The Physics of Airplanes:
Why We Go Up
An old, lofty theory of how airplanes fly

Year 2005 witnessed the 300th birth anniversary of Daniel Bernoulli, the Swiss Mathematician and Polymath, born in 1700, Bernoulli never had a lot to say about airplanes while he was alive, and yet these days he is widely credited with keeping every one of them in the air. In an article on insect flight it was almost off-handedly maintained that an airplane wing diverts air downward and that this "downward flow lowers the air pressure above the wing, lifting it ..."

Air flows faster over the top surface of the wing than below (underneath) the bottom. Bernoulli's principle says that when any fluid moves faster— for instance as it passes a bottleneck in a pipe — the static pressure in it decreases. Therefore, borrowing Bernoulli's logic, the air above the wing must be at lower pressure than the air below that lifts the wing. 

The air flowing over the curved upper surface of the wing travels farther than the air traveling under the bottom, and so it has to travel faster to get to the trailing edge at the same time. The problem is, there is no earthly reason why the air should get there at the same time.  In fact, it doesn't.  Someone, somewhere (and let's hope it wasn't a science journalist) made up the "principle of equal transit times." The air on top actually gets to the trailing edge sooner than the air on the bottom, because it really does travel faster.

'What makes a wing fly?' Lift is a reaction force. The wing pushes the air down, so the air pushes the wing up. To understand ‘lift’ you need only Newton's three laws and something called the Coanda effect. The Coanda effect is just the tendency of air or any even slightly viscous fluid to stick to a surface it is flowing over, and thus to follow the surface as it bends. As air follows the upper surface of a wing, it gets bent downward— because the surface is curved but also because the leading edge is tilted up (especially when ascending) at what is called the angle of attack. The air that is bent downward pulls on the air above it, distending it and creating a low-pressure zone above the wing. To bend the air downward, the wing has to exert a force on it (air). That action inevitably elicits an equal and opposite reaction (Newton's third law). By means of the low-pressure zone above the wing and the higher pressure below it, the air exerts an upward force on the wing: That's lift. The size of the force is equal to the mass of air the wing has diverted downward multiplied by the acceleration of that air (Newton's second law). A pilot can increase the lift by flying faster (adding power) or by increasing the angle of attack (pulling back on the stick); either way the wing diverts more air down and behind the plane. The wings of a 250-ton airliner pump down about 250 tons of air every second. It holds itself up by brute force. In this quick illustration we find that air flows to the right, such as in a wind tunnel.  Inside the air stream is a wing diverting the molecular flow. At the front edge, molecules of air are diverted upward, compressing the atmosphere above.  Incoming molecules, which may not strike this object, are also caused to move upward because of the compressional effect of the molecules below them and already moving upward, low pressure is formed above and behind the wing. Beneath the wing, the molecules have not undergone deflection, and standard air pressure exists, pushing the wing upwards. Once compressed, the airflow above begins to return downward to fill this low pressure, its momentum carrying its molecules below their initial position, building pressure below, which then pushes them back up in a progressively dampened process. A wind tunnel clearly reveals low pressure formed as a boundary layer flow becomes dispersed.