonic airfoil DLR-F15 is equipped with an active internally-blown flap, which consists of a thin air jet tangentially blown over the shoulder of a simple-hinged flap deflected by 65°. To improve the lift generated by the airfoil, the effects of a flexible droop-nose device, wall suction and unsteady blowing are investigated. The fundamentals of gap-less droop-nose design are presented, describing the aerodynamic sensitivities of the main geometrical parameters and the physical phenomena that determine the lift performance. The efficiency of the resulting droop-nose configuration is also tested on a wing-body aircraft model. The analysis reveals three-dimensional flow mechanisms that limit the lift performance in operative conditions. The airfoil efficiency is then further improved by adding a boundary-layer suction device. The effects of shape and location of the suction slot are studied to maximize the lift coefficient and pressure recovery. Finally, the effectiveness of unsteady excitation of the mixing layer by means of dynamic blowing is investigated. As a final result, a target maximum lift coefficient of 5.0 can be achieved with a 43% lower jet-momentum coefficient with respect to the baseline airfoil configuration.