A team of scientists led by the mechanical engineer David Lentink has found the answer to a question which has intrigued aerodynamics researchers for years: what makes bird wings - particularly hummingbird wings - so much more efficient than those found on man-made aircrafts such as airplanes and helicopters?
Lentink, an assistant professor of mechanical engineering at Stanford University, and his team found that the key lies in the hummingbird wings' stubby shape. They believe that the results of the study, published in the Journal of the Royal Society Interface, could significantly advance aerodynamics research and cause future aircraft and drone designs to be increasingly inspired by biological models.
After taking more than a million fine samples of aerodynamic force and airflow, researchers found that the hummingbird's wing aspect ratios - the ratio of the wing's length to its breadth, or chord - are remarkably low. While high aspect ratios - which result in the long and slender wings or blades found on airplanes and helicopters - perform well when the wings are parallel to the ground, airflow over the wing becomes unstable at more aggressive angles. Hummingbirds wings, however, perform exceptionally well in these circumstances. At higher angles, hummingbird wings required 20 percent less energy to hover in comparison to helicopter blades.
By mounting wings from museum specimens of Anna's hummingbirds to a spinning device, Lentink's team found that air flowed around the wing in a pattern that mimicked hovering. They found that the leading edge of the wing created a stable tornado-like vortex in the air.
With the knowledge that low aspect ratios favor this ability to hover, the team studied a hummingbird wings with different aspect ratios, from low to intermediate to large. They found that the ideal aspect ratio for vortex stability is 4 chord lengths from the base of the wing; beyond this, the wing stalls. This explains why most birds, insects and bats have aspect rations that are between 3 and 4 chord lengths.
Besides answering an age-old aerodynamics question, the study also sheds some light on how quadcopters can be designed so that they do not stall in turbulent air or when a gust of air suddenly increases the angle of attack of the rotor blade.
"It can also explain the big divide between engineering and biological wing design. If you operate at low angle of attack, you want to use a super-efficient helicopter blade, but if you need to avoid stall at high angles, you better select a stubby hummingbird wing," said Lentink.
