Wind tunnel measurement of boundary layer transition on a helicopter rotor in forward flight

Abstract

This talk will provide an overview of a well-defined reference data set for code calibration of boundary layer transition on a trimmed, scaled helicopter rotor in forward flight. The boundary layer transition was measured using differential infrared thermography (DIT) on the top (suction) side of the three-bladed NASA/Army "PSP rotor" at an advance ratio of 0.3 (115 kt) in the NASA Langley 14-by-22-Foot Subsonic Tunnel. The tests used a FLIR X8500 SLS long-wave infrared camera. Differential infrared thermography (DIT) refers to a class of image measurements involving the analysis of the difference between two infrared images. While a single infrared image can contain information about seconds or minutes of the history of the surface, subtracting two images reduces the dataset to the differences occurring in the time bracketed by the images. For an attached flow with a moving boundary layer transition position, the most prominent feature in the difference images is the transition point. However, for the helicopter rotor in forward flight, the most prominent feature is the suction peak and identifying the transition position requires additional analysis. For the DIT analysis two images are taken in quick succession, using a rotating mirror system to avoid motion blur. The measurement is repeated over many cycles, and the images are derotated and averaged, and then subtracted from each other. The resulting DIT image is analyzed in chordwise slices to produce a transition line. These can be combined to provide a rotor map of transition.