Coaxial Rotor Flow Phenomena in Forward Flight 2016-01-2009

Coaxial rotors are finding use in advanced rotorcraft concepts. Combined with lift offset rotor technology, they offer a solution to the problems of dynamic stall and reverse flow that often limit single rotor forward flight speeds. In addition, coaxial rotorcraft systems do not need a tail rotor, a major boon during operation in confined areas. However, the operation of two counter-rotating rotors in close proximity generates many possible aerodynamic interactions between rotor blades, blades and vortices, and between vortices. With two rotors, the parameter design space is very large, and requires efficient computations as well as basic experiments to explore aerodynamics of a coaxial rotor and the effects on performance, loads, and acoustics. In this study, the Harrington/Dingeldein rotor forward flight test case from the 1950s is simulated using the incompressible Rotor Unstructured Navier-Stokes (RotUNS) CFD to predict performance and flow field properties using momentum source modeling. Flow field properties from RotUNS are used to investigate coaxial rotor wakes and blade-blade aerodynamic interactions. To further understand these aerodynamic interactions, a simplified OVERFLOW simulation of two airfoils traveling in opposite directions aided the understanding of the pressure field behavior of a coaxial rotor in hover to capture compressible blade crossing effects. A MatLab-based simulation of a coaxial rotor system that generates two super-imposed undistorted rotor wakes is used to estimate the times and locations of blades overlaps and blade-vortex interactions.