What Makes Airplanes Fly? Bernoulli's Principle!

We see air planes nearly every day, flying in and out of airports, leaving vapor trails across the sky.  How can such a large, heavy object fly?  After all, even the smallest of air planes is over 1500 pounds!  We can find the answer in something called Bernoulli’s principle.

To fully understand Bernoulli’s principle, we first have to understand air pressure.  There is air all around us all the time, and this air is always pressing on us.  Because the air is pressing on us evenly from all directions, we usually don’t notice it. 

When objects move through the air they must push it out of the way,  just as they would if they were traveling though a liquid or a solid.  As it is pushed out of the way by the object, the air moving around the object must move faster than the rest of the surrounding air.  Think of it this way:  imagine a shallow swimming pool full of ping pong balls.  If you run from one end of this pool to the other end through the ping pong balls, the balls you push out of the way as you run will need to move faster than the balls farther away from you.  This is exactly what happens to the air as an object moves through it—the air molecules are like the ping pong balls, being pushed out of the way! 

As the air molecules move faster, they are not able to exert as much air pressure—and this is where Bernoulli’s principle comes in.  Bernoulli’s principle states that an increase in the speed of a fluid (either a liquid or a gas) is accompanied by a decrease in pressure.  Therefore, as this fluid moves from an area of high pressure to an area of low pressure, it will speed up—this is why when the atmospheric pressure drops, the wind begins to blow! 

This also means that as fluid moves faster, its pressure—that is, the amount of force with which it pushes against other things—drops.  This effect is the reason why airplanes can fly.  To understand this, we need to look closer at an airplane’s wings.

The wings of an airplane have a very particular shape:  the top of the wing is more curved than the bottom of the wing.  Because of this extra curve on top, the air flowing around the wing has farther to travel around the top of the wing than the bottom of the wing.  This extra distance means the air travels faster, reducing the pressure over the top of the wing.  The reduced pressure on top of the wing allows the pressure on the bottom of the wing to push the airplane upward, causing it to fly!

TRY IT!

You can demonstrate Bernoulli’s principle for  yourself with a very simple experiment.  All you need is a sheet of paper (at least 5 inches wide and 8 inches long), and some clear tape!

Here’s what to do:

1.       Take your sheet of paper, and fold it along its width.  The fold should be about one inch off the middle of the sheet.

2.       Bring the edges of the paper together, and hold them in place.  Use your tape to tape the edges together.

3.       Blow across the edges of the paper, and observe what happens!

What happened when you blew across the paper?  Did it move?  If so, which way?  Be sure to write down all your observations!


References for further reading:

1)      Lord, M.  “Lesson: Get a Lift!”.   eGFI.  March 25, 2011.  teachers.egfi-k12.org/get-a-lift/