Development of an Airborne Ultrasonic Velocimeter and its Application to Helicopters

The Japan Aerospace Exploration Agency (JAXA) has been developing a new air data sensor, called an ultrasonic velocimeter (USV), based on the ultrasonic anemometer commonly used in ground-based meteorological observation. For airborne use, JAXA has modified the probe and realized the operational velocity range of 0–100m/s to cover almost the entire flight envelope of small airplanes and helicopters. The USV has a great advantage particularly as a helicopter airspeed sensor since it measures three-axis airspeed components even at low airspeeds where a conventional Pitot-static system is ineffective. Two USV installations on a helicopter, at the tip of a nose boom and at the bottom of the fuselage, were evaluated by flight test. The USV successfully measured two- or three-axis airspeed components at airspeeds down to 5–10m/s forward speed. For practical use, there still remains a technical problem to find a more suitable location for the USV probe where position error, i.e. the effects of rotor downwash and flow around fuselage, does not change greatly throughout all flight conditions.