Anyone who has flown in an airplane has probably wondered, as they stared down at the ground far below, what it is that keeps an airplane up in the air? The answer is simple: lift. However, understanding what lift is all about is complex, and explanations about lift found in popular science books often include one of the most common misconceptions about airplane flight.
It’s not Bernoulli
Many articles and books that explain lift begin with the Bernoulli effect. The top of an airplane wing is curved, they note, while the bottom is somewhat flat. Air rushing over the wing moves faster over the top than across the bottom. This creates lower pressure on top, which pulls up on the wing, creating lift.
While the Bernoulli effect does occur around wings, it’s not sufficient to explain lift. A little thought and a little observation will reveal why.
First, take a look at a real airplane wing, whether it’s the wing of a huge transcontinental airline or the wing of a tiny private plane. You won’t find a wing that is curved only on the top surface and flat on the bottom as the illustrations of the Bernoulli effect all show. Both surfaces are curved, and the wing is tilted slightly toward the tail end of the plane.
Then, think about the proportional difference between the wing of the little prop plane and the wing of the airliner. If the lower pressure on top of the wing was all that held the plane up, the wing of an airliner would have to be proportionally much, much larger than the wing of the small plane. It would have to be enormous compared with the size of the cabin, as well as have a much higher domed upper surface to create enough of a pressure difference. Clearly this doesn’t describe actual airplane wings.
Consider airspeed as well. Keeping a plane aloft requires that air be moving over the wing. According to the Bernoulli explanation, more lift is created if the plane is moving faster. That would mean that slowing down would always cause the plane to sink, while speeding up would always make it rise. Speeding jets must fly the highest of all, but anyone who has been to an air show can testify that performers in the fastest military jets can, indeed fly close to the ground. Their speed is not tied to stronger lift.
Finally, if wings were shaped as the Bernoulli explanations say they are, and if lift is created by air rushing over the upper, curved surface, then it would be impossible for stunt planes to fly upside-down. The lower air pressure would be on the side facing the ground, and the plane would immediately drop.
In truth, the wing shape doesn’t matter. The famous early barnstormer, Lincoln Beachey, was absolutely right when he said he could fly a barn door if the motor was strong enough. So what is it that creates lift?
A Newtonian Explanation
Newton, rather than Bernoulli, developed the laws that give us a better explanation of lift. Newton’s third law states that for every action there is an equal and opposite reaction.
Take a close look at an actual airplane wing. The surfaces are curved and the wing is angled. As the propeller pulls the plane through the air, a stream of air is carried across the wing. The angle of the wing creates a strong “downwash”: that is, a stream of air that is diverted downward. The steeper the angle of attack, the stronger the downwash.
To experience downwash, take that pair of wings, mount them on a rotor on top of the flying machine, and turn them rapidly. What you have is a helicopter propeller, and the strong downward rush of air is immediately evident. Put those wings back on the plane and use jet engines or a forward propeller to move the plane through the air. Again, there is downwash created by the wing. The downward thrust of air creates the equal an opposite reaction that Newton’s law predicts: lift.
Speed of the airplane, the angle of the wing, and the density of the air all matter in the amount of lift that is created. Higher speed creates a greater amount of air moving over the wing, creating more downwash, while slow speeds reduce the amount of air. A greater angle of attack creates a stronger downwash of air, while changing the shape of the wing by changing the wing flaps can alter the amount of downwash if the pilot needs a greater angle of attack to stay aloft at the same speed. Air density changes with altitude, which affects the speed at which a plane must fly to stay aloft.
The problem is that explaining lift is complicated. Bernoulli diagrams are simple, and that is why popular science books rely on them. For a full explanation of the Newtonian theory of lift, see the article “How Airplanes Fly: A Physical Description of Lift,” originally published in Sport Aviation and now available at http://www.aviation-history.com/theory/lift.htm.