Applied Aerodynamics of Circulation Control Airfoils and Rotors

The basic attribute of circulation control airfoils is high- lift capability with reduced dependence on dynamic pressure and angle of attack for production of that lift. Furthermore, if the airfoil is dual-slotted (bidirectional), performance is independent of flow direction. These capabilities make a lifting stoppable rotor aerodynamically feasible. For such a rotor, the critical flight condition (minimum available lift) occurs in the advance ratio range of 0.85 to 1.10 as a result of the trends in mean dynamic pressure on the retreating side. Analysis of the aerodynamic flow field for each blade element at the critical flight condition indicates that the airfoils operate, on a mean basis, in a low speed (M∞ = 0.25), highly yawed (40 deg) environment at a negative angle of attack (−2.5 deg). Such information provides guidelines for properly designing airfoils and assessing their performance. Recent tests of circulation control airfoils have more clearly defined the stall boundaries and the influence of airfoil geometry on the optimal operating conditions. The increasing knowledge of both rotor and airfoil aerodynamics will be the basis for more advanced circulation control rotors through the integration of airfoil and rotor design.