Stringer Skin Nose rib Front spar Rear spar Ribs Ribs Figure 1-23.  Wing structure nomenclature. Wing Spars supply the necessary contour or shape to the airfoil. Although  the strict monospar wing is not common, this type of design  modified by the addition of false spars or light shear webs  along the trailing edge for support of control surfaces is  sometimes used. corrugated sheets. A combination of corrugated sheets on the  upper surface of the wing and stiffeners on the lower surface  is sometimes used. Air transport category aircraft often utilize  box beam wing construction.  The multispar wing incorporates more than one main  longitudinal member in its construction. To give the wing  contour, ribs or bulkheads are often included. The box beam type of wing construction uses two main  longitudinal members with connecting bulkheads to  furnish additional strength and to give contour to the wing.  [Figure 1-24]  A corrugated sheet may be placed between  the bulkheads and the smooth outer skin so that the wing  can better carry tension and compression loads. In some  cases, heavy longitudinal stiffeners are substituted for the  upper surface of the wing and stiffeners on the lower surface Spars are the principal structural members of the wing. They  correspond to the longerons of the fuselage. They run parallel  to the lateral axis of the aircraft, from the fuselage toward  the tip of the wing, and are usually attached to the fuselage  by wing fittings, plain beams, or a truss. Spars may be made of metal, wood, or composite materials  depending on the design criteria of a specific aircraft.  Wooden spars are usually made from spruce. They can be  generally classified into four different types by their cross- sectional configuration. As shown in  Figure 1-25,  they may  be (A) solid, (B) box-shaped, (C) partly hollow, or (D) in  Figure 1-24.  Box beam construction. 1-13