Aerodynamic performance of the swift unravelled

Exquisite flight control

A swift adapts the shape of its wings to the immediate task at hand: folding them back to chase insects, or stretching them out to sleep in flight.Ten Dutch and Swedish scientists, based in Wageningen, Groningen, Delft, Leiden, and Lund, have shown how 'wing morphing' makes swifts such versatile flyers. Their study, published as cover story in Nature on April 26, proves that swifts can improve flight performance by up to three-fold, numbers that make 'wing morphing' the next big thing in aircraft engineering.

Swifts spend almost their entire life in the air. During flight, they continually change the shape of their wings from spread wide to swept back. When they fly slowly and straight on, extended wings carry swifts 1.5 times farther and keep them airborne twice as long. To fly fast, swifts need to sweep back their wings to gain a similar advantage.

Economic turns During the summer, we can observe swifts circling above town squares, where they catch up to 20.000 insects a day. Swift can triple their efficiency by turning with their wings stretched out.  When chasing rivals and flying insects, swifts also want to make their turns fast and tight.  However, in fast and diving turns, the load on the wings easily reaches more than four times the swift's body weight. So in extreme turns, swifts need to sweep back their wings or else risk breaking them.

Swifts typically sweep their wings back during fast glides and extend them while turning. More photos: www.ezo.wur.nl/UK/Press/Swift+photos/  © Ran Schols - www.pbase.com/ranschols/swift

Night's rest
Swifts do not land to roost, but spend the night at 1.5 km above the ground. To measure their flight speed, Swedish scientists used radar. They found that swifts let the air blow past their wings at 8 to 10 m/s (29-36 km/h). At these air speeds, swift wings deliver maximum flight efficiency.  For the swift that means more gliding and less flapping to maintain altitude.

The scientists figured all this out when they measured just how much lift and drag a swift wing generates. The wings were tested to their limit in a windtunnel at speeds of up to 108 km/h (30 metres per second).Scientists compared extended and swept wings, and learnt that flying slowly with extended wings gives swifts maximum flight efficiency. But swept wings deliver a better aerodynamic performance for flying fast and straight. Swept wings are also better for fast and tight turns; but this time swept wings are better because they do not break as easily as extended wings.

Airplanes
Morphing wings are the latest trend in aviation. The best wing shape to save fuel costs depends on flight speed. In 2003, birds inspired NASA to design a revolutionary "morphing wing" aircraft.  Also so-called micro-aircraft, which are the size of a bird, begin to exploit the benefits of varying wing shape. These tiny flyers, equipped with cameras and sensors to assist in surveillance and espionage, imitate faithfully the flight behaviour and appearance of birds.  In an ongoing project, students at Delft University cooperate with scientist at Wageningen, to make such a small airplane fly like a swift.

The swifts for this study had been brought in dead or dying to seven Dutch bird sanctuaries. Swifts, when forced to land on the ground, cannot take off by themselves and will starve unless a kind and timely passer-by throws them in the air.
Swifts are the most aerial of birds. They migrate annually from South Africa to Europe. Over their lifetime, swifts cover 4.5 million kilometres, a distance equal to six round trips to the moon or 100 times around the Earth.

Observations of swifts in the Netherlands (Natuurkalender)