Leading- and Trailing-Edge Reversal of a Cambered Airfoil for Stopped Rotors

For helicopters in very high-speed flight there is interest in stopping the rotor and having it operate as a fixed-wing. However, with conventional airfoils, one half of the stopped rotor/wing would be in reverse flow. To overcome this challenge, this paper focuses on airfoil reversal, where the sharp trailing-edge morphs into the rounded leading-edge, and vice-versa. Extending on a previous study that reversed symmetric airfoils, the current paper presents a design methodology and solution for reversal of a cambered airfoil. Navier Stokes CFD simulation results show that rather than using straight links to approximate a reference airfoil profile, curved links that exactly represent the nose to mid-chord section reduce aerodynamic penalties considerably despite the “bumps” they produce over the trailing-edge region. The aerodynamic performance of a curved-link reversible NACA 23012 was seen to be close to that of the reference NACA 23012. A curved-link reversible NACA 4-digit airfoil, with the same thickness and camber as the Fairchild Reverse Velocity Rotor (RVR) airfoil performed significantly better, aerodynamically, than the RVR airfoil in reverse flow. The study examined actuator force requirement, and showed that both the magnitude and direction of the actuator force required to resist the aerodynamic loads vary depending on the airfoil angle of attack and the internal mechanism design.