How do airplanes fly?

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I have a fun one today, which includes an experiment so you can understand a little better.

Take a smallish piece of normal copy paper, not much bigger than 3″x5″. Hold the short end under your lower lip, so you can blow over it but no air will go under it. Blow.

Didn’t expect the paper to move, did you?

Another experiment – hold two pieces of paper up to your face, on either side of your mouth, about 3″ apart. Blow between the pieces of paper. My first thought was that they would push apart, but they don’t. They come together.

How is lift generated? How do airplanes fly? Intuitively, at least for me, they shouldn’t. Airplanes are getting pushed forward, not up. This gave me a headache for a while, but then I started to understand a little more. I didn’t have to do the actual physics equations, either, so those of you who have issues with physics don’t have to worry about hard math.

The problem we face when trying to figure this out is that there are two separate camps, both of which have part of the answer. The actual answer is very complex, so we’ll go through it as simply as possible.

Bernoulli’s equation says that air going over a surface that is curved on top and flat on the bottom will have less pressure on the top because the air is going faster there. The lower pressure provides lift. Things will move towards an area of lower pressure. This kind of lift is dependent on the shape of the wing. Gliders are almost completely dependent on this kind of lift. No engines to push them forward and create relative wind. This kind of lift is also very stable.

Newtonian lift is created when air hits the bottom of a tilted surface (remember the angle of incidence?). The air is deflected downward, which creates the equal and opposite reaction of upward lift on the wing. If you have ever stuck your hand out the window of a moving car and tilted your hand, you have felt the air push your hand up as you deflect it down. This kind of lift doesn’t depend on the shape of the wing. It’s somewhat unstable, which, again, you can find out by sticking your hand out the car window – you’ll feel the adjustments you have to make to keep your hand from going in unexpected directions.

Between these two kinds of lift, you have pretty stable flight.

People have huge arguments over which one of these equations is correct. They both are, in combination. Before powered flight, humans used Bernoulli’s lift pretty much on its own. With powered flight, we can use Newtonian lift to get more lift and Bernoulli to stabilize the flight.

See? No physics equations, and a couple of neat things to try. Not as hard as you thought, was it?