Shop Windows to the Universe

Earth Science Rocks! Select one of our four cool NESTA t-shirts from our online store, and express your love of Earth and space science!

Why is it that the air traveling over the top of the airfoil has to reach the end of it at the same time that the air traveling underneath the airfoil reaches it.

The idea that the air traveling over the top of the airfoil must reach the end of the wing at the same time as the air traveling along the bottom of the airfoil is called the 'principle of equal transit times.' This follows from Bernoulli's principle, which says that when fluid velocity increases, pressure decreases. As the air over the top of the airfoil has higher velocity than that of air at the bottom of the airfoil, there is a decrease in pressure at the top of the airfoil, which causes lift. This explanation of lift is actually inaccurate. The reason this explanation has been around for so long is that it is the easiest to understand.

The Bernoulli principle is true, as is the fact that the air at the top of the airfoil goes faster than that at the bottom. However, it is untrue that the seperated air must reach the end of the airfoil at the same time. In reality, the air at the bottom of the airfoil is slowed down.

In order to reach the correct explanation, we need to look at Newton's laws. The third law says that for every action there is an equal and opposite reaction. In order for an airfoil to produce lift, it must obtain an equal and opposite reaction. So, an airfoil changes the direction of the air: it sends a stream of air downward, in order to send the wing upward. (See picture.) The lift of an airfoil is proportional to the amount of air diverted downward times the downward velocity of the air. The more air the airfoil sends downward (by changing the shape or angle of the airfoil), the more lift will be obtained. For a more detailed description, and more information on flight, you might try the ALLSTAR Network.

Submitted by Bob (Maryland, USA)
(August 2, 1999)

Shop Windows to the Universe Science Store!

Learn about Earth and space science, and have fun while doing it! The games section of our online store includes a climate change card game and the Traveling Nitrogen game!

Windows to the Universe Community



You might also be interested in:

What are the retrograde motions of planets in the sky?

It depends on what kind of motion you are talking about. When seen from the north pole of the celestial sphere all planets orbit around the Sun in a counter-clockwise or direct path. Most planets also...more

How do Astronauts Live in Space?

Almost everyone has a question or two about living in space. What do astronauts do in space? How do they do everyday things like eat, sleep and go to the bathroom? Well, this is our attempt to answer...more

How far is the Earth from the Sun, the Moon and all the other planets? How far are all of the planets from the Sun? Do you know of a software that tracks the planets in real-time?

There is a really neat internet program called Solar System Live that shows the position of all of the planets and the Sun for any given day. If you go to that page, you'll see an image similar to the...more

Is it really true that man never really walked on the Moon?

The picture of the American Flag (the one put there by the Apollo astronauts) is waving (or straight out) in the wind. How could that be possible if there is no atmosphere on the Moon? Was it some sort...more

How many planets orbit the sun?

I was wondering if there is a new planet? Are there planets (a tenth planet?) after Pluto belonging to our solar system? What are the names of the new planets discovered in the solar system? Are there...more

According to Stephen Hawking, any object with an energy which equals Plank's energy has to become a black hole.

If that is so, the energy released during the Big Bang must have created many such black holes. Therefore most of the Energy of the Big bang must have disappeared in that form. Then how did the Universe...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF