Law of physics governs airplane evolution

Jul 22, 2014
This graph shows how -- as the years have passed -- bigger and bigger airplanes have joined the ranks of their behemoth brothers. Credit: Adrian Bejan

Researchers believe they now know why the supersonic trans-Atlantic Concorde aircraft went the way of the dodo—it hit an evolutionary cul-de-sac.

In a new study, Adrian Bejan, professor of 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 Boeing 787s. The analysis also provides insights into how aerospace companies can develop successful future designs.

The Concorde, alas, was too far from the curve of these good designs, Bejan says. The paper appears online July 22, in the Journal of Applied 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 encompassing 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 vascular system has evolved to provide blood access to flow through a network of a few large arteries and many small capillaries. River systems, tree branches and modern highway and road networks show the same forces at work, he says.

In the case of , designs have evolved to allow more people and goods to flow across the face of the Earth. Constructal law has also dictated 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 length, and the fuel load has grown in step with the total weight.

This chart shows how the ratio of mass to speed of animals follows the same general rules as airplanes. Note that the Concorde is way off of the historical trend. Credit: Adrian Bejan

"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 analogous 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 airliners 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, lizards, birds, insects and more. Evolutionary constraints found in nature, in other words, can be seen at work in the airline industry.

There was, however, one outlier on the chart—the Concorde.

"The Concorde was too far off from the ratios that evolution has produced in passenger jets," explained Bejan, who points out that the doomed aircraft had limited passenger capacity, a low mass-to-velocity ratio, an off-the-charts fuselage-to-wingspan ratio, massive engines and poor fuel economy. "It would have had to adhere to the constructal design rules to succeed."

Bejan said this analysis shows that the aviation industry has done well with its designs over the decades, and that the trends dominating the industry are indeed the most efficient. They also reveal the general design parameters that future passenger aircraft should follow to succeed economically.

"This study gives the rough sketch of what airplane designs will put you in the game," said Bejan. "For design companies, it is money in the bank."

Jose Camberos, research aerospace engineer and lead of design space exploration at the Multidisciplinary Science & Technology Center of the Air Force Research Laboratory at Wright-Patterson Air Force Base in Dayton, Ohio, said that the work will hopefully give the field better insight into where the design of airplanes is going.

"There is definitely an analogy to be understood and articulated to explain why engines and airplanes are sized the way they currently are and how that has evolved," said Camberos, who was not involved with this study. "By looking at the development of aircraft in a larger context in these terms, it may be possible to gain insights into how best to achieve what nature has been able to accomplish already."

Explore further: Uncovering the forbidden side of molecules

More information: The article, "The Evolution of Airplanes," is authored by A. Bejan, J.D. Charles and S. Lorente. It will appear in the Journal of Applied Physics on July 22, 2014: scitation.aip.org/content/aip/… /4/10.1063/1.4886855

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RichTheEngineer
3.7 / 5 (6) Jul 22, 2014
Perhaps that might apply for commercial aircraft, but does it apply for military, which are designed for different roles than commercial? Does it also apply for trans-sonic, super-sonic, and hyper-sonic designs?
Nik_2213
3 / 5 (4) Jul 22, 2014
Three thoughts...
Politics ? USA & Co. went NIMBY on Concorde, especially when Boeing's plans flopped.
Fuel ? IIRC, there was a *huge* hike in fuel price.
Size ? Really, Concorde was the '707', pathfinder for more efficient craft twice that size...

Now, had the 'Concordski' team solved its 'engine management' issues, things might have been different...

{ Urban Legend held that a prototype R-R EMU was stolen from Toulouse, only for the 'reverse engineering' team to discover it had been re-purposed to play a video game... ;-) }
Johan-C
4.7 / 5 (3) Jul 22, 2014
What is the exact definition of "succesful commercial aircraft"?

It's all too easy to say that Concorde was a "doomed aircraft". All right, one could hardly call it a "commercial" succes, but still, in its own way it was a "popular" aircraft. During air shows it still made everyone's head turn, even many years after its first flight.
Squirrel
5 / 5 (6) Jul 22, 2014
Concorde was not a commercial aircraft. Full stop. Built for political reasons, massively subsidized by the UK and French taxpayers, it has nothing to do with the aircraft in the diagram that were all built to make profits in the real business world.
pandora4real
5 / 5 (3) Jul 22, 2014
What is the exact definition of "succesful commercial aircraft"?

It's all too easy to say that Concorde was a "doomed aircraft". All right, one could hardly call it a "commercial" succes, but still, in its own way it was a "popular" aircraft. During air shows it still made everyone's head turn, even many years after its first flight.


So would a dodo at a livestock show even though they wouldn't be commercially viable. The two factors aren't related.
pandora4real
5 / 5 (2) Jul 22, 2014
Three thoughts...
Politics ? USA & Co. went NIMBY on Concorde, especially when Boeing's plans flopped.
Fuel ? IIRC, there was a *huge* hike in fuel price.
Size ? Really, Concorde was the '707', pathfinder for more efficient craft twice that size...

Now, had the 'Concordski' team solved its 'engine management' issues, things might have been different...

{ Urban Legend held that a prototype R-R EMU was stolen from Toulouse, only for the 'reverse engineering' team to discover it had been re-purposed to play a video game... ;-) }


Did you ever stand under its glide path? It was too noisy. Damned straight, NIMBY...and no one should have to listen to that.
NIS_0
5 / 5 (1) Jul 22, 2014
I wonder how this translates to warplanes. Are they sustainable?
Dunbar
1 / 5 (1) Jul 22, 2014
The man who proposed the "theory" is clearly an idiot.
AJW
5 / 5 (1) Jul 22, 2014
Let's see a graph of spacecraft, length, width/diameter, payload, thrust, cost per ton of payload...
JVK
1 / 5 (7) Jul 22, 2014
Re: "Evolution is a universal phenomenon encompassing technology, river basins and animal design alike, and it is rooted in physics..."

