The Basic Principles Behind Flight
In order for an airplane to be able to fly, thrust has to equal drag, and lift has to equal weight. If, for any reason, the amount of drag becomes larger than the amount of thrust, the plane will slow down. If the thrust is increased so that it is greater than the drag, the plane will speed up. Similarly, if the amount of lift drops below the weight of the airplane, the plane will descend. By increasing the lift, the pilot can make the airplane climb.
Thrust is an aerodynamic force that has to be created by an airplane in order to overcome the drag. Airplanes create thrust using propellers, jet engines or rockets.
Drag is an aerodynamic force that resists the forward motion of an airplane flying through the air. To understand how drag works, stick your hand out of a car window while moving. The amount of drag that your hand creates depends on a few factors, such as the size of your hand, the speed of the car and the density of the air. If you were to slow down, you would notice that the drag on your hand would decrease. This is why, after takeoff, a passenger jet always retracts its landing gear. The amount of drag produced by the landing gear of a jet is so great that, at cruising speed, the gear would be ripped right off of the plane!
This one is the easiest principles of flight to understand. Every object on earth has weight, and it is caused by the downward pull of gravity.
Lift is the aerodynamic force that holds an airplane in the air, and is created by air moving over the wings. The air moving over the top of the wing is moving faster than the air across the bottom, thus a lower pressure is created on the top of the wing. Because of this lower pressure, the airplane will be able to fly.