Figure 1-4.—Wing construction. assembly  or  power  plant  with  the  engine  and  its accessories. Vibrations originating in the engine are transmitted to the aircraft structure through the engine mount. NACELLES In single-engine aircraft, the power plant is mounted in the center of the fuselage. On multiengined aircraft, nacelles are usually used to mount the power plants. The nacelle  is  primarily  a  unit  that  houses  the  engine. Nacelles are similar in shape and design for the same size aircraft. They vary with the size of the aircraft. Larger aircraft require less fairing, and therefore smaller nacelles. The structural design of a nacelle is similar to that of the fuselage. In certain cases the nacelle is designed  to  transmit  engine  loads  and  stresses  to  the wings through the engine mounts. WINGS The wings of an aircraft are designed to develop lift when  they  are  moved  through  the  air.  The  particular wing  design  depends  upon  many  factors  for  example, size, weight, use of the aircraft, desired landing speed, and desired rate of climb. In some aircraft, the larger compartments of the wings are used as fuel tanks. The wings are designated as right and left, corresponding to the right- and left-hand sides of a pilot seated in the aircraft. The wing structures of most naval aircraft are of an all-metal  construction,  usually  of  the  cantilever  design; that is, no external bracing is required. Usually wings are of the stress-skin type. This means that the skin is part of the basic wing structure and carries part of the loads and stresses. The internal structure is made of “spars and stringers” running spanwise, and “ribs and formers” running coordwise (leading edge to trailing edge). The spars are the main structural members of the wing, and are often referred to as “beams.” One method of wing construction is shown in figure 1-4. In this illustration, two main spars are used with ribs placed at frequent intervals between the spars to develop the wing contour. This is called “two-spar” construction. Other   variations   of   wing   construction   include "monospar (open spar), multispar (three or more spars), and  box  beam.”  In  the  box  beam  construction,  the stringers and sparlike sections are joined together in a box-shaped  beam.  Then  the  remainder  of  the  wing  is constructed  around  the  box. The skin is attached to all the structural members and carries part of the wing loads and stresses. During flight, the loads imposed on the wing structure act primarily on the skin. From the skin, the loads are transmitted to the ribs and then to the spars. The spars support  all  distributed  loads  as  well  concentrated weights, such as a fuselage, landing gear, and nacelle. Corrugated sheet aluminum alloy is often used as a subcovering for wing structures. The Lockheed P-3 Orion wing is an example of this type of construction. Inspection and access panels are usually provided on the lower surface of a wing. Drain holes are also placed in the lower surfaces. Walkways are provided on the areas of the wing where personnel should walk or step. The substructure is stiffened or reinforced in the vicinity of the walkways to take such loads. Walkways are  usually  covered  with  a  nonskid  surface.  Some aircraft  have  no  built-in  walkways.  In  these  cases removable mats or covers are used to protect the wing surface. On some aircraft, jacking points are provided 1-4