Airplanes are affected by four major forces when they fly. The first two have been explained here: lift and weight

Another one of the four forces of flight affecting airplanes is called drag. Drag occurs when a solid body moves through a field of particles (a liquid or gas fluid), such as when a swimmer makes a stroke through water, or when a plane moves through the air. This interaction creates friction, which slows the plane down. An airplane must generate more forward motion, called thrust, in order to overcome drag. For older planes, that thrust is generated by the turboprop engine, and in new planes, a jet engine.

On a plane, many things affect drag, including the size and shape of the plane, and the speed of the plane relative to the speed of the air around it. Slim and smooth shapes often reduce drag – many swimmers will shave their bodies and wear full-body wetsuits to increase the speed of their laps. Many airplanes are shaped like a missile because that shape is one of the most effective at reducing drag.

An object must be moving through a fluid in order to experience drag. An object that doesn’t move won’t experience drag, and an object won’t experience drag in an environment devoid of fluids, such as outer space.

Learn more about the forces of flight here: NASA Glenn Research Center