Figure 3-5.  Truss-type fuselage structure.  flight. There are numerous wing designs, sizes, and shapes  used by the various manufacturers. Each fulfills a certain need  with respect to the expected performance for the particular  airplane. How the wing produces lift is explained in Chapter  5, Aerodynamics of Flight.  Wings may be attached at the top, middle, or lower portion  of the fuselage. These designs are referred to as high-, mid-,  and low-wing, respectively. The number of wings can also  vary. Airplanes with a single set of wings are referred to as  monoplanes, while those with two sets are called biplanes.  [Figure 3-6]  Many high-wing airplanes have external braces, or wing  struts that transmit the flight and landing loads through the  struts to the main fuselage structure. Since the wing struts  are usually attached approximately halfway out on the wing,  this type of wing structure is called semi-cantilever. A few  high-wing and most low-wing airplanes have a full cantilever  wing designed to carry the loads without external struts.  The principal structural parts of the wing are spars, ribs,  and stringers.  [Figure 3-7]  These are reinforced by trusses,  I-beams, tubing, or other devices, including the skin. The  wing ribs determine the shape and thickness of the wing  (airfoil). In most modern airplanes, the fuel tanks are either  an integral part of the wing’s structure or consist of flexible  containers mounted inside of the wing.  Figure 3-4.  Airplane components.  Figure 3-3.  Center of gravity (CG).  3-4  Bulkhead  Stringers  Longerons  Struts  Longerons  Empennage  Powerplant  Fuselage  Wing  Landing gear  independent of airspeed  Nose-down force  dependent upon airspeed Nose-up force  Fixed Variable  Vertical forces acting on an airplane in flight.  CG  CL  Lift  CG too far aft  nose-down force  Insufficient elevator  If the CG is too far aft, there might not be enough elevator nose-down  force at the low stall airspeed to get the nose down for recovery.  Lift  CG  CL  CG too far forward  nose-up force  Insufficient elevator  If the CG is too far forward, there will not be enough elevator  nose-up force to flare the airplane for landing.  CG  Lift  CL 