Transient Helicopter Rotor Wakes in Response to Time-Dependent Blade Pitch Inputs

Abstract

A time-accurate, free-vortex method was used to predict the evolution of a helicopter rotor wake and the corresponding unsteady rotor airloads in response to time-varying changes in blade pitch. Both steady and maneuvering flight conditions were examined. The modeling was validated using measured rotor responses to transient increases in collective pitch and also for oscillatory collective and cyclic blade pitch inputs. In each case the results showed that there was a temporal lag in the growth and convection of vorticity into the rotor wake, causing significant unsteady effects at the rotor. For transient blade pitch inputs the calculated results showed the bundling of individual vortex filaments below the rotor into vortex rings, a result also verified experimentally. These vortex rings, however, subsequently break down through the development of Kelvin waves. A simulated piloted pull-up maneuver from descending flight was studied, producing evidence that maneuvers can also cause wake vorticity to bundle below the rotor. Large unsteady rotor airloads were produced as the blades encountered this accumulated wake vorticity