To: Rusty Cessna
It has been done and is in use on at least two helicopters. One system is used on the AH 64 Apache and it sits on top of the rotorhead. It measures the wind direction in relation to the direction of flight. When in operation, I believe it spins at a speed higher than the rotor system. It looks something like an anemometer, which measures wind speed, but instead of cups on the end of the arms it has pressure sensors that measure air impact pressure. It knows which way the helicopter is flying and it measure the pressure differential between itself and the pitot air pressure as it rotates in a circular path. The internal computer derives the direction of the wind and its speed in relation to the helicopter.
The final calculation is fed into the weapons control system and is used to calculate trajectory and deflection.
The other system is used on the Bell AH1 J and possibly others including the Agusta 129. It is usually mounted to the fuselage on the right hand side near the pilots’ position. This unit is free to pick up its’ own position relative to the direction of flight and it has sensors to detect airspeed and position relative to the direction of flight. The position can also be influenced by the downwash of the rotor if the helicopter is stationary when firing its' weapons. The output is also used in weapons calculations.
There is also a third type that was made by a company called J Tech, which was located in the State of Iowa in the USA. This unit looked like a double-ended venturi. In the center of the venturi was a thin metal rod. On opposite sides of the venturi but not quite in line with the rod were two ultrasonic detectors. The operation of this unit is based on a phenomenon called vortex shedding. When air or water passed over the rod the flow would be disrupted and the air or water would form vortices as it passed the rod. The ultrasonic detectors sensed these vortices and recorded the frequency of vortex shedding. This went into a microcomputer and the output was the airspeed or water speed. It could not sense direction but it could record the speed if you knew which direction it was coming from. I believe they developed a multiple unit that was sensitive in several directions and the microcomputer could figure out the direction.
Good luck on your project.
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The Cat