On the wing: Birds, skeeters, jet planes: Same design rule applies!

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On the wing: Birds, skeeters, jet planes: Same design rule applies!
Assembly line of Boeing 787 jumbo jets in the final stage of production
The Boeing 787 under construction at the factory in Everett, Wash. Does aeronautical engineering respond to the same basic physics that governs bird evolution?
Photo: Boeing

Evolution through natural selection governs the “design” of flying creatures. Engineers design flying machines. But flying is about physics, and physics is the ultimate arbiter of both processes, says Adrian Bejan, a professor of mechanical engineering at Duke University.

And that produces parallels in mechanical and animal evolution.

In biology, “fitness” is the gauge of natural selection, says Bejan, “But biology never explains what it mean to be less fit or more fit. Physics is the discipline that states unambiguously what it mean to be more fit; it means to provide greater access to the flow,” in this case, to moving air.

“We can take that physics statement and translate it into other languages,” says Bejan. “In biological language, it means to survive. In engineering, they talk about being more efficient or more economical. In economics, they say being less expensive or more good for you.”

Take airplanes, as Bejan did in a study1 published this week. “The evolution of the airplane is an important story because it … is visible and well known to many people.”

When Bejan and colleagues looked at the speed and range of commercial jetliners, they found that as mass increases, so does velocity. That relationship, he says, mimics what’s occurred among living “flying machines.”

Mass, size and velocity are positively correlated during the history of jetliners. Rollover image to see a similar phenomenon among moving organisms — clams, fish, lizards and people.
Both courtesy Bejan et al., Journal of Applied Physics

Bejan says the “constructal law” that he promulgated in 1996 describes continual change in phenomenon as diverse as lightning, river basins and technology, such as airplanes. “We showed that larger airplanes are faster, more efficient as vehicles, and have greater range,” Bejan said.

We wondered: engineers design planes, but natural selection determines winners and losers in the living realm. Bejan said both process are responding to similar pressures. “Selection mean choice, meaning choice between different configurations,” Bejan responded. “Over time, certain architectures, certain configurations flow more easily.”

Flow happens when creatures or objects move through their environment, he explains. “Flow is relative motion, between something that moves and something that … is not moving. What is not recognized much in physics is that for this movement to happen, the mover, the water, or you or I, has to get the environment out of the way.”

And that produces a set of universal requirements regarding shape, proportion, density, and heat transfer. “Movement in physics means having an impact on the environment. River water changes the landscape, everything that moves reshapes the world, that is basically what life is.”

Photo of Colorado River in a deeply cut canyon with sharp, vertical cliffs.
Late afternoon view of the Colorado River in Marble Canyon near Lees Ferry, Ariz. The formation of rivers, Bejan maintains, is another example of how “flow” governs the configuration of the animate and inanimate realms.
Photo: USGS photo by Stewart Tomlinson

The sum of its parts

The history of airplanes shows change among the components as well as overall size, and here too, Bejan sees parallels with the animal realm. “Engine mass is proportional to overall mass, a design feature that is the same in animals where the mass of the motive organs — muscle, heart and lungs — is proportional to body mass,” Bejan says.

In the evolutionary design of a jetliner, he adds, “every component is getting better, but it’s getting better for the purpose of moving” the whole plane. “The big idea is that the better engine is the one that makes the better airplane.”

With constructal law, Bejan says, “You can predict evolution for configurations, designs that are free to change, and identify the direction in which these change are bound to be occurring.”

A few quibbles

As we looked over the matter, we did notice some facts that failed to jibe with Bejan’s theory:

* Darwinian evolution does have a precise definition of “fitness”: the ability to raise more offspring to reproductive age

* Some pterosaurs that went extinct 66 million years ago had a wingspan exceeding 10 meters

* While jetliners have evolved to larger size, big birds, like the whooping crane and the California condor, are among the most endangered

Birds-eye view of a condor mid-flight highlighting the flying creature’s impressive wingspan
Once near the brink of extinction, the Californian condor has benefited from a multi-decade species restoration.

Still, we did concede the general point: that structures created through biological and technological evolution can face similar constraints, and therefore their change through time does have similarities. “This law is about what moves on the planet and how this movement reshapes the earth’s crust every day,” says Bejan. “We are the latest and biggest movers of stuff on the planet. We do it in a huge number of ways, agriculture, transport, most recently we do it big time with air traffic.”

As jetliners get better and cheaper, “More millions of us are flying without noticing the immense steps in our evolutionary design.”

– David J. Tenenbaum

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Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer


  1. The Evolution of Airplanes, A. Bejan, J.D. Charles and S. Lorente, Journal of Applied Physics, July 22, 2014
  2. That’s no bird: This ‘four-winged’ dinosaur looked like an airplane
  3. Why The Evolution Of Flight May Be Universal