Coaxial Rotor Flow Phenomena in Forward Flight

Coaxial rotors are nding use in advanced rotorcraft concepts. Combined with lift offset rotor technology, they offer a solution to the problems of dynamic stall and re-verse ow that often limit single rotor edgewise forward ight speeds. Lower tip speed means reduced high-speed impulsive noise. The need for an anti-torque tail rotor is eliminated, a major boon during operation in conned areas. However, the operation of two counter-rotating ro-tors in close proximity generates many possibilities for aerodynamic interactions between rotor blades, blades and vortices, and between vortices. The parameter de-sign space is very large, and requires efcient computations as well as basic experiments to explore important physics to determine performance, loads, and acoustics. Computations are done on the classic HarringtonDingeldein rotor test case from the 1950s using the ROTUNS Navier Stokes code. Two regimes are explored: very low advance ratio as a perturbation from hover, and high advance ratio. Flow eld properties from RotUNS are used with 2-D OVERFLOW computations to capture blade crossing effects including those of higher subsonic Mach numbers. Bladeblade and bladevortex intersection events are captured using a MatLab-based predictor.