How Does an Airplane Stay in the Air?
September 13, 2015
For the longest time, I thought I knew why an airplane stayed in the air. It was because the wings were curved, and in such a way that the top was more curved than the underside. Air moving over the top of the wing had to move faster to keep in sync with the air below, resulting in less pressure (same amount of air over a larger volume), thus creating an uplift, called the Bernoulli Effect.
I remember reading about that in some popularizing books, and even right now you can find plenty of websites—even at NASA's—touting this description, which is illustrated below.
While I was researching other topics, I stumbled upon the real explanation. Now, the Bernoulli Effect is indeed present, but it isn't enough (by far) to lift a modern airplane.
Even if you don't do any calculating, there are a couple of things apparently missing from the explanation. First of all, it would exclude airplanes doing rolls or flying upside down and staying in the air. Secondly, as any child knows, a paper airplane flies very well, but it doesn't have any curved wings at all. And thirdly, the airplane of the Wright brothers didn't have curved wings either (as far as I can tell from photos and their patent application). Furthermore, it seems a bit odd that air would "know" how fast it should go to keep up with its counterpart below the wing.
So, what then is the full explanation? Two things: first, some thrust (an engine for a mechanical plane, your arm for a paper airplane), and then "angle of attack," or how the wing is positioned against the air. At horizontal, no lift is generated. When the wings are positioned slightly against the air, there is lift. The air is pushed downwards (that's why you need the thrust), and you create lift.
Now note that the air is pushed downwards below the wing but also the air flowing over the top of the wing is being pushed down. This is called the Coanda effect, and it can also be illustrated by a spoon or a glass against a water flow: the water "follows" the outside of the spoon. It can be shown that it is actually the air going over the wing that creates the most lift. That's why you'll find the engines and weapons attached under the wings and not above for a more limited loss of lift.
There is a limit however. If the airplane is pulled up too much, there's no longer sufficient air being pushed down, and the result is known as a "stall," a dangerous condition at lower altitudes as it takes some time to recover from it.
There are also a couple of other interesting things to look out for when flying or when observing airplanes taking off and landing. When an airplane takes off, it will increase its lift by making the wings longer using its flaps. As soon as the airplane is in the air, it retracts the flaps to reduce drag and because the speed is now sufficient to push down enough air to stay in the air. When approaching the airport, the flaps again go out to increase lift, but now it's so that enough air gets pushed down as the airplane decreases its speed. In the final phase, the spoilers (see drawing) go up; "spoiling" the airflow and reducing the lift even more.
I love airplanes, but it strikes me now that I didn't really know a basic aspect of how they stay in the air. I wonder what other things are out there that I think I know, but that I don't really know. Science can surprise even the most avid knowledge seekers.
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