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Why do two air molecules, separating at the same time, and one going along the "straight" bottom of an airplane wing and one going along the curved top of the wing have to arrive at the back of the wing at the same time? Or do they? Isn't that what creates lift?
Question Date: 2011-10-11
Answer 1:

That's an excellent question! This is, in fact, a common misconception: an air molecule traveling along the bottom of the wing does not, in general, need to meet up with the other molecule that traveled along the top of the wing (this is the so-called "equal transit time" theory). In general, lift is a very complicated thing to understand (for me as well!), but here's a more physically correct understanding: because of the shape of the top surface of a wing, a tube of air flowing along the top is "squished" by the shape of the wing, while a tube of air flowing below the wing is relatively unchanged. But when a tube of fluid (air, in this case) is narrowed, its speed must increase, and therefore the pressure decreases. Therefore, the compressed tube of air flowing along the top of the wing generates a lower pressure than the tube underneath, and the wing is lifted.

Another understanding of lift follows from Newton's third law of motion: a wing is generally shaped and oriented so that the air that passes the wing gains some downwards velocity; this means that the wing had to push down on the air. But Newton's third law of motion says that for every action there is an equal and opposite reaction, so if the wing pushed down on the air, then the air had to push up on the wing, creating lift. You can actually try this yourself some time by holding your hand flat outside of a car window; as you tilt your hand up and down, you'll push the air either up or down, and that will create a lift force on your hand.

Answer 2:

I can tell you that as the wing moves through the air it will create a void behind it, which will suck air ahead of it in.

Answer 3:

This is one of those classic misconceptions you learn in school. I am not an aeronautics guy so I'll be careful about saying what really gives an airplane lift but I promise you it isn't Bernoulli's law. I'm sure different planes use various designs, but the general idea is that the tips of the wings create an awkward surface aerodynamically which cause little eddies of air to spin off of them. The wings are generally tapered backwards, not straight 90 degrees from the plane's body which guides massive currents of air towards the tips where large turbullent buffets of air are "thrown" from the wing creating lift in the same way you glide backwards on ice if you push off someone else. It should be noted that no single effect explains lift in planes and vortices coming off the edges near the tips is only part of the story, but it is a big part!

I urge you to Google around for some diagrams explaining this effect. "What causes airplane lift" should do it.

airfly that site has a nice picture of vortices behind a plane going through fog.

Hope this helps!

Answer 4:

Excellent question! This is actually a rather famous misconception (or over-simplification) of the fluid dynamics behind lift. A google search for airfoil myth is a good place to start if you'd like to look into the topic futher. I'll try to provide the best explanation I can here. The truth is there is no reason why the air on the top of the wing and the bottom of the wing need to re-unite at the back at the same time. The classical proofs of this are 1) an airplane is able to fly up-side down for long periods of time, and 2) airplanes with flat wings are able to fly, without having the airfoil shape. How is lift, then, actually created? It's a combination of two forces. One, very simply, is that airplane wings have an angle of attack which deflects the incoming air down. Pushing down on the air, the plane is lifted up. The second force is where the reference to Bernoulli's principle enters. The wing, while cutting through the air, creates a wake, just like you would find behind a speeding boat. (Air, being a fluid, behaves a lot like water!) The airfoil shape helps to focus this wake on the upper edge of the wing. This turbulent section is moving around a lot faster than the rest of the air, resulting in a low pressure pocket above the wing, pulling the wing up. Of course, an answer to your question which didn't mention the Navier-Stokes equations would be incomplete! These are the (very-complicated) equations which describe all fluid motion. In all but the simplest cases, they cannot be solved without a computer. As a bit of a side note, aircraft designers have to balance the wing shapes against two dangers: a very smooth wing cuts fast through the air, but doesn't generate much lift. A more aggressive wing can generate a lot of upward force, but also requires much more power to push through the air! They can use computer simulations to find the optimum shape between these two extremes.

Answer 5:

There are two parts to your answer. 1) Why is the air on the top side of a wing less dense and than the air on the bottom of the wing? 2) How do two molecules separated by the wing moving through the air meet on the other side.

1) The lift generated by a wing (force that pushes a plane into the sky) has been described by two scientists. Newton and Bernoulli, both were very smart and both created mathematical equations to describe how air flow over a wing creates lift. Both were right, however they took two different ways to get to the same answer. Both answers are tricky to explain exactly without math that you will learn about in ~ 7th grade. A better explanation for you maybe that while a wing on an plane is being pushed forward by the engines through still air, some of the air that meets the front edge of the wing gets pushed under the wing and some air is pushed above it. Because the wing is at an angle where the front edge is higher than the back edge, more air is pushed below the wing than above the wing. The more air that is pushed down the more the plane is pushed up. While this pushing of air is going on, the smaller portion of air that was pushed over the top of the wing has fewer other air molecules to bump into since some of the air was pushed under the wing. This means the air above the wing is now less dense, and the air below more dense. The difference in density helps make it easier to push the plane up. Also, if gas molecules, like air, are less dense they tend to speed up since they don't have other molecules to bump into and slow them down. So, the molecules on the top of the wing move faster than the ones on the bottom of the wing.

2) You can see that it takes many words to explain lift without math. One way that can show how lift works is to say two molecules separated by the wing, one goes faster than the other and one goes slower and they meet again after the wing has past by. This picture really isn't physically true. On average the air on the top of the wing is traveling faster and the air on the bottom is slower, but the chances of two molecules that were next to each other on one side "meeting up" on the other is very slim.

I hope this helped.

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