Humans have always been fascinated by flight. As Ty Hedrick says `over the last century we've become pretty good at flight on a large scale', so now we defy gravity with propellers and jets. But understanding the forces that keep our feathered friends aloft; that's a different matter. Hedrick explains that until recently, only one study had measured the forces acting on a bird in flight directly,when a team in Germany fitted accelerometers to a pigeon. But that was 20 years ago, and the technology only allowed them to analyse one wing beat. Since then, no one had succeeded in directly measuring the net forces acting on a flying bird, until Hedrick and his colleagues, Jim Usherwood and Andy Biewener, fitted accelerometers to cockatiels. By convincing the birds to fly steadily in a wind tunnel while the team videoed the bird's flight, Hedrick and his colleagues have collected the first instantaneous measurements of the net forces that keep birds aloft(p. 1689).
For Hedrick, the birds were the least of his problems; cockatiels are enthusiastic fliers. The difficulties were in synchronising the video and accelerometer data. Hedrick explains that he needed to know the position of the bird's body at each instant of the wing beat, so that he could correct the accelerations measured on the body as the bird twisted, tilted and moved,ultimately measuring the true aerodynamic forces. Hedrick remembers that the first acclerometer trace `was full of crazy peaks', and nothing like the smooth traces he was expecting. But after laboriously correcting the acceleration data according to the bird's orientation and movements, all the peaks vanished leaving Hedrick with a realistic record of the accelerations and forces the bird experienced. `Now we knew we could do this, and it was going to work' says Hedrick.
After putting the birds through their paces at speeds ranging from 1 m s–1 to 13 m s–1, Hedrick noticed that the cockatiel's flight pattern was smoothest around their cruising speed of 7 m s–1. At low and high speeds, the lift produced by each upstroke was a fraction of the downstroke's force, so the birds were bounced along by each wing beat. But at intermediate speeds, the down and upstrokes were better matched, giving the birds a smoother ride.
Hedrick adds that although it was widely accepted that the downstroke generates a significant amount of lift during a wing beat cycle, there was some debate about the upstroke's contribution to aerodynamic forces. Was the upstroke generating lift, or simply a way of returning the wing back to its starting point? The Harvard team's results suggest that the upstroke's main contribution to flight is simply to return the wing to the top of the cycle,while minimising the effects of drag.
Knowing that the upstroke appears to contribute little to the aerodynamic forces of bird flight, the team wondered whether the upstroke comes with an energetic penalty; after all it takes energy to lift the wing's weight. The accelerometer data showed Hedrick and his colleagues that although the birds used all of the kinetic energy of the downstroke producing aerodynamic force,the same was not true for the upstroke, the energy was not recovered from the wing for later use. In fact the upstroke cost the cockatiels a significant 14%of their energy in flight.