Shorter queues for taxiing planes?

Spinning around: The air flow from the wing of a plane, made visible by using coloured smoke. (Courtesy: NASA)

By Ian Randall

If you’re as impatient as I am, the worst part about flying off for your summer vacation is the interminable hold-up that sometimes occurs right before take-off – waiting for the plane to taxi onto the runway and desperately hoping the in-flight entertainment will kick off soon. But these annoying delays may soon be cut down thanks to Georgios Vatistas and colleagues at Concordia University in Montreal. The team has developed a new mathematical airflow model to help refine the safe separation distances needed between planes during take-off and landing.

As an aeroplane moves along, the lift-generating difference in pressure between the top and bottom surfaces of its wings causes air to flow out from beneath each wing and up around the wing tip. This creates a circular vortex pattern behind each tip (pictured above), with a downwash in-between – forming a turbulent wake that can be hazardous to any craft that passes through it. If large enough, this turbulence can roll the next aircraft, faster than they can resist – leading to a crash.

“That’s why there are often large separation distances between the planes as they line up for take-off,” says Vatistas, as the pauses need to be long enough to allow the vortexes time to dissipate. This, he explains, is “a major cause of delays on the runway”.

The strength of each wing-tip vortex is related to the lift generated by each wing, and so it depends on the size, shape, speed and weight of the aircraft that creates it. Previous studies used to determine the current safe-separation distances have typically relied on models which assume aircraft generate laminar vortexes. In reality though, the vortices created by many aeroplanes, especially the large and heavy ones, are highly turbulent. With such vortexes decaying faster than their laminar counterparts, past models may have overestimated wake dangers, resulting in unnecessarily long delays on the airport tarmac.

To address this, the researchers have developed a new airflow model, recently published in the Journal of Aircraft, with a simple algebraic formula that can account for both laminar and varyingly turbulent vortexes. The team found that this new model performs better than the traditional formulae in approximating the wake of both small and large-scale vortexes generated in the laboratory. The model could allow separation distances to be precisely tailored to particular aircraft designs – with the potential to shave down waiting times while still ensuring aircraft safety. It could also be used to check that correct separation distances are being applied to the increasingly large and heavy airliners that are operating today – the powerful vortexes in the wake of which, Vatistas notes, “must resemble mini-tornadoes”.

The researchers’ work could also improve our understanding of the turbulence created by helicopter blades, inside turbomachinery and also within atmospheric vortexes such as tornadoes and tropical cyclones. Vatistas’ group is now looking into the half-lives of the vortexes generated by variety of aircraft in real-life.