Drone Research Shows Why Albatross Wings Are Black On Top | @GrrlScientist
A team of aerospace engineers looking to improve marine drone design accidentally discovered why large soaring seabirds, like albatross, have wings that are black on top and white underneath
Nature is humanity’s most reliable teacher. For example, I recently shared a video report about how imitating the structure of a bumble bee wing could make drone wings less likely to become damaged by a collision (more here). And now, I stumbled across another study that explores how albatross can help us make drones more efficient fliers: this serendipitous discovery was published by a team of aerospace engineers at New Mexico State University.
“Albatross, shearwater, black skimmer, and sooty terns […] can fly thousands of kilometers without stopping,” said drone bioengineer, Abdessattar Abdelkefi, an assistant professor at New Mexico State University, and senior author of the study. Professor Abdelkefi and his students study nature, seeking new ideas for how to design and construct more efficient and durable marine drones.
Naturally, they looked to birds for inspiration.
Albatrosses are a group of very large seabirds that can soar as far as 16,000 kilometers (10,000 miles) in a single journey (ref). Albatross wings are amongst the longest for any living group of birds, with the wandering albatross’s wings reaching 3.5 meters (11.5 feet) — the longest wingspan of any living bird species.
To fly such long distances, albatross rely upon soaring and gliding flight. These modes of flight are unpowered — instead of flapping to stay aloft, the birds merely extend their wings away from their body so they can ride the wind. Similar to aircraft, soaring birds rely upon both speed and lift: speed is caused by gravity as the birds fall towards Earth, and lift is produced by the difference in air pressure flowing over the wing’s upper and lower surfaces. Together, speed and lift keep soaring birds airborne with a minimal expenditure of energy.
Because albatross operate so close to the structural limits for wing length and wing loading, Professor Abdelkefi and his collaborators thought these birds might have some interesting lessons to teach them about making flight more energetically efficient. Whilst designing their studies, the team couldn’t help but notice that the black-over-white color pattern of albatross wings was repeated across a variety of other seabirds — many of which are known to soar for long distances.
“They had a common feature: their wings are black on the upper side and white on the lower side,” Professor Abdelkefi explained in email. The researchers reasoned that this color scheme is probably important to a soaring lifestyle because so many unrelated species of birds that soar for long distances show it — but why is this particular color pattern so special?
Originally, ornithologists thought this color pattern acted as camouflage; reducing the chances that the birds’ prey might spot them as they soar over the surface of the ocean, and reducing the chances that potential predators might spot these birds on their nests. But Professor Abdelkefi and his team suspected another explanation could be at work, too, so they investigated.
“The question was: does this coloration have any specific effect on the flight endurance? Therefore, we started investigating the color effects on the flight aerodynamics,” Professor Abdelkefi explained in email.
In their study, Professor Abdelkefi and his team examined how sunlight and wing surface color influence each other. They found that the dark upper wing surface absorbs sunlight very efficiently — so efficiently, in fact, that the dark upper wing surface could become as much as 10° Celsius (18° F) warmer than the pale underside. This temperature difference lowers air pressure on the upper side of the wing, which reduces drag and generates additional lift, according to the researchers. This serves to make soaring less energetically expensive for these large seabirds, particularly over long distances.
“The obtained results showed that birds with darker colors are more efficient (constant lift to drag ratio and drag reduction) and have better endurance,” Professor Abdelkefi said.
“Simulations on unmanned air vehicles show that these dark colors can decrease their drag,” Professor Abdelkefi said. “These bio-inspirations are more suitable for drones [than aircraft], since they have closer sizes to the migrating birds.”
Have Professor Abdelkefi and his team precisely measured the effects of sunlight on other wing color patterns?
“We are trying to compare the skin drag generated using white-white, white-black, black-white, and black-black in order to identify the best arrangement of colors from aerodynamic and thermal points of view,” Professor Abdelkefi said, adding that this particular study is already in progress.
“Preliminary results show that, overall, black on the top yields the least skin friction drag. We are still investigating whether black-black or black-white is better, trying to investigate all the intervening parameters,” Professor Abdelkefi said.
There still are some questions that remain unanswered. For example; skimmers and terns are flapping — not soaring — seabirds, and skimmers do not fly long distances in one go, yet both have the black-over-white wing color pattern. On the other hand, frigatebirds, which are the “ultimate soarers” according to an ornithologist with whom I discussed this study, lack white underwings. Why?
Professor Abdelkefi and his students think their findings will help them design more efficient and durable drones for use at sea.
“Drag reduction means less fuel consumption,” Professor Abdelkefi explained. “That may also mean that the battery can persist longer, which allows longer non-stop missions.” Almost like an albatross soaring halfway around the world in one go.
M. Hassanalian, H. Abdelmoula, S. Ben Ayed, and A. Abdelkefi (2017). Thermal impact of migrating birds’ wing color on their flight performance: possibility of new generation of biologically inspired drones, Journal of Thermal Biology, published online on 10 October 2017 ahead of print | doi:10.1016/j.jtherbio.2017.03.013
Daniel T. Ksepka (2014). Flight performance of the largest volant bird, Proceedings of the National Academy of Sciences 111(29):10624–10629 | doi:10.1073/pnas.1320297111
Enjoy my writing? Please give me a few handclaps to recommend this piece. Follow me on Medium for more like this.
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
Originally published at Forbes on 16 October 2017.