Circulation control has been applied to airfoils since the late 1960's, and has been proven to change the aerodynamic performance by altering the interaction of the streamlines without changing the physical characteristics of the airfoil itself. This has many applications in fluid dynamics; the focus of this application is for the replacement of the conventional helicopter rotor blade system with a fly-by-wire, active circulation controlled system. Conventional helicopters use a swashplate and a series of mechanical linkages, bearings, and dampers to create a fully articulated rotor hub system. This system is required to achieve the blade characteristics required for stable flight. The need for such a system stems from the asymmetric lift developed in maneuvering flight conditions, which requires the angle of attack of the blades to be changed based on the rotational position within the rotor plane, also known as the azimuth angle. By alternatively activating blowing slots along the leading and trailing edges of the airfoil, the aerodynamic parameters (i.e. lift and drag) can be changed, effectively changing the angle of attack through streamline alteration thus eliminating the need for physical blade pitch changes.

Mathematical models/codes are used to model and simulate the complex blade dynamics of a full-scale rotorcraft. Many of these codes use a blade element method that separates the rotor into small segments and evaluates the aerodynamic characteristics of these elements as two-dimensional airfoils at different local conditions. These local conditions include, but are not limited to, extreme angles of attack, reverse flow, compressibility effects, dynamic effects, and other aerodynamic phenomena.

This research investigated the reverse flow aerodynamic characteristics of a 10:1 elliptical airfoil with various leading and trailing edge blowing pressures. The testing conditions for the aerodynamic investigation were: angle of attack [154 to 196 degrees]; blowing coefficient [0 to 0.009] and [0 to 0.014] for the leading and trailing edge blowing slots, respectively; and Mach number [0.073 and 0.109].

It is concluded that the potential exists for increasing forward flight speeds for helicopters using circulation control in the reverse flow regions of the helicopter rotor environment. Specifically, it is concluded that positively altering the aerodynamic characteristics, primarily the lift coefficient, in reverse flow, is possible through circulation control. Through this investigation, the general trends were found for the aerodynamic characteristics of a 10:1 circulation controlled elliptical airfoil in reverse flow. These trends led to the selection of blowing configurations to decrease the asymmetric loading condition based on the condition of the local blade environment.