# how does airplane flies .?

i mean why it does not falls down when in the air.

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You're probably going to get a number of different looking answers to that question. The fact is, aeronautical engineers don't have a fully-general answer. They know that in order to lift the airplane up, it's wings must push the air down. They know that certain wing cross sections are better at pushing the air down under certain conditions, while other cross sections work better under other conditions. They have mathematical models that work (within some narrow range of parameters) for predicting how well a new type of wing will work. They have, literally, a hundred years-worth of that kind of knowledge. But they don't have one, undisputed, fully-general theory of how air flows over an aircraft's wings and body and other surfaces.*

Here's a test you can apply to separate the wheat from the chaff. Have you ever seen an airplane fly upside down? If you haven't, then that means you've never been to see an air show. You should go some time. It's fun. A skilled pilot can fly an airplane upside down. Not just loop-the-loop, but I'm talking about straight and level inverted flight.

According to some of the theories that some people will give you, it should be impossible for an airplane to fly up-side down. Who are you going to believe? Them? or your own lying eyes? Be especially skeptical of any theory that has the word "Bernoulli" in it. Look at it and ask yourself, If this theory is true, can airplanes fly up-side down? If the answer is no, then throw that one out.

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* There's not much incentive for anybody to even LOOK for a theory like that anymore. Computer simulations (virtual wind tunnels) that model the behavior of air-flow down to the level of individual molecules, now exist. Engineers can now build and test whole virtual airplanes without ever getting up from their desk.

When you look at at cross-section of an airplane wing you can distinguish two streams of air: one above the wing and one below the wing. Because the top side of the wing is curved, this route is longer for the air then the route below the wing, but because the wing flies through the air, the air takes the same time to pass these two routes. With a larger distance to travel with the same time, the air will go faster at the top of the wing.

Now knowing that we can take Bernoulli's formula for pressure of gasses and liquids:

letters: Pa + rho*g*h + rho*v^2 = constant

words: Atmospheric pressure + density times gravitation acceleration times height + density times the square of speed is constant.

The Pa + rho*g*h is the static term, and the rho*v^2 the dynamical.

At the top of the wing the speed of air is higher then below the wing, so the dynamical term, rho*v^2 is there larger as the dynamical term under the wing. If you accept that the difference in height is very small above and under the wing, so rho*g*h above = rho*g*h below, then the atmospheric pressure must decrease at the top of the wing because the static+dynamic = constant, dynamic increasing, static decreasing. Dynamical pressure doesn't excert a force on the wing, but static is, so the pressure at the top of the wing is decreased. With the pressure at the bottom of the wing still the same (or less decreased then the top of the wing), the total force is directed above and thus the plane begins to fly.

Source(s): http://en.wikipedia.org/wiki/Bernoulli%27s_princip... Applied Physics classes :-)

I grew up learning it was the Bernoulli effect, that faster air flow over the top of the wing created a little bit of vacuum. Here's a discussion of that effect.

http://www.allstar.fiu.edu/aero/fltmidfly.htm

But many people say this is wrong. If you angle something flat into the wind, the wind will lift it, simply because the wind is blowing on the bottom, not the top.

The actual story is more complicated, a combination of several effects. That same website gives a pretty complete explanation here.

http://www.allstar.fiu.edu/aero/airflylvl3.htm

The basic idea is that if the plane pushes air down, then by Newton's laws there will be an equal and opposite force up (lift).