Ever wondered how those magnificent steel birds keep themselves in the air?

Flying an airplane is a delicate balancing act. The pilot is responsible for balancing all four of the major forces which have an effect.

Thrust

The first force is thrust. Thrust is created by the engine or the propeller, which scoops air from the front of the plane, and forces it toward the rear. This creates air an area of high pressure behind the propeller, and an area of low pressure in front. The faster the propeller spins, the greater the difference in pressure. If the pressure behind the propeeler becomes strong enough, the plane is forced forward.

The plane will only continue to accelerate as long as the thrust is greater than the drag. When the thrust and drag are equal, the plane will maintain a constant airspeed.

Drag

Drag is the resistance created by air particles striking the plane. It pushes against the plane and counter balances the effect of thrust. The amount of drag is proportional to the amount of thrust. If an aircraft moves twice as fast, there is four times as much drag.

Lift

Much like thrust, lift is created by a difference in air pressure.

The wings of a plane are curved on top, and flat underneath. This means that as the wings push through the air, the air is split. The air that passes over the top of the wing moves faster than the air underneath, making the air above thinner. Because the air under the wing is more dense, it pushes up on the wing, and therefore the rest of the plane as well.

Gravity

Isaac Newton is given the credit for first discovering gravity. And just like the apple that fell on Newton's head, if the lift generated by the wings becomes insufficient, the aircraft will fall. But in normal flight, gravity serves to counter-act the lift of the wings, allowing the plane to maintain controlled level flight.

By understanding how these forces affect each other, and how they work together, you can understand how an aircraft stays in the air.