How does a hummingbird hover so perfectly? That’s the question a group of Stanford engineers set out to answer. The researchers say their findings could offer significant insight to aerodynamic research and the ever-growing drone industry.
Led by Stanford’s David Lentink, a mechanical engineer, the research team investigated wing aspect ratio (the ratio of the wing’s length to its cord). Flying creatures such as the hummingbird use stubby wings with relatively low aspect ratios. They tend to be quite different from wings utilized by helicopters and airplanes, which generally boast high aspect ratios.
Birds vs. aircraft
The latter is best suited for low angles of attack (when the wings are parallel with the ground). Wings with high aspect ratios also come in handy when an aircraft rotates or glides. However, long and slender wings are not ideal for aggressive angles. That’s why, the researchers say, birds can offer tremendous insight and improve our man-made vehicles.
"Scientists have long observed how these animals' wings allow them to hover or fly very slowly, and even avoid stalling when beating at a steep angle of attack, but how they manage to do so has remained a complicated mystery," Lentink said in a statement.
Studying Anna's hummingbirds
The researchers summoned the help of an unidentified museum and mounted wings from its specimens of Anna’s hummingbirds (a type that’s native to the West Coast of North America) onto a spinning device. Lentink and his team then analyzed the way air flowed around the wing as it beat during a hovering-like movement. They discovered the wing’s leading edge created a vortex in the air that resembled a tornado.
The team proceeded to analyze other model hummingbird wings of various kinds to understand the importance of stubby wings in this phenomenon. They discovered intermediate hummingbird wings used significantly less power (approximately 20 percent) than helicopters and airplanes to hover at higher angles. The engineers then looked into a number of other birds and flying insects, realizing that flies have the least effective wings for hovering at higher angles.
Ultimately, the ideal aspect ratio for vortex stability is four chord lengths from the wing’s base. The wing stalls beyond that point, which might explain why most insects and birds generally have aspect ratios between three and four.
Evolutionary convergence
"More research with flapping wings is needed, but this finding points toward a remarkable example of evolutionary convergence," Lentink said. "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."
The study was recently published in the Journal of the Royal Society Interface. Lentink said his research will be useful for designing quadcopters that are capable of hovering during turbulence.