Researchers believe they now know why the supersonic trans-Atlantic Concorde aircraft went the way of the dodo（古代巨鸟） -- it hit an evolutionary¹ cul-de-sac. In a new study, Adrian Bejan, professor of mechanical engineering and materials science at Duke University, shows that a law of physics he penned more than two decades ago helps explain the evolution of passenger airplanes from the small, propeller-driven DC-3s of yore to today's behemoth（巨兽，河马） Boeing 787s. The analysis also provides insights into how aerospace² companies can develop successful future designs.

 

The Concorde, alas³, was too far from the curve of these good designs, Bejan says. The paper appears online July 22, in the Journal of Applied⁴ Physics.

 

"The evolution of Earth's species occurred on a timescale far too large for humans to witness," said Bejan. "But the evolution of our use of technology and airplanes to transport people and goods has taken place in little more than a single lifetime, making it visible to those who look. Evolution is a universal phenomenon encompassing⁵ technology, river basins and animal design alike, and it is rooted in physics as the constructal law."

 

The constructal law was developed by Bejan in 1996 and states that for a system to survive, it must evolve to increase its access to flow. For example, the human vascular⁶（血管的） system has evolved to provide blood access to flow through a network of a few large arteries⁷ and many small capillaries⁸（毛细血管）. River systems, tree branches and modern highway and road networks show the same forces at work, he says.

 

In the case of commercial aircraft, designs have evolved to allow more people and goods to flow across the face of Earth. Constructal law has also dictated⁹ the main design features needed for aircraft to succeed; the engine mass has remained proportional to the body size, the wing size has been tied to the fuselage（机身） length, and the fuel load has grown in step with the total weight.

 

"The same design features can be seen in any large land animal," said Bejan. "Larger animals have longer lifespans and travel farther distances, just as passenger airplanes have been designed to do. For example, the ratio of the engine to aircraft size is analogous¹⁰ to the ratio of a large animal's total body size to its heart, lungs and muscles."

 

To apply his theories to airplane design, Bejan teamed up with Jordan Charles, a researcher and development engineer, and Sylvie Lorente, a professor of civil engineering at the University of Toulouse, to mine the historical databases of successful commercial aircraft. As they plotted thousands of statistics including year of introduction, size, cruising speed, engine weight, fuel weight, range, wingspan and fuselage length, many patterns began to emerge.

 

But two in particular stood out.

 

In one chart, a clear curve tracks the increasing size of commercial airplanes through nearly a century of aviation. As time moves on, new commercial airliners¹¹ come in all sizes but the biggest are joined by even bigger models. In another chart, the line that best tracks the relationship of body mass to airplane speeds is nearly identical to mass and speed statistics from various mammals, lizards¹², birds, insects and more. Evolutionary constraints¹³ found in nature, in other words, can be seen at work in the airline industry.

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