DIRECT PERIODIC-SOLUTIONS OF ROTOR FREE WAKE CALCULATIONS

Periodic Inversion is a new approach for the calculation of rotor free wakes which solves directly for periodically steady-state solutions, at all advance ratios including low speed and hover. This is a unique capability as traditional time marching solutions can fail to converge at low advance ratios. The method is based on enforcing a periodic bo undary condition on the wake over one rotor revolution. The resulting system of nonlinear equations is linearized, then iteratively solved f or perturbative corrections to the wake geometry. The method is used t o examine the low speed wake structure, which exhibits three distinct forms depending on advance ratio: a helical structure from hover to so me lower bound advance ratio, a roll-up structure above some higher bo und advance ratio, and a rapid transition region between the two. Perf ormance curves are generated by varying trim parameters between iterat ions.