CH-47D Tandem Rotor Outwash Survey

The outwash flowfield generated by a free-hovering CH-47D was quantitatively measured at a target GW of 41,000 lb and a 20ft AGL hover height during low ambient wind conditions. Testing employed a mobile cart conveying a vertical array of ultrasonic anemometers to make rotor-centric velocity measurements at radial distances ranging from 24 ft (0.80R) to 232.5 ft (7.75R), at 30 deg azimuth increments around the aircraft port side and 45 deg azimuth increments around the starboard side. Additional measurements were acquired along the lateral axis midway between the tandem rotors to quantify the high velocity gradients found abeam the rotor overlap region. Supplemental surveys were conducted at the cardinal azimuths to examine the effects of variations in disk loading and hover height on the outwash flowfield characteristics. The outwash velocity field is analyzed in the areas of flowfield unsteadiness, flowfield direction, mean and peak height-velocity profiles, and the potential impact of outwash forces on ground personnel.