If you’ve ever watched a hummingbird sipping away at nectar, you’ll have noticed those incredible wings that beat so fast, up to 80 flaps per second for some species, that the bird hovers seemingly motionless in the air. In fact, hummingbirds are actually the only birds that can sustain hovering; a unique feature that comes at a high energetic cost.
But what determines the efficiency of this behavior, and how does that compare with man-made flying machines? These are questions Stanford and Wageningen University scientists were keen to answer, who recently published their findings in the Journal of the Royal Society Interface.
Using helicopter and airplane design theory, the researchers predicted that the hummingbird wing aspect ratio (ration of the length of wings to their width) determines aerodynamic efficiency. However, the flight performance of a hummingbird is particularly difficult to measure given the fact that the forces generated are so small.
To overcome this, the researchers used detached wings from museum specimens and placed them in an apparatus called a wing spinner in order to measure drag. The team then combined this data with recordings taken from wild hummingbirds which allowed them to determine how much power the hummingbird muscles required to sustain hover. Next, they compared performance measurements with that of the man-made blades of a micro helicopter called a Black Hornet in order to determine which was more efficient.
The researchers report several interesting findings. They discovered that, true to their original hypothesis, the power required to sustain hover is highly dependent on wing aspect ratio. During the down stroke, wings that had larger aspect ratios required much less power than those with smaller ratios.
“This shows that if we design the wings well, we can build drones that hover as efficiently, if not more efficiently, as hummingbirds,” lead researcher David Lentink told BBC News. “Clearly we are not even close to hummingbirds in many other design metrics, such as wind gust tolerance, visual flight control through clutter, to name a few. But if we focus on aerodynamic efficiency, we are closer than we perhaps ever imagined possible.”
[Header image "male Broad-tailed Hummingbird at feeder," by Mark Watson, via Flickr, used in accordance with CC BY-NC-ND 2.0]