Figure 3-1.  The four forces.  Figure 3-2.  Illustrates the pitch, roll, and yaw motion of the aircraft along the lateral, longitudinal, and vertical axes, respectively.  Weight is the combined load of the aircraft itself, the crew,  the fuel, and the cargo or baggage. Weight 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).  Lift opposes the downward force of weight, is produced by  the dynamic effect of the air acting on the wing, and acts  perpendicular to the flight path through the wing’s center  of lift (CL).  An aircraft moves in three dimensions and is controlled by  moving it about one or more of its axes. The longitudinal,  or roll, axis extends through the aircraft from nose to tail,  with the line passing through the CG. The lateral or pitch  axis extends across the aircraft on a line through the wing  tips, again passing through the CG. The vertical, or yaw, axis  passes through the aircraft vertically, intersecting the CG. All  control movements cause the aircraft to move around one or  more of these axes and allows for the control of the aircraft  in flight.  [Figure 3-2]  The wings are airfoils attached to each side of the fuselage  and are the main lifting surfaces that support the airplane in  The fuselage is the central body of an airplane and is designed  to accommodate the crew, passengers, and cargo. It also  provides the structural connection for the wings and tail  assembly. Older types of aircraft design utilized an open truss  structure constructed of wood, steel, or aluminum tubing.  [Figure 3-5]  The most popular types of fuselage structures  used in today’s aircraft are the monocoque (French for  “single shell”) and semimonocoque. These structure types  are discussed in more detail under aircraft construction later  in the chapter.  Although airplanes are designed for a variety of purposes, most  of them have the same major components.  [Figure 3-4]  The  overall characteristics are largely determined by the original  design objectives. Most airplane structures include a fuselage,  wings, an empennage, landing gear, and a powerplant.  One of the most significant components of aircraft design is  CG. It is the specific point where the mass or weight of an  aircraft may be said to center; that is, a point around which,  if the aircraft could be suspended or balanced, the aircraft  would remain relatively level. The position of the CG of  an aircraft determines the stability of the aircraft in flight.  As the CG moves rearward (towards the tail), the aircraft  becomes more and more dynamically unstable. In aircraft  with fuel tanks situated in front of the CG, it is important  that the CG is set with the fuel tank empty. Otherwise, as the  fuel is used, the aircraft becomes unstable.  [Figure 3-3]  The  CG is computed during initial design and construction and  is further affected by the installation of onboard equipment,  aircraft loading, and other factors.  Major Components  Fuselage  Wings  3-3  Lift  W eight  Drag  Thrust  Yawing Rolling  Pitching  Lateral axis  Longitudinal axis  Vertical axis 