China ducks are out of fashion. Some decades ago it was the epitome of good taste to have a set of these ornaments arranged diagonally across the living room wall above the sofa or the sideboard.
And why diagonally? Because this, as everybody knows, is how ducks fly, spread out in V-shaped formation as they migrate northwards or southwards, depending on the season, to find themselves a more attractive temporary home. There are good reasons why ducks, geese and other large birds arrange themselves across the sky like this, rather than flying side by side or one behind the other in a line.
The formation saves energy. Each flapping wing creates a vortex of air, a whirlpool with a horizontal axis which spreads out rear-wards from the wing-tip. A bird suitably positioned with respect to this vortex has the advantage of an upwash of air that allows it to orientate itself so there is less drag, and therefore less power required to maintain a given speed.
A study on pelicans, for example, in a recent edition of the scientific journal Nature, examined the behaviour of these birds flying both solo and in combination. The birds were fitted with tiny instruments to monitor their heart rate, and the results showed that when flying alone, the pelicans flapped their wings more frequently than when flying in formation. When flying in formation their heartbeat was on average 10 per cent lower than when flying alone at similar speed, a clear indication of less energy involved.
This study was reported in Nature in October. Then, just a week later, Flight magazine coincidentally reported the results of another study intended to explore if this trick could be put to use in aviation. The idea was that if the same phenomenon applied to aeroplanes flying in formation, then perhaps V-shaped convoys of cargo aircraft over large distances could save substantially on fuel.
The results showed that in principle, the theory works. The idea is feasible nowadays because satellite-based navigational systems, linked to the controls of participating aircraft, can position them very close to one another with great accuracy. Test flights showed that an aircraft availing of the upwash from the wing-tip vortex of a leader could be "re-trimmed" to fly with its nose pointing slightly downwards, thus reducing the amount of drag. The best results are achieved with the trailing aircraft about one aircraft's length behind its leader, and with the wing-tips overlapping by about two metres.
"It is like flying downhill," said one researcher. And the fuel savings achievable, it seems, are of the order of 15 per cent.