Why doesn't gravity make planes and birds fall out of the sky

Well, no one has really answered you yet. The answer is a little complicated, but not sooo bad, and I think you should be given a chance to understand it.

You know, I think, that air is pressing down on us all the time. It is called atmospheric pressure. It is really just the effect of the weight of all the air that is above us in the atmosphere, just like water pressure gets greater when you go deeper in the sea. We are at the bottom of a sort of sea of air, and that air creates a pressure all around us.

You may have seen what happens when a can is filled with steam, then closed and allowed to cool, and how it collapses when the steam condenses to water and the atmospheric pressure outside crushes it.

Now, a wing on an aeroplane or bird has a special shape. It has a curved upper surface and a flat lower surface. That means that when it is moving forward, the air has to move faster over the top then when it moves under the bottom: that the same amount of air is stretched over the longer upper surface.

Since there is sort of less air for each length on the upper surface, the upper surface will have less air pressure than the lower surface, so the air below the wing will create a net upward force on the wing. And that is what we call "Lift" and that is how the plane flies. So you could say the the top surface of the wing sort of sucks the plane up.

It sounds unlikely, and in fact, even in to the late 30s, people were sceptical that this was REALLY what was going on. But in World War 2, when planes went out of control in dives, we found that it was the upper wing surfaces that were sort of burned.

This lifting effect is called the Bernoulli Effect. It is also responsible for the way roofs are blown (lifted) off of buildings, and even how fuel is pulled into an engines carburetor - used before fuel injection became popular.

http://inventors.about.com/library/inven…

Here is a 50 year old film about how planes fly. If you skip the first few minutes, it may not be the worst introduction,
http://www.youtube.com/watch?v=_LXW3pHNn…