Aerodynamics of Flight  Chapter 5  Forces Acting on the Aircraft  Thrust, drag, lift, and weight are forces that act upon all  aircraft in flight. Understanding how these forces work and  knowing how to control them with the use of power and  flight controls are essential to flight. This chapter discusses  the aerodynamics of flight—how design, weight, load factors,  and gravity affect an aircraft during flight maneuvers.  The four forces acting on an aircraft in straight-and-level,  unaccelerated flight are thrust, drag, lift, and weight. They  are defined as follows:  •  Thrust—the forward force produced by the powerplant/  propeller or rotor. It opposes or overcomes the force  of drag. As a general rule, it acts parallel to the  longitudinal axis. However, this is not always the case,  as explained later.  •  Drag—a rearward, retarding force caused by disruption  of airflow by the wing, rotor, fuselage, and other  protruding objects. As a general rule, drag opposes  thrust and acts rearward parallel to the relative wind.  •  Lift—is a force that is produced by the dynamic effect  of the air acting on the airfoil, and acts perpendicular  to the flight path through the center of lift (CL) and  perpendicular to the lateral axis. In level flight, lift  opposes the downward force of weight. 