Suitability of a Drag Sphere Anemometer for Measurement of VTOL Aircraft Downwash

Tests were conducted on a simple, low-cost drag sphere anemometer to determine its suitability for measuring wind velocities in the vicinity of VTOL aircraft and helicopters. A drag sphere anemometer is a device for determining wind velocity by measuring the drag force acting on a spherical body of known drag coefficient. The drag sphere anemometer, as tested, was found to be capable of measuring wind velocities and direction in one plane over a speed range of 10 to 110 mph. Instrumentation accuracy was found to be plus or minus 2.5 mph in the speed range of 10 to 50 mph and plus or minus 7 in the speed range of 50 to 110 mph. Directional accuracy was found to be approximately plus or minus 30 deg. at low wind speeds, plus or minus 10 deg. for speeds from 30 to 60 mph, and plus or minus 5 deg. above 60 mph. On the basis of the relatively unsophisticated tests performed, the drag sphere anemometer is considered to be suitable for measurement of downwash velocities in close proximity to hovering VTOL aircraft. If required, the upper end of the usable speed range could be extended through additional wind-tunnel calibration.