How is the unexplained universal phenomenon of "evolution" linked to physics, chemistry, and biology without first addressing the biophysical constraints on nutrient-dependent life that is controlled by the metabolism of nutrients to species-specific pheromones?

http://www.ncbi.n...24693353
Burnerjack
5 / 5 (1) Jul 22, 2014
I wonder how this translates to warplanes. Are they sustainable?

Warplanes operate on a 'design for mission' basis. Different animal. They don't need to be comfortable, economical, etc. While range is always a factor, in flight refueling can and has modified the priority. Payload, speed, crew survivability, ability to satisfy performance envelope as required for the intended mission is what counts. No galley service, No drink carts. No in flight entertainment. Short take off/landing roll etc.
AJW
5 / 5 (4) Jul 22, 2014
"...JVK: Re: "Evolution is a universal phenomenon encompassing technology, river basins and animal design alike, and it is rooted in physics..."

How is the unexplained universal phenomenon of "evolution" linked to physics, chemistry, and biology without first addressing the biophysical constraints on nutrient-dependent life that is controlled by the metabolism of nutrients to species-specific pheromones?..."

"evolution" as used in this sense refers to anything that changes and subsequent version are based on a previous version without resorting to a complete re-engineering or re-design. Non-replicating things require an engineer or designer. Your use of the term "evolution" refers to biological things which are self-replicating and as a result do not need a designer.
JVK
1 / 5 (5) Jul 22, 2014
Your use of the term "evolution" refers to biological things which are self-replicating and as a result do not need a designer.


Self-replication is an interesting phenomenon that appears to integrate ecological variation and result in ecological adaptations, which are manifested in biodiversity due to nutrient-dependent pheromone-controlled amino acid substitutions that differentiate cell types in species from microbes to man.

What experimental evidence of biologically-based cause and effect suggests to you there is no need for a designer?

Dobzhansky (1973) appears to disagree with your conclusion that no designer is required. See: Nothing in Biology Makes Any Sense Except in the Light of Evolution. Link opens pdf: http://img.signal...nsky.pdf "I am a creationist and an evolutionist. Evolution is God's, or Nature's, method of Creation."
t_d_lowe
5 / 5 (3) Jul 22, 2014
Totally disagree, concord was a strategic design to appeal to a specific and well paying customer (for business deals between NY and London). As such it was very successful and only closed down because of the terrible PR of the crash and the overheads of lining all wings with flame proof material.
That scientists can try out a thousand relationships between plane parameters and pick one where concord stands out, it is a classic case of retro-analysis and hindsight. Is anyone surprised that no-one wrote such an article before concord stopped running? Would the article have been published if concord was still running?
Retro-analysis is not science.
jimbo92107
5 / 5 (1) Jul 22, 2014
So if some engineer comes up with an airliner design that is radically faster, cheaper, more fuel efficient, etc, sorry Charlie, it doesn't fit the chart.
antialias_physorg
5 / 5 (1) Jul 23, 2014
Even aircraft follow the law of the minimum: You can expand only to the limits of the least abundant resource.

Currently that resource is the size of plane an airport can handle.

Planes 'evolve' to fit the environment ('environment' in this case being: fuel costs, demand for travel and available cash for travel by air, available infrastructure, legislation). in the end it always boils down to one of 2 factors why a plane gets built
1) maximum profit
or
2) national independence concerns (read: political showpiece)

The Concorde could well be a good design if you think in terms of a (near?) future world where only a small, hyper-rich elite can afford to fly - but don't really care how much a ticket costs if it means getting there fast.
antialias_physorg
5 / 5 (1) Jul 23, 2014
So if some engineer comes up with an airliner design that is radically faster, cheaper, more fuel efficient, etc, sorry Charlie, it doesn't fit the chart.

The chart works under the assumption that the change in the 'environment' is slow and without any obvious breaks.
(Such a break could be the invention of a new propulsion system, the global switchover to a new fuel type, or a radical shift in customer availability due to a global market crash, etc. )
The chart might work for conservative planning (i.e. big companies like Boeing and Airbus). Big companies hate to take risks,as the payoff for the CEOs is minimal to non-existent if it works and hugely negative if it doesn't.
ab3a
not rated yet Jul 23, 2014
As Ray Kurzwiel proved, you too can be a "visionary" if you plot your data on log/log scales.

The problem is that if you extrapolate outside the line you have drawn, you end up with weird predictions that may have no basis in reality.

That's the problem here. The article does not describe the physics of this issue. It describes the results of many engineering projects using similar technologies with similar expectations of range and speed.

So of course the Concorde is off the line.

Whether it has a chance of making money depends upon what the market demands. That's an economic issue, not an engineering issue.
marraco
not rated yet Jul 23, 2014
Is not valid to take conclusions for the Concorde, because it was a supersonic plane.

A supersonic plane flights on supercritical flow. It belongs to a different cloud of points. Air behaves on some opposite ways, when subsonic and supersonic regimes are compared.

Best design rules for subsonic planes may be the worst rules for supersonic.
Nik_2213
not rated yet Jul 23, 2014
And, watch out, here comes Reaction Engines' Mach 5 cruise Lapcat...
http://www.reacti...s.co.uk/

This would not fit on a linear graph due to the novel heat exchanger they've developed, with a truly remarkable power/weight ratio and wondrous efficiency...
EWH
not rated yet Jul 27, 2014
Cost per ton-mile scales linearly with peak power for everything from a Piper Cub to the Saturn V.