# why do planes stay in the air

Airplanes need four forces to fly. Lift is one of them. Image Credit: NASA
How do airplanes stay in the air? Four forces keep an airplane in the sky. They are lift, weight, thrust and drag. Lift pushes the airplane up. The way air moves around the wings gives the airplane lift. The shape of the wings helps with lift, too. Weight is the force that pulls the airplane toward Earth. Airplanes are built so that their weight is spread from front to back. This keeps the airplane balanced. Don't forget the pilot! Image Credit: NASA Thrust is the force that moves the airplane forward. Engines give thrust to airplanes. Sometimes an engine turns a propeller. Sometimes it is a jet engine. It doesn't matter as long as air keeps going over the wings. Drag slows the airplane. You can feel drag when you walk against a strong wind. Airplanes are designed to let air pass around them with less drag. An airplane flies when all four forces work together. But, most airplanes need one more thing: They need a pilot to fly them! 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,б andб even right now you can find plenty of websitesБeven atб Бtouting 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,б but it isnБt enough (by far)б. Even if you donБt do any calculating, there are a couple of things apparently missing fromб 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 doesnБt have any curved wings at all. And thirdly, the airplane of the Wright brothers didnБt 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 БknowБ how fast it should go 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 Бangle of attack,Б 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 (thatБs 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, and it can also be against a water flow: the water БfollowsБ 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 sб why youБll 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 Бstall,Б 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 asб the airplane decreases its speed. In the final phase, the spoilers (see drawing) go up; БspoilingБ the airflow and reducing the lift even more. I love airplanes, but it strikes me now that I didnБt 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 donБt really know. Science can surprise even the most avid knowledge seekers.

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