The HART II test – measurement of helicopter rotor wakes

A key parameter to reduce rotor noise is a wake location relative to a blade. During the data analysis in the years after the HART test of 1994 it was found that comprehensive wake data are needed to understand wake development, vortex creation and aging, their motion and their re-development after an interaction with a rotor blade. Therefore, DLR, ONERA, NASA Langley, US Army AFDD and DNW again came together and generated a follow-on program named HART II. In this program the rotor wake was intensively measured with a novel double-stereo PIV (Particle Image Velocimetry) system. This provides instantaneous flow field data in both a large observation area and in a smaller close-up view to the vortex core. Additionally, rotor blade deformation, airloads, and noise radiation were measured to form a complete, consistent, and comprehensive data base. Another “first time” was the application of the blade position measurement technique SPR (Stereo Pattern Recognition), which provided instantaneous optical measurement of the blade bending in flap, lead-lag and torsion. In this paper, the HART II test is described in detail, including a description of all measurement techniques applied and with some results from each of them.