Flags flap in the wind. What if wings also did? A recent paper from Oscan Curet, Sharon Swartz and Kenneth Breuer at Brown University shows that an aeroelastic instability, similar to flag flapping, could have provided an impetus for some gliding animals to shift to actively flapping their wings during flight.

Scientists have long debated how animals first evolved the strong wing muscles and shoulder joints necessary for flapping flight. Gliding might seem to be a natural first step on the way to fully powered flight, but gliders typically don't have the types of shoulder girdle or muscles required for flapping motions. So how could flapping evolve from gliders? Scientists believed that strengthening the wing muscles wouldn't be a sufficient starting point for evolution of active flight, as slightly stronger wing muscles still wouldn't be strong enough to power lift-off. But what if the wing muscles weren't for powering take-off, but rather were for controlling passive flapping?

Curet and his colleagues decided to examine how passive flapping motions – generated without muscle, solely due to interactions between the flexible wing and the air around it – might arise in a wing-like model, and what their effects might be on lift and drag forces. They built a simplified physical model of a wing, with flexible joints: a ‘shoulder’ joint that let the wing flap up and down, and a flexible flap on the trailing edge of the wing that allowed it to curve towards its trailing edge. Then they attached the wing to a load sensor that could measure forces, and put the whole setup in a wind tunnel.

At relatively low wind speeds, the wing behaved much like a classical airplane wing and remained stationary. Furthermore, like a normal wing, as they tilted and increased the angle of the leading edge, it generated more and more lift. As they increased the wind speed, though, all of a sudden the wing would start flapping. It began to oscillate up and down, though at a relatively low frequency, lower than birds or bats use when flying. Airplane engineers are quite familiar with this ‘flutter’ instability. However, if an airplane wing starts flapping, it often falls off, which is generally frowned upon and so, no one had considered the biological implications of fluttering wings.

When Curet measured the forces on the wing after it started fluttering and compared them to those obtained when the wing was stationary, he found a substantial increase in both lift and drag forces. As the wing flaps, a large vortex forms on the leading edge of the wing, enhancing lift, but also increasing drag. The transition to fluttering wings happened at wind speeds that would occur during typical flight speeds of flying animals. Only very slow gliders would not have to deal with the instability; others would have to pass through the transition as they changed their flight speed. But sudden changes in forces, which occur when fluttering starts or stops, tend to make it hard to control and stabilize flight.

If gliding animals want to avoid this instability, they face a choice: they can either attempt to suppress the transition (as airplane engineers do) by making the wing and shoulder stiffer or they can enhance it, by developing muscles that can flap the wing and maintain the fluttering instability below the speed at which the transition occurs passively. Both strategies would allow them to avoid the unpredictable changes in forces at flight speeds that induce the transition. In the latter option however, once muscles had evolved to control the passive flapping motion, the muscles might gradually increase in strength and this could then allow the animal to increase the flapping frequency, increasing lift even further, until fully powered flight became possible.

O. M.
S. M.
K. S.
An aeroelastic instability provides a possible basis for the transition from gliding to flapping flight
J. R. Soc. Interface