The end of the tube

UK engineers are developing a new generation of air-speed sensors that use the latest laser technology to improve on traditional methods. Jon Excell explains.

The pace of technological change – from passenger jets to helicopters – has been staggering over the past 100 years, yet there are still a few critical components that have remained unchanged.

One is the Pitot tube – a simple mechanical device at the heart of the speedometer on most modern aircraft – designed almost 300 years ago by French inventor Henri Pitot, to measure fluid velocity. The basic instrument consists of two coaxial tubes: an interior tube that is open to the flow, and an exterior tube open at 90 degrees to the flow. By measuring the difference between these two pressures the flow rate of the fluid can be calculated.

But the tube’s lack of accuracy at low speeds and poor aerodynamic performance has led some in the aerospace industry to question its validity in the 21st century.

When mounted on an aircraft, the central passage in the tube points in the direction of travel. Meanwhile, a number of openings in the outside wall of the main tube lead to a second set of smaller passageways. These two sets of tubes are typically connected to either side of a pressure transducer attached to the base of the unit. The transducer measures the difference in atmospheric pressure in the two groups of tubes, and this pressure difference can be used to calculate the aircraft’s speed.

While the tube is accurate at high speeds, its ability to resolve differences in pressure at low speeds is limited which compromises its performance.

An additional problem is that on fixed wing aircraft its profile significantly increases the amount of drag and therefore has an impact on fuel efficiency.

With these problems in mind, engineers at BAE systems Advanced Technology Centre have launched The Laser Air Speed Sensor Instrument programme (LASSI) a two and half year project aimed at developing air speed sensors that are both accurate at all speeds and won’t contribute towards drag.

With the programme still in its infancy, LASSI project leader Leslie Laycock would not go into technical details. He did however reveal that the system will use a compact, short-pulse UV laser and a fibre-optic system. By firing the laser into the atmosphere, the nature of the light reflected from air molecules will change according to speed. This variation is measured by LASSI and used to calculate airspeed.

Laycock said that while engineers have been aware of the Pitot tube’s drawbacks for some time, there hasn’t really been a viable alternative. Engineers have looked at using lasers before, but they’ve been too bulky. Laycock claimed that it is largely due to cutting edge work in the UK on compact high-power lasers that laser-based systems are finally becoming viable.

Although LASSI has yet to yield any solid test data, Laycock said that initial estimations indicate that over the operating life of a long-haul plane, significant fuel savings could be achieved by replacing the Pitot tube with something that’s flush to the airframe.

Clearly, the manufacture and installation of a system that requires compact lasers and fibreoptic cables will initially be more expensive than the Pitot tube. Laycock is confident however, that LASSI’s ultimate benefits will far outweigh the initial cost. Laycock’s team is also looking into the application of the technology in other areas. It could, for instance, be used to model airflow around buildings.

There’s still a long way to go and at the end of the project Laycock hopes to be able to unveil a demonstrator that will be deployed either on an aircraft, or in a wind tunnel to both prove the principle and show just how small the system could be. If all goes smoothly, he said that the system could be appearing on planes in about five years.

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