Simulation of a rotor in forward flight to characterize helicopter wake signature and encounter severity

Abstract In this work, Large Eddy Simulations of the wake shed by a helicopter rotor in forward flight are performed using a hybrid Eulerian-Lagrangian Vortex Particle-Mesh (VPM) method. Fundamental wake properties are measured. The influence of flight speed on the wake is investigated, from a high speed of 100 kt to a very low speed of 12.5 kt. At high airspeeds, the wake rolls up into a typical compact two-vortex system. The vortex circulation is thus inversely proportional to the flight speed. At low airspeeds, the wake departs from this simple behavior: the vortex spacing increases, and the wake becomes overwhelmed by turbulence. The circulation is then no longer directly related to the flight speed. The VPM method is also used to measure the aerodynamic loads experienced by a rotorcraft when crossing a wake. The main perturbation is found to be a pitching moment. The results show a proportional relationship between this pitching moment and the airspeed of the follower rotorcraft.