Correct answer: nobody really knows.
No One Can Explain Why Planes Stay In The Air
- On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don’t explain why aerodynamic lift occurs.
- There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.
- Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What Does Keep Them Up There?” The point of the piece was a simple question: What keeps planes in the air? To answer it, the Times turned to John D. Anderson, Jr., curator of aerodynamics at the National Air and Space Museum and author of several textbooks in the field.
What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,” he told the Times. People give different answers to the question, some with “religious fervor.” More than 15 years after that pronouncement, there are still different accounts of what generates lift, each with its own substantial rank of zealous defenders. At this point in the history of flight, this situation is slightly puzzling. After all, the natural processes of evolution, working mindlessly, at random and without any understanding of physics, solved the mechanical problem of aerodynamic lift for soaring birds eons ago. Why should it be so hard for scientists to explain what keeps birds, and airliners, up in the air?
Even as extraordinarily broad and supple an intellect as Einstein’s couldn’t suss it all out:
In Germany, one of the scientists who applied themselves to the problem of lift was none other than Albert Einstein. In 1916 Einstein published a short piece in the journal Die Naturwissenschaften entitled “Elementary Theory of Water Waves and of Flight,” which sought to explain what accounted for the carrying capacity of the wings of flying machines and soaring birds. “There is a lot of obscurity surrounding these questions,” Einstein wrote. “Indeed, I must confess that I have never encountered a simple answer to them even in the specialist literature.”
Einstein then proceeded to give an explanation that assumed an incompressible, frictionless fluid—that is, an ideal fluid. Without mentioning Bernoulli by name, he gave an account that is consistent with Bernoulli’s principle by saying that fluid pressure is greater where its velocity is slower, and vice versa. To take advantage of these pressure differences, Einstein proposed an airfoil with a bulge on top such that the shape would increase airflow velocity above the bulge and thus decrease pressure there as well.
Einstein probably thought that his ideal-fluid analysis would apply equally well to real-world fluid flows. In 1917, on the basis of his theory, Einstein designed an airfoil that later came to be known as a cat’s-back wing because of its resemblance to the humped back of a stretching cat. He brought the design to aircraft manufacturer LVG (Luftverkehrsgesellschaft) in Berlin, which built a new flying machine around it. A test pilot reported that the craft waddled around in the air like “a pregnant duck.” Much later, in 1954, Einstein himself called his excursion into aeronautics a “youthful folly.” The individual who gave us radically new theories that penetrated both the smallest and the largest components of the universe nonetheless failed to make a positive contribution to the understanding of lift or to come up with a practical airfoil design.
Can’t recollect via whom I found this one; I suspect it was probably Insty, but a bit of searching around at his place didn’t turn it up. Whoever it was, my abjectest apology for failing to acknowledge the find with a return link. It’s a fascinating article all around, if you’re into the whole aviation thing. Which, y’know, I am.
Tx. Most interesting link I’ve read in a long time. Digging up the Physics Teacher article referred to in the link page .
It seems to me that it’s just a matter of vectors and energy allocations. Think about the energetics involved. Is there more energy in the air immediately above a Bernoulli surface in horizontal motion than in the air immediately below it? No. Yet the air immediately above that surface, by the nature of transit through it, must be moving faster horizontally than the air immediately below it. If energy has been diverted to horizontal movement, it follows that it must have come from somewhere — and in this case the “somewhere” is the potential energy of the air that arises from gravitation: i.e., the downward, gravity-induced pressure on the Bernoulli surface. If there’s a mystery of any sort remaining, it would be in the micro-causal realm: the transfer of some of the air’s gravitational potential energy to the kinetic form.
This would also serve to explain why a Bernoulli surface must be rather subtle. If the upper curve is too dramatic, the disruption of the air’s energetic balance becomes violent. Laminar air flow breaks down and turbulence sets in. Your aircraft is no longer controllable. So you must be satisfied with a modest degree of lift from the Bernoulli Effect and achieve any greater degree of lift from propulsive mechanisms.
I’m reminded of a gag-line that’s common among physicists: “That’s all very well in practice, but it won’t work in theory.” Sometimes theorists get hung up on ferreting out mechanisms they can’t see and neglect what’s right in front of them. I suppose it can happen to anyone.