Flight details

HERE’S a brainteaser: a driver with a cargo of birds approaches a bridge with a limit of 5,000kg. His lorry weighs 4,800kg and his load 400kg. The driver then has an idea: he strikes the side of the lorry, frightening the birds into flight, and quickly drives across while they are airborne. Does the ploy work?

This question is the stuff of late-night bar discussions among physics students. The answer is that physics is not fooled. A bird’s flapping produces a downdraft that pushes on the lorry’s floor with a force, on average, the same as the bird’s weight.

That simple explanation, though, did not satisfy David Lentink, an engineer at Stanford University. Dr Lentink applies insights from flying animals to the design of drones. Yet the methods used until now to unpick the particulars of powered flight are either crude (measuring the tug on a tether tied to a flying animal) or unduly complex (feeding supercomputers with data gained from dissections on the masses and densities of bird parts).

Dr Lentink’s paper, just published in the Journal of the Royal Society Interface, shows how to measure the quickly changing forces of flight without even touching an animal. This is not easy. The scales used must measure forces with great precision but also bounce back rapidly, to track fast changes. This requires them to be both light and stiff. Dr Lentink’s team therefore built a box whose floor and ceiling (each hooked to a precise sensor) were made of balsa wood. They then taught Gaga and Ray, two small parrots, to fly across it.

The musculature of many birds suggests that their wings create more lift on the downstroke than the up, but what the team measured was extreme. Gaga and Ray created lift equivalent to twice their body weight on the downstroke and virtually none on the up. Were they to flap in synchrony, then, the apparent weight of the birds in a lorry could briefly double. That finding complicates matters for physics undergrads, but the approach makes studying real birds—and drones—far easier.