Main Rotor Trimming Using CFD Method in Forward Flight

Abstract

In this paper a CFD trimming method was proposed and compared with wind tunnel experiment and blade element method. NASA’s generic ROBIN helicopter model was adopted for transient simulations to get the final main rotor trimming condition. Totally three steps were applied to CFD method, one being no cyclic pitch motion, one being pure longitudinal cyclic pitch motion and the other one being pure lateral cyclic pitch motion. At the same time, a simple linear equation system between the roll and pitching moment was adopted to get the final longitudinal and lateral cyclic pitch angles for the main rotor through these three steps. An overset grid approach was used where the volume around each blade was modeled in an individual overset grid region. The rotor rotation was resolved with 3 degree per time-step. Turbulence was modeled with the well known SST K-omega model with 2nd order convection. The helicopter was in straight forward flight with an advance ratio of μ=0.151. Three sources of stick angles, which are also calling rotor trimming angles, were shown and compared with each other. And the corresponding results were also plotted with type of history plot in CFD condition. In the simulations the results became quasi periodic after about 1.5 rotations and 4 rotor rotations were simulated for each case. The pitch moment coefficient and roll moment coefficient were all trimmed to zero by CFD trimming method while moments were not be removed thoroughly in other two sources condition. So, it is concluded that CFD trimming method has an ability to trim helicopter main rotor in forward flight.