This paper describes human application of their knowledge and technological ability to fly and continue to enhance that power. Through application of their knowledge and technological ability, human beings have evolved the ability to fly and continue to enhance that power. Aircraft technology evolution is about the evolving design of the human movement on the Earth’s surface: people, goods, materials, and everything else. As the whole vehicle or animal evolves toward becoming better at moving mass on the landscape, the organs remain imperfect, because each represents a compromise. The whole vehicle or animal is a construct of organs that are ‘imperfect’ only when examined in isolation. The vehicle design evolves over time and becomes a better construct for moving the vehicle mass on the world map. Flow architectures are evolving right now, throughout nature and in technologies. The legacy of all flow systems (animate and inanimate) is: they have moved weight horizontally and improved the efficiency of that movement because of design evolution.
On a Continuum
Houseflies, falcons, and jetliners are all subject to the same evolutionary rules involving moving a weight through the air.
Sadi Carnot the French engineer did not build engines. He built something much more lasting: a theory that became thermodynamics. He studied the engines coming from Britain and invading the Continent, and questioned why they were becoming more efficient. Is there a ceiling to engine efficiency?
Carnot's answer is now history. The most efficient is, at best, imaginable in designs where all kinds of “friction” are avoided by making changes, purposeful choices.
The example of Sadi Carnot is a lesson worth teaching to the new generations: take the bird's-eye view. This need is acute today, because we tend to get lost in the deluge of data and information—mountains of empiricism calling for a theoretical framework.
The recorded evolution of flying machines calls for the bird's-eye view and its theoretical, predictive principle. Along with my colleagues Sylvie Lorente and Jordan Charles, I took this step in an extensive study published in the Journal of Applied Physics. In this essay we introduce readers to the predictions explained in detail in our study: the broad lines of the evolution of the airplanes and the law of physics, the Constructal Law, which governs evolution everywhere, animate, inanimate, and man-made.
To predict evolution is an important step. The prevailing view in science is that one cannot witness biological evolution because it occurs on a time scale immensely greater than our lifetime. I argue, though, that we can witness evolution in our lifetime.
Humanity is the species that created the engineered world. The modern human is not only the result of slow biological adaptation, but also of human technology.
We can watch the evolution of the flying “human-and-machine species”: people encapsulated in and empowered by airplanes, and sweeping the globe. Through application of their knowledge and technological ability, human beings have evolved the ability to fly and continue to enhance that power.
We can document this evolution, and we can also predict it based on physics.
Humans and Machines
Look around: the things we see and touch are changing. Some change from day to day; others from year to year, and many from decade to decade. Then there are broad-based changes that can take centuries or millennia to develop.
Airplanes, too, are evolving to carry more and more people all over the globe.
The advancement of airplanes is toward ever-greater efficiency. Each successful new model was presumably more economical than its predecessors of the same size, because otherwise it would not have been economical to adopt it. One of the important trends is that, although new models come in all sizes, the big airplanes of one decade are joined by even bigger models in the next decade.
We can consider the steady advance of aircraft as an example of the natural phenomenon of evolution. Evolution as physics, not as metaphor. Evolution means flow organization (or flow design) that changes over time. Evolution happens everywhere you look, in biological and nonbiological systems. This is why evolution is a phenomenon of physics, of all nature.
In biology, evolution is a descriptive narrative built on imagination, because the time scale of animal evolution is immense relative to the time available to us for observations. One cannot witness animal evolution, and this places the biology argument for evolution at a disadvantage. It would be useful to have access to the evolution of one species in real time.
The history of the airplane satisfies precisely this need. The species to watch is us. New airplane models do not happen by themselves. They are extensions, enclosures of the humans who come together to move on the globe more easily.
The species to watch is the human-and-machine species.
The human species is not defined only by its advanced biology—bipedal walking and nimble opposable thumbs—but also by the technology that is the fruit of human knowledge, which means action with purpose.
The word “machine” in the human-and-machine species requires some explaining. It is not about automobiles, power plants, refrigerators, and manufacturing. “Machine” in this instance is used in accord with its oldest meaning, which is “contrivance” (mihaní in old Greek), a sophisticated tool that allows for more effective use of human effort.
Every artifact that we attach to ourselves is a contrivance: the shirt, the harvested food, and the power drawn from an animal or an electrical outlet. Over the centuries new contrivances have made us much more powerful, bigger, and longer lived.
Every model of aircraft is a new human-and-machine design for moving our bodies, groups, and belongings over the entire globe. This design is changing, and what evolves with it is the movement of humans. This spreading flow gets better, faster, more efficient, and farther reaching.
This is just like the evolution of animal fliers. The bigger fly faster. The slow evolution of animal fliers has brought numerous forms of animal movement to converge on the same design features as the evolution of human fliers.
(In this evolution “movie” there are images that deviate from the plot, some faster and others slower, from the hawk and the chicken, to the Concorde. The physics law of evolution is not about outliers. It is about the holistic view, which is the evolving organization, the morphing flow architecture. The fact that the theoretical thread is hairy in some spots is nature. Evolution is a movie in two colors: predictable organization hand-in-glove with random details.)
Equally important is the observation that over time the cloud of flying designs has been expanding. In the beginning were the insects; later came the birds, and even later, the airplanes.
The animal mass that sweeps the globe today is a weave of a few large species and many small. The new are the few and large. The old are the many and small. The new do not displace the old. The new add themselves to the old.
