Tiny helicopters can’t out-hover hummingbirds

"A helicopter is really the most efficient hovering device that we can build," says David Lentink. "The best hummingbirds are still better, but I think it's amazing that we're getting closer." Above, the Black Hornet nano unmanned air vehicle. (Credit: Richard Wyatt/UK Ministry of Defence)

The spinning blades of micro-helicopters are about as efficient at hovering as the average hummingbird, which have had more than 42 million years of natural selection to hone their energetically efficient flight.

That said, hummingbird wings can still generate lift more efficiently than the best micro-helicopter blades, according to a new analysis led by David Lentink, an assistant professor of mechanical engineering at Stanford University. The findings could lead to more powerful, bird-inspired robotic vehicles.

male Anna's hummingbird
“I think it’s nice that there are still a few things about hummingbirds that we don’t know,” says David Lentink. (Credit: John Florence/Flickr)

Black Hornet vs. hummingbird wings

The experiment involved spinning hummingbird wings—sourced from a pre-existing museum collection—of 12 different species on an apparatus designed to test the aerodynamics of helicopter blades.

The researchers used cameras to visualize airflow around the wings, and sensitive load cells to measure the drag and the lift force they exerted at different speeds and angles.

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Lentink and his colleagues then replicated the experiment using the blades from a ProxDynamics Black Hornet autonomous microhelicopter. The Black Hornet is the most sophisticated microcopter available—the UK army uses it in Afghanistan—and it’s about the size of a hummingbird.

Even spinning like a helicopter, rather than flapping, the hummingbird wings excelled: If hummingbirds were able to spin their wings to hover, it would cost them roughly half as much energy as flapping.

The microcopter’s wings kept pace with the middle-of-the-pack hummingbird wings, but the topflight wings—those of Anna’s hummingbird, a species common throughout the West Coast—were still about 27 percent more efficient than engineered blades.

Hummingbirds still win

Hummingbirds acing the test didn’t particularly surprise Lentink—previous studies had indicated hummingbirds were incredibly efficient—but he was impressed with the helicopter.

“The technology is at the level of an ‘average Joe’ hummingbird,” Lentink says. “A helicopter is really the most efficient hovering device that we can build. The best hummingbirds are still better, but I think it’s amazing that we’re getting closer. It’s not easy to match their performance, but if we build better wings with better shapes, we might approximate hummingbirds.”

Based on the measurements of Anna’s hummingbirds, Lentink says there’s potential to improve microcopter rotor power by up to 27 percent.

The study appears in the Journal of the Royal Society: Interface.

Serious muscle

The high-fidelity experiment also provided an opportunity to refine previous rough estimates of muscle power. Lentink’s team learned that hummingbirds’ muscles produce a surprising 130 watts of energy per kilogram; the average for other birds, and across most vertebrates, is roughly 100 watts/kg.

Although the current study revealed several details of how a hummingbird hovers in one place, the birds still hold many secrets. For instance, Lentink says, we don’t know how hummingbirds maintain their flight in a strong gust, how they navigate through branches and other clutter, or how they change direction so quickly during aerial “dogfights.”

He also thinks great strides are possible from studying wing aspect ratios, the ratio of wing length to wing width. The aspect ratios of all the hummingbirds’ wings remarkably converged around 3.9. The aspect ratios of most wings used in aviation measure much higher—the Black Hornet’s aspect ratio was 4.7.

“I want to understand if aspect ratio is special, and whether the amount of variation has an effect on performance,” Lentink says. Understanding and replicating these abilities and characteristics could be a boon for robotics and will be the focus of future experiments.

“Those are the things we don’t know right now, and they could be incredibly useful. But I don’t mind it, actually,” Lentink says. “I think it’s nice that there are still a few things about hummingbirds that we don’t know.”

Source: Stanford