Technology : Safe landings on a wing and some air

Technology 14 December 1996

BY BLOWING bursts of air through slots on aircrafts’ wings, Israeli
researchers say they can make planes fly more slowly without stalling. As well
as making landing safer, this might also allow aircraft to land on shorter
runways, say Israel Wygnanski and his colleagues at Tel Aviv University.

The lift that keeps a plane airborne comes from air moving swiftly over the
wings. Air flows faster over the top of the curved wings than under the bottom,
a phenomenon that sets up a difference in pressure and generates lift.

Friction works against this process, dragging back and slowing down the
“boundary” layer of air closest to the surface of the wing. During takeoff, the
effect of friction is relatively insignificant. As an aircraft comes in to land,
however, it is typically travelling at less than half the takeoff speed, and the
loss of momentum caused by friction in the boundary layer is more significant
because the air is already moving more slowly over the wings. This, says
Wygnanski, can result in the boundary layer losing so much momentum that it
literally slips off the back of the wings, causing a potentially fatal loss of

Boosting the momentum of this surface layer is the key to making it stick to
the wings during landing, claim the researchers. The system they have devised
consists of a pump that fits inside the wing and blows out pulses of air through
a thin slot along the wing’s leading edge. This creates small pockets of
turbulent air or vortexes on the surface. The vortexes mix the slower air at the
boundary with the layer of fast air directly above it. This, says Wygnanski,
transfers momentum from the fast stream to the surface layer and has been found
to delay and even prevent flow separation.


A prototype wing has been tested in a wind tunnel at the Illinois Institute
of Technology. The pressure distribution around the wing was found to improve
lift at speeds approaching half the speed of sound, which is at the slower end
of typical aircraft speeds.

Engineers have used various methods to control the boundary layer before. For
example, a system used with the Lockheed F-104 Starfighter jet blew a continuous
stream of air over the wings. This system proved too power-hungry and expensive
for commercial airliners, however.

Wygnanski says his system is much more energy efficient. The oscillating
pulses mean that only a small amount of air needs to be blown through the
system—equivalent to around 10 per cent of the extra momentum a steady
blower would need to supply to achieve the same mixing effect.

Richard Wlezien at NASA Langley Research Center’s flow modelling and control
department in Virginia says the technology is very promising, but needs to be
validated in conditions which simulate flight more closely before it can be used
on commercial aircraft. “The Illinois tests were at the lower end of real flight
conditions,” he says. NASA plans to carry out trials of the technology next

An improved type of aircraft wing