Bird flight aerodynamics researchers often base their flight models on man-made flying machines. This may work for soaring and gliding birds, but as Jim Usherwood points out, `slow-flapping birds really don't resemble planes'. So Usherwood and his colleagues came up with an innovative approach involving pressure sensors to calculate the aerodynamic power of slow-flapping pigeons(p. 355).
`To calculate the aerodynamic power output of a bird's flight muscles you need to know the forces acting on a bird's wing and the speed of the wing'says Usherwood. He explains that there are two non-independent types of force acting when you see a bird flying - those that keep the animal in the air, and those that require muscular work to keep the animal flying. These forces result from pressure differences. `If you know the pressure difference between the top surface and the bottom surface of a bird's wing, you can calculate the force acting on the wing,' says Usherwood. To calculate the forces acting during slow-flapping flight, he needed to measure the pressure at different points on pigeons' wings during flight.
Teaming up with Tyson Hedrick, Craig McGowan and Andrew Biewener, Usherwood began adapting pressure sensors so that he could attach them to the birds. The challenge was modifying pressure sensors to stay in place despite the large accelerations of the flapping wings. The team eventually succeeded in pinning pressure sensors through eight feathers on the birds' wings. With the sensors firmly in place, the team used high-speed digital video to record the pigeons'flapping flight as they flew between two perches. The team calculated the birds' aerodynamic power output by combining measurements of the birds' wing speed from the digital videos with simultaneous pressure sensor measurements.
The team's novel approach provides a valuable test of the accuracy of direct wing muscle measurements, which until now provided the only method of calculating power output directly. But the team's calculations of the power output of slow-flapping flight are higher than those obtained from direct wing muscle measurements in previous studies. Usherwood emphasizes that each component of his team's power output calculation is reasonable, and slow flight may simply be more energetically expensive and aerodynamically inefficient than previously thought, resulting in higher power outputs than had previously been predicted.
Usherwood is enthusiastic about this new approach. `Now that we know that the pressure sensors work, we want to take the set-up outside,' he says;`eventually, we'd like to be able to calculate power output while birds are behaving naturally, such as during migration or foraging.'