High-flying birds need more wingbeats to stay aloft

Birds have evolved for optimal flight, so understanding how they adjust their flight patterns can help improve our own aerodynamics in many different fields.

Beaumont et al. used a computer simulation to test if migratory birds flap their wings more quickly at higher altitudes, a common hypothesis for bird flight. Their research examined the dynamics of a flapping wing and determined that, indeed, flapping frequency is positively correlated with altitude.

“Our study demonstrated the magnitude of the forces exerted on a bird in flight decreased as the altitude increased,” said author Fabien Beaumont. “Our simulations suggest that increasing the frequency of wing flapping would allow the bird to maximize lift while minimizing drag.”

The researchers modeled their wing after a Canada goose and created a simplified flapping mechanism. At an altitude of 100 meters, their simulated wing behaviors agreed with observations from actual geese. After confirming the model’s accuracy, they tested it in different air densities mimicking those found at different altitudes.

At high altitudes, both lift and drag forces decreased dramatically. Less drag means less strain on the bird’s body, but less lift means the bird must work harder to keep itself aloft. The scientists tested the effects of flapping frequency, amplitude, and transverse speed and found increasing frequency provided birds with the greatest benefit.

The team plans to explore the effects of flying in formation and apply these results to problems in other fields, such as sports.

“For example, and by analogy with migratory birds, cyclists ride in a pack forming an echelon in crosswind conditions.” said Beaumont.

Source: “Aerodynamics of a flapping wing as a function of altitude: New insights into the flight strategy of migratory birds,” by Fabien Beaumont, Sebastien Murer, Fabien Bogard, and Guillaume Polidori, Physics of Fluids (2021). The article can be accessed at https://doi.org/10.1063/5.0071910 .