Spars are the principal structural members of the wing.
They
correspond to the longerons of the fuselage. Spars run from the base
of the wing toward the tip and are usually attached to the fuselage
by wing fittings, plain beams, or part of a truss system. The I-beam
construction for a spar consists of a web, a deep wall plate, and
capstrips. These are either extrusions or formed angles, as shown at
Detail A in Figure 1-7. The web, the principal depth portion of the
spar, is attached to the capstrips that carry the loads caused by the
wings bending. When joined, the web and capstrips form a foundation
for attaching the skin.
Stiffeners give additional strength to the
spar structure.
These may be beads pressed into the web or
extrusions or formed angles riveted to the web vertically or
diagonally.
Wing Ribs.
In the framework of a wing, ribs are the crosspieces
running from the leading edge to the trailing edge of the wing. The
ribs give the wing its contour and shape and transmit the load from
the skin to the spars.
Ribs are also used in ailerons, elevators,
fins, and stabilizers.
Figure 1-7 shows three general rib
constructions: the former, reinforced, and truss ribs. Each type is
discussed separately in the following paragraphs.
Former ribs, located at frequent intervals throughout the wing, are
made of formed sheet metal and are very lightweight.
The bent-up
portion of a former rib is the flange and the vertical portion is the
web.
The latter is generally made with beads pressed between the
lightening holes.
These holes lessen the rib's weight without
decreasing its strength. Lightening hole area rigidity is ensured by
flanging the edges of the holes.
The reinforced rib is similar in construction to the spar,
consisting of upper and lower capstrips joined by a web plate.
Vertical and diagonal angles between the capstrips reinforce the web
plate. The reinforced rib is used more frequently than the truss rib.
Vertical and diagonal cross members only are used to reinforce and
join the capstrips in constructing truss ribs. These and reinforced
ribs are heavier than former ribs and are used only at points where
the greatest stresses are imposed.
Empennage. The aft end of the fuselage, or tail section
of the aircraft, includes the rudder or rudders,
elevators, stabilizers, and trim tabs, and it is called
the empennage.
Figure 1-8 shows the empennage construction.
Airplane stabilizing units consist of vertical and horizontal
surfaces at the aft end of the fuselage.
In many respects,
construction features are identical with those of wings. Empennage
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