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.
Airplanes create thrust using propellers, jet engines , or rockets .
In the figure above, the thrust is being created with a propeller, which acts like a very powerful version of a household fan, pulling air past the blades. If you stick your hand out of a car window while moving, you will experience a very simple demonstration of this effect. 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 also reduce. We see another example of drag reduction when we watch downhill skiers in the Olympics.