Flight-Test Measurement of the Aero-Optical Environment of a Helicopter in Hover

Abstract

Methods and results are described for flight-test measurements of the aero-optical effect of the near-field flow surrounding a medium-sized helicopter in hover. The data were acquired using a novel, passive measurement approach in which optical wave-front aberrations were computed from the displacements of light-emitting diodes attached to a target array mounted under the helicopter fuselage. The resulting aero-optical data are shown through the rotor downwash and through the engine exhaust of the helicopter, and they are compared to estimates computed using parametric models for the helicopter wake structure and rotor–vortex diffusion. The experimental data provide additional information on the magnitude and frequency content of the aero-optical aberrations, as well as vibration data, that will be valuable for future beam-control and modeling studies.