![]() ![]() Kennedy Space Center Pad 39B on July 26, 2005Ī popular translation of Newton’s second law is, “The alteration of motion is ever proportional to the motive force impressed and is made in the direction of the right line in which that force is impressed.” This is the basic principle of acceleration: the speed of an object depends on the force applied. This is the shuttle Discovery, launching from John F. This image of a space shuttle launch gives us a great example of the interactions between Newton’s second and third laws, and adds, the promise of even Bernoulli getting into the act on the craft’s return from orbit. Newtonian physics works well to explain the behavior of objects moving at well below relativistic speeds, so using it on a piston single shouldn’t present a problem. Sir Isaac Newton first published what has become known as his laws of motion in 1687. And we haven’t even gotten to inverted flight. Bernoulli also doesn’t explain how non-cambered wing designs-those lacking a curved upper surface, or nearly so-can generate lift, or how symmetrical airfoils, with identical cambers on the top and bottom, also create it. Bernoulli doesn’t really tell us why this happens, only that it does. Yes, the wing’s curved upper surface establishes an area of lower-pressure air above it, but there’s no venturi effect because there’s no venturi. In fact, according to NASA’s Glenn Research Center, “The actual velocity over the top of an airfoil is much faster than that predicted by the ‘Longer Path’ theory and particles moving over the top arrive at the trailing edge before particles moving under the airfoil” (emphases added). This is commonly known as the “longer path” or “equal transit time” theory.īut there’s no science that says the air particles have to arrive simultaneously. We probably were told in ground school that the low-pressure area on top of the wing results from the air particles passing over it having to accelerate in relation to the air below the wing so that they both arrive at the trailing edge at the same time and rejoin. But we still don’t know exactly why the air on top of the wing is at a lower pressure than the air underneath it. Still, once we put Bernoulli and Newton in the same room, then sprinkle some Cayley throughout, we have a working idea of how to build and fly an airplane. But they don’t have the details we need from Bernoulli. Taken together, Newton’s laws describe how we can fly inverted and how angle of attack works. That’s where Newton’s second and third laws (see the sidebar on the opposite page for details) come into play. Bernoulli’s principle-that the faster air on top of the wing experiences reduced pressure-is correct but doesn’t explain why it’s correct. The basic problem is that neither theory completely explains real-world observations. Also today, we teach that the theories of Sir Isaac Newton (1642-1726) and Swiss mathematician Daniel Bernoulli (1700-1782) provide the detailed science that explains lift. His three-part work, On Aerial Navigation, published in 18, is often cited as the first description of what we today call an airplane. Sir John Cayley, an English engineer who also first identified the four forces of flight-lift, drag, thrust and weight-developed the cambered airfoil through detailed experimentation. ![]() Air Force's Thunderbirds demonstration team illustrate one of the problems with Bernoulli's principle when explaining lift production: How does the inverted wing - and the airplane to which it's attached - stay aloft?Ī popular misconception is that the Wright brothers, in addition to all of their other achievements, invented the airfoil. ![]()
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