If Active Flow Control (AFC) is to Become a Design Tool in Aerodynamics, Do We Need To Depart From Prandtl’s Classical Approach?

Boundary layer control by blowing enabled early combat jet airplanes to operate from shorter runways by increasing their lift coefficient at low speeds. The concepts of boundary layer and circulation control evolved at that time however, blowing was never considered interactively with other wing design parameters that were separated from each other due to the boundary layer approximation. The need for increased speed resulted in swept-back planforms that were more recently modified to trapezoidal and lambda shapes for enhanced maneuverability and reduced observability, while commercial transport wings hardly changed during the past fifty years. Sweep back resulted in a pitch-up problem at low speeds that seriously limited cruise performance. It was traditionally alleviated by swinging the wing forward and adding leading edge devices that increased the weight and the complexity of the airplane. This problem could be alleviated by AFC. Furthermore, AFC can provide control authority around all three axes by itself and in conjunction with conventional control surfaces. To that end, AFC should be considered at the beginning of the design process and not after its completion.  Such a change in approach recognizes the interactions among the traditional design parameters and AFC, simply stating that forces and moments acting on a wing depend on its shape, on its attitude and on AFC concomitantly. These variables are of comparable value and they should not be treated separately no matter how many parameters are required to define them. Experiments carried out on swept wings and tailless aircraft models indicate how AFC has to be tailored in order to avoid pitch-up with an increase in incidence. Such tailoring does not have to be limited to momentum or mass-flow input levels that may be traded off in favor of a change in location and/or orientation of the AFC input. However, the payoff is large because a small input may double the usable lift coefficient when the latter is needed on a landing approach.