Plane & Pilot
Saturday, July 1, 2006

Bernoulli Or Newton: Who's Right About Lift?

Misconceptions abound about one of the most important forces in flying

Just about every pilot would agree that studying certain aspects of flight can be a time-consuming mental workout. Any attempt to master complex aviation subjects can be frustrating, if not impossible, when pilots are given conflicting or incorrect data. One topic in particular, how lift is generated, tends to muster a tremendous amount of heartache among aviators and aerodynamicists alike. In fact, if you look at five different aviation references, you’re likely to find five different explanations about how lift comes to be. Even worse, some sources advocate a specific theory, while rejecting the premises favored in others. Groundschools, instructors and mainstream pilots have a tendency to unquestioningly embrace what’s written in certain texts, each of which is quick to defend its theory. This shouldn’t be much of a surprise, as it’s said that “arguing with a pilot is like wrestling a pig in mud—after a while you begin to think they like it.”

The primary conflict about how lift is created centers on two white-wig-wearing historical figures—Sir Isaac Newton (1642-1727) and Dr. Daniel Bernoulli (1700-1782). Those who prefer Newton’s ideas (i.e., Newtonian lift) believe that air is forced downward behind the wing. Simultaneously, the wing is forced upward with the famous “equal and opposite reaction” described in Newton’s third law. Then there are those who favor Bernoulli’s celebrated principle that as airflow accelerates, the static pressure within the airflow drops. We all know that the air flows faster over the top of the wing than the bottom. Bernoulli’s equation therefore concludes that the wing gets “sucked” upward by the reduced pressure.

All right, so you’re tired of the guys at the hangar making fun of you for preferring one theory over another. Off you go to prove them wrong. You pull out the classic Stick and Rudder by Wolfgang Langewiesche and quickly find the statement “forget Bernoulli’s theorem.” Still unsatisfied, you reach for Aerodynamics for Naval Aviators. There you find nothing but Bernoulli and something called “circulation.” You find much of the same while inspecting Aerodynamics for Engineers by John Bertin and the more readable Illustrated Guide to Aerodynamics by Skip Smith. In fact, Newton isn’t even mentioned during the lift discussion in the latter book. To add more fuel to the fire, prestigious academic institutions seem to favor one theory over another. Portions of Harvard’s and Princeton’s Websites discuss Bernoulli as though his ideas are the only ones available.

Don’t feel sorry for Newton just yet—he has his group of supporters. The recently published Understanding Flight by David Anderson and Scott Eberhardt dedicates two pages to Bernoulli, mostly about how his theory doesn’t appropriately describe lift, followed by nearly 200 pages about Newton. The Pilot’s Handbook of Aeronautical Knowledge and Jeppesen’s Private Pilot Manual mention that both Newton and Bernoulli have to be considered to accurately define lift (frustratingly, though, an earlier edition of Jepp’s Instrument/Commercial Manual leaves Newton out of the picture entirely). Nonetheless, even among those who prefer Newton, there’s some disparity. Cranfield University, one of the U.K.’s leading aeronautical institutions, offers the suggestion that, contrary to popular belief, the wing pushes upward, providing the Newtonian action, while “the reaction to this would be for the wing to push the air down,” thus producing the requisite lift.

When I asked Dr. Sheila Widnall, professor of Aeronautics and Astronautics and Engineering Systems at MIT, about this argument, she responded, “It’s scary to think there might be controversy about this issue. This is the basis of all subsonic aircraft design.” Indeed, it is scary. Pilots at all levels are receiving conflicting information about how their favorite toys stay in the air. Equate this to some doctors thinking the heart works one way, while others believe it works in an opposing manner. Yikes!

If you’ve been standing on a soapbox supporting one theory over another, there’s no need to hide your face the next time you pass the flight-school water cooler. The truth is that, among the Newton-Bernoulli disputers, neither party is wrong. According to Dr. Jean-Jacques Chattot, professor of Mechanical and Aeronautical Engineering and director of the Center for Computational Fluid Dynamics at the University of California-Davis, the descriptions of lift advocated by Newton and Bernoulli “are actually the same thing, just from two different perspectives.” How is this possible? Take another look at the dates when Newton and Bernoulli lived.


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