• Weight—the combined load of the aircraft itself, the  crew, the fuel, and the cargo or baggage. Weight is  a force that pulls the aircraft downward because of  the force of gravity. It opposes lift and acts vertically  downward through the aircraft’s center of gravity (CG).  • The sum of all upward components of forces (not just  lift) equals the sum of all downward components of  forces (not just weight)  • The sum of all forward components of forces (not just  thrust) equals the sum of all backward components of  forces (not just drag)  In steady flight, the sum of these opposing forces is always  zero. There can be no unbalanced forces in steady, straight  flight based upon Newton’s Third Law, which states that for  every action or force there is an equal, but opposite, reaction  or force. This is true whether flying level or when climbing  or descending.  It does not mean the four forces are equal. It means the  opposing forces are equal to, and thereby cancel, the effects of  each other. In  Figure 5-1,  the force vectors of thrust, drag, lift,  and weight appear to be equal in value. The usual explanation  states (without stipulating that thrust and drag do not equal  weight and lift) that thrust equals drag and lift equals weight.  Although true, this statement can be misleading. It should be  understood that in straight, level, unaccelerated flight, it is  true that the opposing lift/weight forces are equal. They are  also greater than the opposing forces of thrust/drag that are  equal only to each other. Therefore, in steady flight:  This refinement of the old “thrust equals drag; lift equals  weight” formula explains that a portion of thrust is directed  upward in climbs and slow flight and acts as if it were lift  while a portion of weight is directed backward opposite to the  direction of flight and acts as if it were drag. In slow flight,  For an aircraft to start moving, thrust must be exerted and be  greater than drag. The aircraft continues to move and gain  speed until thrust and drag are equal. In order to maintain a  By using the aerodynamic forces of thrust, drag, lift, and  weight, pilots can fly a controlled, safe flight. A more detailed  discussion of these forces follows.  Discussions of the preceding concepts are frequently omitted  in aeronautical texts/handbooks/manuals. The reason is  not that they are inconsequential, but because the main  ideas with respect to the aerodynamic forces acting upon  an aircraft in flight can be presented in their most essential  elements without being involved in the technicalities of the  aerodynamicist. In point of fact, considering only level flight,  and normal climbs and glides in a steady state, it is still true  that lift provided by the wing or rotor is the primary upward  force, and weight is the primary downward force.  Another important concept to understand is angle of attack  (AOA). Since the early days of flight, AOA is fundamental to  understanding many aspects of airplane performance, stability,  and control. The AOA is defined as the acute angle between the  chord line of the airfoil and the direction of the relative wind.  In glides, a portion of the weight vector is directed along  the forward flight path and, therefore, acts as thrust. In other  words, any time the flight path of the aircraft is not horizontal,  lift, weight, thrust, and drag vectors must each be broken down  into two components.  thrust has an upward component. But because the aircraft is in  level flight, weight does not contribute to drag.  [Figure 5-2]  Thrust  Figure 5-1.  Relationship of forces acting on an aircraft.  Figure 5-2.  Force vectors during a stabilized climb.  5-2  Thrust  Drag  Weight Lift  Rearward component of weight  Thrust  Drag  Flight path Relative wind  Component of weight  opposed to lift  Lift  CL  CG 