How do Airplanes fly?

Two primary principles contribute to the creation of lift, which is what makes flight possible. Those two principles are Bernoulli's Principle and Newton's 3rd Law of Physics

Bernoulli's Principle

By definition, Bernoulli's Principle states: For an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in Pressure.

From a practical standpoint, this basically means that as a fluid (air, water, etc) moves faster, it's internal pressure decreases.

Well, let's think about this. Picture an airplane's wing - but cut in half so we can see the shape of it (referred to as an airfoil). The top of the wing is more curved than that of the bottom of the wing. The reasoning behind this is that the increased curvature on top of the wing will take advantage of something called magnus effect.

Let's define magnus effect in a nutshell. I'll do this through an example. Close your eyes and envision a baseball game. How does the pitcher get the ball to move in a desired direction? He or she can curve the direction of the ball's flight left, right, down, or even up if desired.

Well, magnus effect states that a rotating ball or cylinder moving through a fluid (air, water, etc) will create faster moving fluid in the direction of rotation, thus lowering pressure and "pulling" the ball or cylinder in that direction. This force is not created when the object is stationary, which is why a baseball pitcher puts a "spin" on the ball when he or she wants a curveball.

So we know the top of the wing is more curved than the bottom. But how does that have anything to do with magnus effect? Basically, the shaping of the wing "fools" the air around it into thinking it is a long rotation cylinder, and forces the air to travel faster over the top of the wing than that of the bottom. And according to Bernoulli's Principle, faster moving air = lower pressure. If we have lower pressure on top of the wing than we do on the bottom of the wing, we now have an inequality of pressures acting on the wing. There is more pressure pushing up on the bottom of the wing than there is on the top pushing down, which means we now have a total net force pushing UP. And voila… we have LIFT.

Newton's 3rd Law

This one is much less complicated than Bernoulli's Principle.

Newton's 3rd Law is defined as: To every action there is always an equal and opposite reaction.

Airplane wings are fixed onto the airframe of the plane at a slight angle. It may not be easy to see for the untrained eye, but upon a close examination of a wing's attachment point to the body of a plane, one will see a slight angle.

This angle creates a deflection of air downward. As air hits the underside of the wing, even in straight-and-level cruise flight, it is forced downward. And according to Newton's 3rd Law, the forcing of the air downward causes an equal and opposite upward force on the wing, thus contributing to the creation of lift.

Now, one thing worth mentioning about Newton's 3rd Law is that its contribution to lift is highly debated within the industry. Some say it only has a tiny effect, while others argue it has barely any effect.

I will say this much; without Newton's 3rd Law the creation of lift would be MUCH more difficult. It contributes greatly to the creation of lift. To make an example out of this, picture an aerobatic airplane in knife-edge flight (flying on its side). Bernoulli's Principle doesn't properly work in this configuration because the wing is at an unusual attitude. So I ask you this: how does the plane stay aloft? Newton's 3rd Law is the only explanation for such a flight scenario. As air is hitting the airframe and control surfaces of the plane, it is being forced downward at a great rate, creating an opposing force - LIFT - in the opposite direction.

Source: http://wiki.answers.com/Q/How_do_airplanes_fly