A Comparison of Helicopter Rotor Inflow Predictions in Turbulence Using Blade Element and Vortex Lattice Models

A comparison and evaluation of the results obtained using two inflow prediction methods are presented for a rotor operating in a spatially varying velocity field. Inflow predictions from a free-wake vortex lattice rotor aerodynamic analysis code modified to permit the introduction of local freestream velocity variations are compared to those produced by a less intensive blade element rotor simulation model for real-time helicopter/ship dynamic interface simulation. In the vortex wake model, local velocity variations modify individual blade segment angles of attack as well as individual wake vortex segment trajectories. The second rotor model utilizes a finite state dynamic inflow model. This model also permits the modification of individual blade segment angles of attack, but inflow variations result primarily from the loading variations produced by the modified blade segment angles of attack. Spatially varying velocity fields produced by deterministic functions were used to highlight the characteristics of the inflow predictions and isolate their differences. An additional case represented the steady velocity distribution in the wake of a ship pointed into a starboard headwind as predicted by computational fluid dynamics (CFD). Comparisons were made between the time histories of thrust, loading distributions, induced velocity distributions and harmonic inflow states predicted by the two schemes while they traversed identical paths through the input flowfields. Induced velocity distributions are used to display differences between the solutions at specific times during the runs. Examination of the results revealed substantial differences in Vortex Wake and Dynamic Inflow induced velocity prediction related to the character of the input velocity fields. The results also indicated that for the specific low-speed flight cases investigated, the impact of the local wake displacement on induced velocity prediction was not significant.