Do airplanes evolve like birds?

The development of passenger aircraft over the past century mirrors the evolution of flying animals, and shows that evolution is not just a biological phenomenon, according to a paper published today in the Journal of Applied Physics.

Adrian Bejan, a mechanical engineer at Duke University in Durham, North Carolina, argues that evolution is a physical phenomenon, with changes in animals driven by physical laws. In the case of birds, the factors include aerodynamics.

"I want to persuade people that evolution – that is the change in [body shape] over time – recognizes no distinction between the two camps of biology and physics," he said.

The argument is based on "constructal law," a theory developed by Bejan in the 1990s that states that the shape and structure of all flow systems, living and inanimate, evolve in such a way as to provide progressively easier access to the flow of currents within them – be that air, water, blood or electricity.

In the case of airplanes and flying animals, that "flow" is the movement of people or animals around the world. And the rules of aerodynamics forced airplanes to solve the problem in pretty much exactly the same way as birds. For example, larger planes are faster, more efficient and have greater range. The mass of their engines is proportional to their body size, in much the same way that the muscles, heart and lungs that drive flight in birds scale up with body size. And, like birds, all airplanes exhibit a proportional relationship between wingspan and body length, and between fuel load and body size.

Furthermore the evolutionary path for each is the same. Over time a few small planes have "evolved" into an array of different sizes, each filling a particular niche, while the largest ones have tended to get larger.

The similarities mean that although evolution is too slow to observe in action in animals, the evolution of the "human-and-machine species" of the airplane can be witnessed in the span of a human lifetime. And it can be used to predict how both airplanes and animals will evolve in the future. This, said Bejan, means that biologists should be the chief beneficiary of his work.

"Without a physical law, and the ability to observe it, the argument for evolution is weak … with physics it is stronger," he said.

But not many would agree with him.

Robert Blake, who studies comparative physiology at the University of British Columbia in Vancouver, Canada, is not convinced by Bejan's argument that technological development offers a way to study biological evolution.

"Biological evolution is not directed, there is no goal, no conscious decision as there is in technology, where things are done for a reason," he said.

That lack of planning in biology is what sets the two worlds apart. No engineer, after all, would try to make an airplane shaped like a cube. But the fantastically un-aerodynamic bumblebee did evolve to get off the ground. Bejan's idea of technological evolution is more like artificial selection, said Blake, through which people have produced farm animals and ornamental goldfish – goal-oriented and purposeful.

There are problems with some of the comparisons between birds and planes as well, he adds. The aerodynamic models Bejan used are for steady flows and wings that don't flap. That's fine for big birds, like an albatross.

"They flap infrequently enough that they're like a plane," said Blake. "But when you compare it to a hummingbird, it all breaks down. You get ludicrous results."

Axel Kleidon, a geophysicist at the Max Planck Institute in Jena, Germany, said he thinks the interesting ideas in Bejan's work lose something by being linked to his constructal law. Kleidon has always been a bit leery of constructal law because unlike other physical laws, it is not a mathematical statement you can quantify.

"It's quite vague," he said. "If you want to apply constructal law to a new area, you don't know what to do."

But both Kleidon and Blake say the work contains fascinating ideas, and offers an interesting new way to think about technological development.

"These kinds of ideas are good to share, even though I don't agree with them," said Blake. "They get people thinking."

Brian Owens is a freelance science journalist in St. Stephen, New Brunswick, Canada.

Originally posted on Inside Science.