Physics Friday: Why do airplanes fly?

I recently graduated with my bachelors in physics.  I also tutored physics for a couple of years. I'm thinking of including short posts with quick fun physics explanations sometimes.

Air travel has been in the news a lot lately because of the new TSA regulations. I'm not going to go into it, although I would like to know for sure if I can take some knitting needles with me to keep busy in the terminal and on the plane, but that's neither here nor there. I want to take a moment and talk about what keeps planes in the air. I promise not to get technical, because the principle is really simple.

You've probably seen the illustration in Figure 1. The wing is curved on top and flat on the bottom. As it slices through the air, adjacent air particles are moved apart. One goes above the wing and one goes under then they meet up on the other side. The one traveling above the wing goes farther in the same time than the one that goes under, therefore it must have gone faster. Speed = distance/time. If the distance is bigger but the time is the same, the speed is bigger. If the air speed above is faster, then there is less pressure above the wing. Things like to move from areas of high pressure to areas of low, so the wing moves up. You can test this by holding two strips of paper in front of your face and blowing between them. See what happens. This can also be tested by running past a door, especially one that is relatively light. Try it, it's fun. The door should move toward you and the strips should move toward each other.

Fig. 1

This is all well and good, but there is no reason for the air particle on top of the wing to meet up with the one below it. They don't need to stick together. So why does a plane fly?

The simple answer is conservation of momentum as illustrated in Figure 2. The wing is angled as shown. As the wing moves forward, it hits air. The air is forced downward because of the angle of the wing. The downward momentum of the air must be conserved, so the wing must move up. Next time you're driving in the car, put your hand out the window. See what effect angling your palm different ways has on the direction it's forced.

Fig. 2

It's more complicated than that. If you watch the wings while flying, you'll see flaps moving up and down especially during landing, but this is the basic physics concept behind flight. 

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Happy flying!