Birds often flap their wings as they approach a landing, even though powered flight uses more energy than gliding. Research from the University of Manchester suggests this behavior may help them slow down safely within the limited space available in real landing environments.
The study, published in Proceedings of the Royal Society A Mathematical Physical and Engineering Sciences, used mathematical modeling to examine how a bird could land without flapping. In theory, an unpowered bird could reach zero speed as its feet touched down. However, achieving that result would require a much faster and longer approach than birds typically use.
For a pigeon-sized bird, the model indicated that the maneuver would begin at more than twice its usual cruising speed, cover about 60 meters horizontally and involve climbing roughly 20 meters. The findings suggest that the lowest possible touchdown speed is not always the most practical objective. Flapping may allow birds to reduce speed while retaining control in a confined area.
The research also offers an explanation for the swooping approaches used by some birds when landing on perches. Earlier work on Harris's hawks found that they could swoop below the perch before gliding upward toward it, a maneuver thought to reduce the distance traveled after their wings stalled and aerodynamic control declined.
The new model suggests that these trajectories reflect a balance between arriving at the perch slowly and minimizing the time spent stalled. According to the researchers, touchdown speed may have a stronger influence on swooping behavior than previously recognized.
Landing on the ground presents a different physical problem. A bird cannot continue below the landing surface or rise through its destination, so an unpowered approach cannot remove all speed before touchdown. The remaining energy must be absorbed by the wings, legs or both.
That requirement may place opposing pressures on bird evolution. Stronger muscles and bones, particularly in the legs, can help an animal withstand landing, but additional mass can reduce flight performance. The researchers suggest that the balance between being light enough to fly efficiently and robust enough to land safely may have influenced the evolution of birds.
The same physical principles could also help explain how other flying animals perform landings without powered flight. The study was led by Dr. Ben Parslew, a lecturer in aerospace engineering at the University of Manchester, and is titled “Flight dynamics and optimal control of unpowered avian landings.”
Sources: Phys.org.