The airplane models evolved in the same way. In the beginning was the Douglas DC-3 and many smaller airplanes; then the DC-3 was joined by the DC-8 and the Boeing B737; next the B747 joined the smaller and older models still in use. In this evolutionary direction, the size record is broken every time. This trend unites human fliers with animal fliers.
Think of a vehicle that consumes fuel and moves on the world map, and ask how large one of the organs of this vehicle should be—the engine, for example, or a duct that carries fluid, or the heat exchanger surface of the environmental control system. Because the size of the organ is finite, the vehicle is penalized (in fuel terms) by the component in two ways.
First, the organ is alive with currents that flow by overcoming resistances of many kinds. In thermodynamics, this universal phenomenon is called irreversibility, or destruction of useful energy (exergy), or entropy generation. This fuel penalty is smaller when the organ is larger, because larger means wider ducts and larger heat-transfer surfaces. In this sense, larger seems better.
Second, the vehicle must burn fuel and destroy more exergy in order to transport the larger organ. The fuel penalty for carrying the organ is proportional to the weight of the organ. This second penalty suggests that smaller is better, and it comes in conflict with the first penalty.
From this conflict emerges the discovery that the organ should have a certain, finite size, not too large, not too small, but just right for that particular vehicle.
The organ size recommended by this trade-off is such that larger organs, from engines to fuel loads, belong on proportionally large vehicles, and small organs belong on small vehicles.
During the evolution of airplanes, proportionalities have emerged among the mass of the heat engine (Me), the mass of the whole aircraft (M), and the fuel load (Mf). The engine data are correlated in a statistically meaningful way as Me = 0.13 M0.83, where both M and Me are expressed in tons.
The sizes of engines and airplanes increased by factors of order 20 from 1950 to 2014.
Larger vehicles also travel farther, just like the bigger animals and rivers. The range L is predicted to vary in proportion with Mα, where the value of the α exponent is smaller than 1. This is confirmed by the L vs. M data for airplane evolution, which are correlated as L = 324 M0.64, with L in kilometers and M in tons.
Commercial air travel is becoming more efficient. The trend of fuel spent per seat is visibly downward. The unit cost expressed as liters of fuel spent per seat for 100 kilometers flown has decreased by one order of magnitude during the past half century. On average, every year there has been a 1.2 percent decrease in fuel burn per seat.
In sum, aircraft technology evolution is about the evolving design of the human movement on the Earth's surface: people, goods, materials, and everything else. As the whole vehicle or animal evolves toward becoming better at moving mass on the landscape, the organs remain imperfect, because each represents a compromise. The whole vehicle or animal is a construct of organs that are “imperfect” only when examined in isolation. The vehicle design evolves over time and becomes a better construct for moving the vehicle mass on the world map.
Like the Animals
Everything that we can say about vehicles applies to animals. The organs that constitute the motor system of the animal (muscles, heart, lung) are the counterparts of the engine of the vehicle.
In biology, it is well known that muscle mass, heart mass, and lung volume are empirically proportional to the animal's body mass. The animal organ scaling is the same as the engine mass versus vehicle mass.
Small or large, airplanes do not look like animals. They do not flap wings or spread tails. They have engines that provide steady power for cruising speed and constant altitude.
Yet, airplanes exhibit features (body-size rules) that unite them with birds and other animals. Their engines scale with their body sizes and with their fuel loads. The larger airplanes are more efficient vehicles of mass, and travel farther, just like the larger animals.
Small or large, airplanes are evolving such that they look the same. The airplane body has two main parts, a fuselage that carries passengers and freight, and wings that lift the fuselage. Every shape (aspect ratio) of this structure is predictable from the same law of physics (the Constructal Law) that predicted the evolutionary trends.
The primary objective of commercial airplanes is to carry as many people as possible to a specified distance while using as little fuel as possible. The fuel consumed is proportional to the work delivered by the engine over the distance, and the work is equal to the total force overcome by the airplane times the distance traveled. In sum, to reduce the fuel requirement of an airplane of specified size is to reduce the total force subject to two constraints: the total mass (fuselage and wings) is fixed, and the wings must be strong to support the weight of the whole.
The key prediction is that the wingspan should be proportional to the fuselage length. In addition, the fuselage cross section must be rounded, and the fuselage and the wing must have slender profiles that are geometrically similar.
Everywhere you look, the fuselage length is one order of magnitude (that is, roughly ten times) greater than its diameter. The swept length of the wing is roughly ten times its thickness. Seen from the side, these many shapes are one shape, which is reminiscent of everything else that has evolved to penetrate its medium: bird or fish.
Evolution as Physics
Technology evolution is about us, about the evolutionary design of all the flows and movements that facilitate human flow, which means life on the Earth's surface (people, goods, and so on). The evolution of airplanes illustrates this convincingly.
What works is kept. Flow architectures that offer greater access persist, and are joined by even better ones. Together, the vascular tapestry of old and new carries the human flow easier and farther than the old alone. Air mass transport with new and old airplane models mixes the global sphere more effectively than in the absence of new models.
Flow architectures are evolving right now, throughout nature and in our technologies. The legacy of all flow systems (animate and inanimate) is this: they have moved weight horizontally and improved the efficiency of that movement because of design evolution.
The view that emerges is that the evolution phenomenon is broader than biological evolution. The evolution of technology, river basins, and animal design is one phenomenon, and it belongs in physics.
This research is supported by the National Science Foundation.