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Sensor takes the drag out of flying

11 Jun 2004

An optical airspeed detector being developed by a UK team promises huge cost savings to the airline industry.

Changing the way an aircraft measures its airspeed could bring huge cost savings to the airline industry. That's the view of BAE Systems and Advanced Optical Technology (AOT), two firms who have teamed up with Hull University to develop an optical airspeed detector.

Airspeed measurements are currently made using a number of finger-sized pitot tubes that protrude from the aircraft. They measure airspeed (the speed of the aircraft relative to the surrounding air) by sensing the impact pressure, the difference between static and total pressure. But this well established technology comes at a price. The probes cause drag and with aircraft operating lifetimes estimated at between 20 - 25 years, the associated fuel cost to the airline industry is huge.

BAE Systems is leading the Department of Trade and Industry funded project to replace pitot tubes with a laser air speed sensor instrument (LASSI). The unit would sit behind an aerodynamic window and detect airspeed remotely. Unlike pitot probes, LASSI would be accurate across a range of airspeeds and could also measure negative velocities for use on helicopters.

LASSI works on the principle of Doppler shifting. This involves firing laser pulses into the atmosphere and measuring the tiny Doppler wavelength shift in the reflected signal.

The current testbed measures 60 x 60 x 15 cm and BAE engineer Hywel McArdle believes there is scope to reduce this. “LASSI only needs to measure the airspeed a few meters ahead of the aircraft,” he explained. “The larger backscatter signal at this range helps us compensate for smaller collection optics.”

AOT is developing the laser. “By using a short cavity to produce the pulse, we were able to reduce the size of the Nd:YAG laser” development engineer Stuart Pearce told Optics.org. “We had to use additional optics to drop down to the third harmonic [355 nm], but it’s still a compact unit.”

The team chose an ultraviolet wavelength source to maximize the backscatter signal. Propriety fast switching technology allows the laser to generate very short 1 ns pulses. These are around 30 cm long in actual space and give the unit excellent distance resolution.

With a flyable demonstrator due at the end of 2005, the team is currently working towards intermediate targets of wind-tunnel and ground vehicle testing.

This work was presented at Photonics Focus, a dti sponsored event, held in London on 2 June.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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