In order for us to understand these four forces thoroughly we must know how these forces originate. The first, and most important force acting on airplanes is that of lift.
Lift is the force that acts upward on the airplane. The wing, often referred to as an airfoil, is the primary source of lift for airplanes. More specifically, the shape of the wing is responsible for the lift produced by airplanes. An airfoils shape allows the pressure exerted by air to be greater on the lower surface of the wing than on the top surface. Lift is only produced after the pressures acting on the top and bottom surface of an airfoil leave the state of equilibrium. Higher pressure being placed on the bottom of an airfoil will result in a net upward force, which is referred to as lift. To better understand why the pressures acting on an airfoil can leave the state of equilibrium we will refer to a principle discovered by a Swiss physicist.
Daniel Bernoulli, the man for which Bernoulli's Principle is named, made a discovery which aviation is dependent upon. His principle simply says that, "as the velocity of a fluid increases, its internal pressure decreases" ("Private," 1996, p. 17). The fluid that we are concerned about in aviation is the air we breathe. Bernoulli's findings allowed engineers to manipulate the shape of an airfoil in order to create these unequal pressures. The shape of an airfoil is somewhat difficult to describe. ("Private," 1996).
An airfoil has a curve on both the top and bottom surfaces. These curves are often referred to as camber, the greater the curvature of the surface, the higher the camber("Private," 1996). Airfoils were, and are to this day, designed with much more camber in the top surface of the airfoil than there is in the bottom. We must note that lift itself is a function of velocity and is therefore absent without some source of velocity or speed. This means that an airplane must also have a source of thrust, which will be discussed shortly, in order for lift to be produced.