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Turbulent Times

The proposed development of a new Airbus Industries aircraft, roughly 1.5 times the weight of a 747 and capable of carrying 500-600 passengers, is not without its problems. One of these is the wake vortices it leaves behind on landing. The wake vortices arise because, to stay airborne, the aircraft must effectively push down on the air below it, causing the surrounding air to swirl around.

This swirling air represents a potentially serious hazard to a following aircraft, and internationally agreed separation distances must be maintained for safe operation. At present, no aircraft may follow closer than 5 miles behind a Boeing 747, and if this distance were to increase significantly for the new, heavier plane, then the advantage of having an increased passenger carrying capability would be outweighed by the increase in waiting time needed between landings. Dr Will Graham and Dr David Maull are currently looking at this aspect of aircraft design as part of the European Airbus project. 'Wings are generally designed to give low drag and good lift. If what you want is a more sympathetic vortex distribution, as in this case, then the design problem changes. For instance, the Boeing 757 used to be one of the heavier aircraft in the middle category of aircraft separation distances. It has recently been up-rated to the "heavy" category with a longer separation distance, after a number of incidents involving following aircraft. This may simply be because it was one of the heaviest aircraft in the middle category, but equally it could be that the 757 wing design produces a more dangerous vortex distribution for its weight than other aircraft,' they explain.

Visualised pressures in the wake behind a wing in high-lift configuration. The aircraft is travelling away from the viewer, with wing tip to the right and fuselage to the left. A tip vortex and a flap vortex are clearly visible as circular regions of low pressure. Courtesy of Daimler-Benz Aerospace Airbus.

Since wake vortices are mainly a problem on landing, when the wing is in a high-lift configuration with flaps out, designing for a safe wake need not compromise drag performance during cruise. For instance without changing the cruise configuration, it would be possible to alter the size, number and location of flaps on a wing, still giving the area required for lift while altering the pattern of vortices.

Preliminary work, using a simplified model, has shown that this could ameliorate the problem. Further research into the accuracy of the model predictions and the downstream evolution of the vortices will be carried out in the Engineering Department's aerodynamics laboratory, with a view to developing a prediction method suitable for design purposes.

Further information from Dr Will Graham, on (01223) 332634.