Analytical method of rotor aerodynamic characteristics by coupling a high-efficiency trim strategy

A numerical analytical methodology was developed for aerodynamic characteristics analysis of helicopter rotor by coupling a high-efficiency trim strategy and the computational fluid dynamics（CFD) technology. Firstly, using the CFD solver to calculate the Jacobin matrix and rotor flowfield, an analytical methodology with direct trim method was established. Then, by employing the “delta method”, the computation of Jacobin matrix was simplified and a delta trim method was developed such that the calculation time of rotor performances was significantly decreased. For the calculation of rotor flowfield, the Navier-Stokes equation was selected as the governing equation, and a second-order upwind scheme, i.e. Roe scheme, was used for spatial discretization. Implicit lower-upper symmetric Gauss-Seidel (LU-SGS) scheme was adopted for time marching, and the Baldwin-Lomax （B-L） model was selected as the turbulence model. The computation efficiency and accuracy of the “delta” trim method was compared with that of the direct trim method by numerical calculations based on experimental states of “AH-1G” and “Helishape 7A” rotors. In addition, effects of the background grid quantity on rotor trimming were also analyzed. It is shown that, compared with the direct trim method, the delta trim method can save at least sixty percent of computing time to make the trimming procedure converged. A coarse background grid is not helpful for the capture of flowfield details, but has nearly the same accuracy as a refined background grid in the calculation of rotor control settings; furthermore, the computing efficiency of a coarse grid is higher than that of a refined grid due to the lower grid quantity.