A revolutionary idea could prevent accidents and cut airport congestion.
It's every air passenger's nightmare: as the plane is about to land, sudden turbulence flips it over and smashes it onto the tarmac. The cause is wake vortices, powerful swirling streams of air billowing from a plane that landed earlier. Now a pair of engineers from Boeing have patented a way of stifling these deadly vortices. As well as making flying safer, it promises to ease congestion at busy airports by reducing the gap between planes as they land.
Wake vortices are generated by the pressure difference between the top and bottom of an aircraft's wings and tail. Vortices from the wing tips are the strongest, followed by those nearer the fuselage. Wind speeds around the core of these swirling currents can reach 350 kilometres per hour, and they can extend for many kilometres behind the aircraft.
The danger comes when a plane coming along behind hits one of these powerful, invisible vortices. "The following aircraft can be rolled to one side, which is particularly hazardous when flying close to the ground," say Boeing engineers Jeffrey Crouch and Phillippe Spalart in their patent (US 6082679), which was granted early this month. "Several accidents have been attributed to wake turbulence," they warn. Such accidents happen out of the blue and are often hard to recognise (New Scientist, 16 November 1996, p 28).
Until now, the only way to avoid the problem of wake vortices has been for air-traffic controllers to keep incoming aircraft up to 11 kilometres apart. This limits the number of planes that can land each hour. Airlines complain that this leads to delays at busy times as planes queue up to land.
Boeing set out to solve the problem by destroying the vortices at source. In earlier wind tunnel tests, engineers found that by wiggling the plane's control surfaces they could produce a wave-like disturbance in the airflow over the wing that broke up the vortices. In these attempts, engineers oscillated the inner and outer ailerons out of phase. But this caused too much stress on the wings for it to be used routinely.
The breakthrough came when Crouch and Spalart found they could make the idea work if they combined small perturbations of the outer ailerons and the spoilers. This makes the inner and outer vortices on each wing interfere with each other, causing instabilities that destroy them both. They say in their patent that a sensor on-board the plane will measure the strength of the vortices it is leaving behind. A computer will then use this information to calculate the strength and frequency of the movements needed for the spoilers and ailerons of the following plane.
"Wake vortices are an immense problem," says Neil Halsey, an aviation specialist at Britain's Meteorological Office. "Linking two vortices will disrupt them more quickly, so it will be interesting to see how Boeing is doing this."
Boeing's is not the only attempt to tackle wake vortices. NASA and Britain's Defence Evaluation and Research Agency are developing laser systems to let airports predict wake turbulence as planes come in to land: vortices are strongest in calm air, but are quickly broken up in windier weather. This will allow controllers to adjust the separation between planes depending on conditions at the time.
Larger planes create stronger turbulence, so Boeing's rival Airbus is designing the wings for its giant A3XX super jumbo to make "short-lived vortices", says Airbus spokesman David Velupillai.
Author: Paul Marks, New Scientist
New Scientist issue: 22nd July 2000
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