There are four forces that act upon an aircraft during straight- and-level flight. They are lift, gravity, thrust, and drag. Lift  counters gravity, and drag counters thrust. When all four  forces are in balance, straight-and-level flight is sustained.  Engine-powered gliders obtain thrust from the engine. Once  in flight and the engine has been shut off, or the glider has  been launched, towed, or winched, the need to obtain thrust  is still there. The glider does this by converting the potential  energy that it has accumulated into kinetic energy as it  glides downward, trading height for distance. In essence, the  gravity vector becomes the horizontal forward thrust vector  component. We measure the force of gravity as the weight in  pounds or kilograms. This explains why the faster the glider  flies, the faster it also descends.  (lifting) reaction. The wing’s construction is designed to  take advantage of certain physical laws that generate two  actions from the air mass. One is a positive pressure lifting  action from the air mass below the wing, and the other is a  negative pressure lifting action from the lowered pressure  above the wing. As the airstream strikes the relatively flat lower surface of the  wing when inclined at a small angle to its direction of motion,  the air is forced to rebound downward, causing an upward  reaction in positive lift. At the same time, airstream striking  the upper curve section of the leading edge of the wing is  deflected upward, over the top of the wing. The increase in  airspeed on the top of the wing produces a sharp drop in  pressure. Associated with the lowered pressure is downwash,  a downward backward flow. In other words, a wing shaped to  cause an action on the air, and forcing it downward, provides  an equal reaction from the air, forcing the wing upward. If  a wing is constructed in such form that it causes a lift force  greater than the weight of the glider, the glider flies. If all the required lift were obtained from the deflection of air  by the lower surface of the wing, a glider would need only  a flat wing like a kite. This, of course, is not the case at all.  The balance of the lift needed to support the glider comes  from the flow of air above the wing. Herein lies the key to  flight. Lift is the result of the airflow above and over the wing  lowering the air pressure above the wing, which pull the wing  upwards and the downwash from below the wing pushing  the wing upward. This fact must be thoroughly understood  to continue in the study of flight. Figure 3-1  shows a basic vector diagram for an unpowered  glider with all forces in equilibrium. The lift vector is  effectively split into two components: one part is opposing  the weight force (gravity in straight-and-level flight), and  the other component of the lift vector opposes drag by  supplying thrust by the conversion of potential energy of  the elevated weight of the glider into kinetic energy. This  conversion continues until the airframe comes to rest on the  surface. A glider is always descending in the air. This allows  development of thrust by the energy conversion process.  The objective of a glider pilot is to remain in air rising faster  than the glider must descend to maintain flying speed. The  same is true for a powered aircraft with its engine turned  off. These forces are explained in greater detail in the Pilot’s  Handbook of Aeronautical Knowledge (FAA-H-8083-25)  and by examining Newton’s laws of motion. Lift opposes the downward force of weight (gravity) and is  produced by the dynamic effects of the surrounding airstream  acting on the wing. Lift acts perpendicular to the flightpath  through the wing’s center of lift. There is a mathematical  relationship between lift, angle of attack (AOA), airspeed,  altitude, and the size of the wing. In the lift equation, these  factors correspond to the coefficient of lift, velocity, air  density, and wing surface area. These relationships are  expressed in  Figure 3-2 . For a complete explanation of  the lift formula and terms refer to the Pilots Handbook of  Aeronautical Knowledge. This shows that for lift to increase, one or more of the  factors on the other side of the equation must increase. Lift  is proportional to the square of the velocity, or airspeed;  therefore, doubling airspeed quadruples the amount of lift if  everything else remains the same. Likewise, if other factors  remain the same while the coefficient or lift increases, lift  also increases. The coefficient of lift goes up as the AOA is  increased. As air density increases, lift increases. However,  According to Newton’s Third Law of Motion, for every  action there is an equal and opposite reaction. Thus, the  air that is deflected downward also produces an upward  Figure 3-1.  Vector components of lift, drag, and weight (gravity). Newton’s Third Law of Motion Lift 3-2 Forces of Flight a Lift Thrust Vertical Weight Flightpath a a Horizontal Drag Aircraft angle