Design and Evaluation of Slatted Airfoils for Improved Rotor Performance

Computational and experimental studies have been conducted to design and evaluate slatted airfoil configurations for use on helicopter blades. The improved lift characteristics of these slatted or multielement airfoils (MEA) can improve rotor dynamic characteristics and aerodynamic performance by increasing blade lift and delaying the onset of stall. Initial airfoil design studies maintained main element and slat shapes and varied their relative position. Airfoil experiments demonstrated significant steady and unsteady stall benefits for some slat positions, and minimal drag penalties for others. Further Navier-Stokes CFD based optimization added the ability to vary slat and main airfoil geometries. Following rotor design studies, a model rotor was tested with interchangeable slats mounted between 50 and 90 % radius. The most effective slats substantially improved the rotor stall boundary, but there was still a torque penalty at low thrust. Post-test computational and analytical studies attributed much of this penalty to slat support drag and the effects of reduced Reynolds number. An active slat concept has also been proposed that would move the slat to avoid having to compromise between improving retreating blade stall and minimizing advancing side drag.