Why don鈥檛 planes fall out of the sky? If you believe Boeing, Airbus and just about every textbook on the subject, it鈥檚 because air travels faster over the 鈥渉ump鈥� of a wing, which creates lift. But Fermilab physicist David Anderson is convinced they鈥檙e all wrong. Forget the fancy fluid dynamics, he says. Newton鈥檚 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鈥檚 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鈥檚 just wrong, but it has such a life of its own that you even see it on NASA websites and in physics books.
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So what鈥檚 the thrust of your explanation?
It鈥檚 very simple: lift is a reaction force. The wing pushes air down, so in turn air pushes the wing up (see 鈥淔lying lesson鈥�). It鈥檚 based on Newton鈥檚 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鈥檚 because there is a hump and air must go farther than the air that flows underneath the bottom of a wing. Bernoulli鈥檚 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鈥檙e 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鈥檛 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鈥檚 laws of motion, and not Bernoulli鈥檚 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鈥檛 seem right. I never accepted the Bernoulli explanation. My training is in physics. I鈥檝e worked at Los Alamos, CERN and Fermilab, mostly designing instruments for high-energy physics. I鈥檓 an inventor. One of my talents is visual perception. So when I was told how wings flew, I couldn鈥檛 visualise it. When I got my bachelor鈥檚 degree, lift was one of the few things I didn鈥檛 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鈥檚 third law help to explain lift. Why can鈥檛 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鈥檚 all true. But that鈥檚 mathematics. It鈥檚 not a physical description. Bernoulli is useful in calculation but it doesn鈥檛 explain lift.
What about the Wright brothers. Didn鈥檛 they use Bernoulli鈥檚 principle?
No. If they did, their wing would never have looked like it did. They just did it by trial and error.
But didn鈥檛 they vary the curvature of the 鈥渉ump鈥� on the aeroplane they flew at Kitty Hawk until they got the desired lift?
Yes, but if you look, their wing doesn鈥檛 have a bottom. The Wright brothers鈥� earlier plane and the Curtis flyer were just warped boards. They worked like many aeronautical engineers-it was all experiment. You鈥檇 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鈥檙e going to put the fuel tanks. The shape of the wing is often a compromise.
What are the implications if everyone accepts that Newton鈥檚 laws are the only explanation of lift? Will it change the way in which aircraft are designed?
No. It won鈥檛. If I needed to design a wing I would still hire an aeronautical engineer. They don鈥檛 need to understand lift to do their calculations. And pilots don鈥檛 need it to fly.
So if the definition makes no difference to aircraft design or the work of pilots, does it matter if you鈥檙e right or wrong?
That鈥檚 like asking 鈥淒oes the truth matter?鈥� 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鈥檚 a lot of jargon involved, but Bernoulliites believe lift doesn鈥檛 require work. My explanation is more than a happy story-it makes you stop and think.
So who鈥檚 at fault for fuelling an incorrect idea. Is it textbooks?
Yes, for the most part. John D. Anderson鈥檚 Introduction to Flight (McGraw Hill, 1978) is a classic, spectacular book, yet it never explains why a wing flies. He says it鈥檚 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鈥檒l cover their hind ends by saying it鈥檚 consistent with Bernoulli. They鈥檙e afraid to say that the emperor has no clothes. It鈥檚 dogma. To some it鈥檚 like a religion and usually these are the people that know just enough to be dangerous.
Aren鈥檛 you overreacting a little? School physics texts don鈥檛 write about quantum theory, but that doesn鈥檛 mean physics professors only believe in classical mechanics
Now that鈥檚 true, but they don鈥檛 teach wrong quantum mechanics. It鈥檚 one thing to not teach something, it鈥檚 another to teach it wrong. The advantage of the Bernoulli theorem is that it鈥檚 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鈥檚 not the explanation of lift.
Have you tried to convince the aeronautics community that they鈥檝e 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鈥檝e 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鈥檙e willing to let go of Bernoulli. They say, 鈥淚 thought there was something wrong with that.鈥�
You have a pilot鈥檚 licence鈥�
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鈥檛 preach. It鈥檚 a battle that鈥檚 just not worth fighting one soldier at a time.
If Bernoulli were alive today, do you think you could convince him that you鈥檙e 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鈥檇 have no trouble convincing him. I have no problem with engineers or physicists as long as I can talk to them. I鈥檝e never had an educated person that didn鈥檛 accept it as being right.