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Photogrammetry aids to reduce helicopter noise

US: UH-60A Blackhawk helicopter rotor, NASA’s Subsonic Rotary Wing Project completed in collaboration with the US Army and the US Air Force at NASA’s Ames Research Center in Moffett Field, California. The project aims to reduce the helicopter noise by applying photogrammetry technique.

The test series, conducted from January to May 2010, was designed to gather data on the rotor blades and air movement that the blades create. Since most helicopter noise and vibration can be blamed on the interactions between the blades and the air. To avoid touching or interfering with the spinning blades, NASA used photogrammetry, to identify the blades’ shape, It employed high speed cameras to take pictures of small reflective targets on the blades and the wind tunnel ceiling. On the rotor blade itself, more than 200 sensors were used to measure pressure that creates the rotor lift.

To obtain the data, NASA designed new measurement techniques for the rotor system and for the world’s largest wind tunnel, the National Full-Scale Aerodynamic Complex, or NFAC, in which the rotor system was tested.

NASA researchers needed to develop techniques to make measurements over large distances both on and around the rotating blades, so they customised a system that uses laser light to measure the air flow without touching the blades. This technique, called particle image velocimetry, is commonly used, but not over areas this large. The blade area of 4 feet by 13 feet, approximately the size of two twin mattresses end to end, was the largest measurement area ever attempted using this technique.

Researchers measured the wake of the rotor using a technique known as retro-reflective background oriented schlieren. This technique shoots light across the wind tunnel to visualise the air as it rotates and provides a complementary view of the rotor wake.

Source: India Blooms