Why don’t planes fall out of the sky? If you believe Boeing, Airbus and just about every textbook on the subject, it’s because air travels faster over the “hump” of a wing, which creates lift. But Fermilab physicist David Anderson is convinced they’re all wrong. Forget the fancy fluid dynamics, he says. Newton’s three laws of motion are all you need. In his quest for the truth he learned to fly, and has co-written a new book claiming to set the record straight. So will pilots and engineers have to be retrained? And will tomorrow’s aircraft look any different? Diane Martindale asked Anderson if he thinks the Wright brothers got it wrong.
What motivated you to write the book?
It took me 25 years to figure out what was wrong with what I had been taught. After I figured it out, I kept being confronted by the mythology. So I wrote Understanding Flight with Scott Eberhardt to strike a blow for logic. The standard explanation of how we fly is mostly myth-it’s just wrong, but it has such a life of its own that you even see it on NASA websites and in physics books.
Advertisement
So what’s the thrust of your explanation?
It’s very simple: lift is a reaction force. The wing pushes air down, so in turn air pushes the wing up (see “Flying lesson”). It’s based on Newton’s laws of motion. In our book we threw away all of the math and we explained it with physics. In aeronautics textbooks, they squeeze all the juice out and give you the pulp. We reconstituted the juice. No pulp.
Fair enough. And what is the standard explanation for flight?
The standard explanation is based on the Bernoulli principle and goes like this: when an airplane is in flight, air flows faster over the top, curved surface of a wing. That’s because there is a hump and air must go farther than the air that flows underneath the bottom of a wing. Bernoulli’s principle says that when any fluid moves faster, it exerts less pressure. So the air above the wing exerts a lower pressure than the air below. This lifts the wing.
And you’re saying that this is all wrong . . .
There are two main problems. First, the Bernoulli explanation fixates on the shape of the wing, but this is the least significant part in determining lift. If the shape of a wing determined lift you could never fly upside-down. Second, the Bernoulli explanation doesn’t tell you why air travels faster on the top surface of a wing. If air travels faster, a force has to be involved. To explain that force, you need to invoke Newton’s laws of motion, and not Bernoulli’s principle.
When you studied at the University of Washington decades ago, you were taught Bernoulli. Did you challenge that explanation?
No, even though it didn’t seem right. I never accepted the Bernoulli explanation. My training is in physics. I’ve worked at Los Alamos, CERN and Fermilab, mostly designing instruments for high-energy physics. I’m an inventor. One of my talents is visual perception. So when I was told how wings flew, I couldn’t visualise it. When I got my bachelor’s degree, lift was one of the few things I didn’t understand, even though I had passed all the tests and done all the math. I just needed to know how wings flew.
Aircraft engineers agree that both the Bernoulli principle and Newton’s third law help to explain lift. Why can’t both explanations be right?
They are two completely different explanations. The Bernoulli explanation came from aeronautics. Aeronautical engineers calculate the lift on a wing using computer simulations. These simulations tell them how the air travels around a wing. They then use Bernoulli to calculate the pressure difference between the top and bottom of a wing. This gives them a value for lift. That’s all true. But that’s mathematics. It’s not a physical description. Bernoulli is useful in calculation but it doesn’t explain lift.
What about the Wright brothers. Didn’t they use Bernoulli’s principle?
No. If they did, their wing would never have looked like it did. They just did it by trial and error.
But didn’t they vary the curvature of the “hump” on the aeroplane they flew at Kitty Hawk until they got the desired lift?
Yes, but if you look, their wing doesn’t have a bottom. The Wright brothers’ earlier plane and the Curtis flyer were just warped boards. They worked like many aeronautical engineers-it was all experiment. You’d be surprised how little calculation and how much experience goes into designing an airplane. Most of it has to do with strength and weight and where you’re going to put the fuel tanks. The shape of the wing is often a compromise.
What are the implications if everyone accepts that Newton’s laws are the only explanation of lift? Will it change the way in which aircraft are designed?
No. It won’t. If I needed to design a wing I would still hire an aeronautical engineer. They don’t need to understand lift to do their calculations. And pilots don’t need it to fly.
So if the definition makes no difference to aircraft design or the work of pilots, does it matter if you’re right or wrong?
That’s like asking “Does the truth matter?” The Bernoulli explanation is wrong and it stymies you-it prevents you from understanding the phenomenon. Many explanations say that a wing holds itself up without doing work, like you holding yourself up by your bootstraps. There’s a lot of jargon involved, but Bernoulliites believe lift doesn’t require work. My explanation is more than a happy story-it makes you stop and think.
So who’s at fault for fuelling an incorrect idea. Is it textbooks?
Yes, for the most part. John D. Anderson’s Introduction to Flight (McGraw Hill, 1978) is a classic, spectacular book, yet it never explains why a wing flies. He says it’s because the air goes faster on the top surface of a wing, but he never explains why. There are books that do get it right. But then they’ll cover their hind ends by saying it’s consistent with Bernoulli. They’re afraid to say that the emperor has no clothes. It’s dogma. To some it’s like a religion and usually these are the people that know just enough to be dangerous.
Aren’t you overreacting a little? School physics texts don’t write about quantum theory, but that doesn’t mean physics professors only believe in classical mechanics
Now that’s true, but they don’t teach wrong quantum mechanics. It’s one thing to not teach something, it’s another to teach it wrong. The advantage of the Bernoulli theorem is that it’s very simple and it satisfies the person. I can teach the Bernoulli explanation in three minutes and everybody goes away with a smile on their face. But that’s not the explanation of lift.
Have you tried to convince the aeronautics community that they’ve got the story wrong?
The reaction has been very good. Two-and-a-half years ago my co-author Scott Eberhardt and I wrote an article in the trade magazine Sport Aviation that was the precursor to the book. Afterwards, a senior member of the aeronautics department at the University of Washington, where Scott teaches, said that every aeronautical engineer should read the article. I’ve even lectured at the Illinois Institute of Technology in Chicago and at the US Federal Aviation Administration at Oshkosh, and when I was through they all agreed I was right. Only a small percentage get upset, the rest are all excited they finally understand flight. They’re willing to let go of Bernoulli. They say, “I thought there was something wrong with that.”
You have a pilot’s licence…
Yes. My wife and I used to fly a Cessna 182 in New Mexico. Part of the reason why I learned to fly was to try and figure this out. I thought about it when I was flying. Learning to fly was one of my greatest investments in life.
Do you take it on yourself to set the record straight. For example, when you travel by air, do you tell the flight crew?
No. I don’t preach. It’s a battle that’s just not worth fighting one soldier at a time.
If Bernoulli were alive today, do you think you could convince him that you’re right?
Bernoulli is an innocent man. He never saw an airplane. But if he did, he would agree with me. If I could talk to him I’d have no trouble convincing him. I have no problem with engineers or physicists as long as I can talk to them. I’ve never had an educated person that didn’t accept it as being right.
