AIRCRAFT
STRUCTURES
Start from Lesson 1: Principle of Airframe
Go to Lesson 2 : PRINCIPLES OF AERODYNAMICS
Go to Lesson 3 : AIRFOIL CHARACTERISTICS
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An airplane has six main parts—fuselage, wings, stabilizer (or tail plane), rudder, one or more engines, and landing gear.
The
fuselage is the main body of the machine, customarily streamlined in form. It
usually contains control equipment, and space for passengers and cargo. The
wings are the main supporting surfaces. Modern airplanes are monoplanes
(airplanes with one wing) and may be high-wing, mid-wing, or low-wing (relative
to the bottom of the fuselage). At the trailing edge of the wings are auxiliary
hinged surfaces known as ailerons that are used to gain longitudinal control
and to turn the airplane.
The lift of an airplane, or the force that supports it
in flight, is basically the result of air moving over the surfaces of the
wings. The lift varies with velocity (airspeed); there is a minimum velocity at
which flight can be maintained. This is known as the stall speed. Because speed
is so important to maintain lift, objects such as fuel tanks and engines, that
are carried outside the fuselage are enclosed in structures called nacelles, or
pods, to reduce air drag (the retarding force of the air as the airplane moves
through it).
Directional stability is provided by the tail fin, a
fixed vertical airfoil at the rear of the plane. The stabilizer, or tail plane,
is a fixed horizontal airfoil at the rear of the airplane used to suppress
undesired pitching motions. To the rear of the stabilizer are usually hinged
the elevators, movable auxiliary surfaces that are used to produce controlled
pitching. The rudder, generally at the rear of the tail fin, is a movable
auxiliary airfoil that gives the craft a yawing (turning about a vertical axis)
movement in normal flight. The rear array of airfoils is called the empennage,
or tail assembly. Some aircraft have additional flaps near the ailerons that
can be lowered during takeoff and landing to augment lift at the cost of
increased drag. On some airplanes hinged controls are replaced or assisted by
spoilers, which are ridges that can be made to project from airfoils.
Airplane engines may be classified as driven by propeller, jet, turbojet, or rocket. Most engines originally were of the internal-combustion, piston-operated type, which may be air- or liquid-cooled. During and after World War II, duct-type and gas-turbine engines became increasingly important, and since then jet propulsion has become the main form of power in most commercial and military aircraft. The landing gear is the understructure that supports the weight of the craft when on the ground or on the water and that reduces the shock on landing.
The airframe components are constructed from a wide
variety of materials and are joined by rivets, bolts, screw, and welding or
adhesives. The aircraft components are composed of various parts called
structural members. Aircraft structural members are designed to carry a load or
to resist stress. A single member of the structure may be subjected to a
combination of stresses. In most cases the structural members are designed to
carry end loads rather than side loads; that is, to be subjected to tension or
compression rather than bending.
Strength may be the principal requirement in certain
structures, while others need entirely different qualities. For example,
cowling, fairing, and similar parts usually are not required to carry the
stresses imposed by flight or landing loads. However, these parts must have
such properties as neat appearance and streamlined shapes. Every square inch of
a wing and fuselage, every rib, spar and even each metal fitting must be
constructed in relation to the physical characteristics of the metal from which
it is made. Every part of the aircraft must be planned to carry the load
imposed upon it. Although planning the design is not the function of the
mechanic, it is important to understand and appreciate the stresses involved in
order to avoid changes in the original design through improper repairs.
AIRCRAFT
FUSELAGE.
See Figure 1-31. The word fuselage originated during
the early developments in French aviation and is still used to designate the
body portion of an aircraft. The original purpose of the fuselage was to
support the empennage for directional control and stability. Later, the
fuselage was used as a primary structure to which all other components, such as
engines, landing gear, and wings were attached. This is still the primary
function of the fuselage.
The second
purpose of the fuselage is to carry the crew and cargo. The type of
configuration is often determined by the basic mission of the aircraft. The
three types of fuselage construction are the truss, monocoque, and the
semi-monocoque.
Figure 1-31, Major Structural Components of Cargo Type Aircraft
Construction
style:
AIRCRAFT
TRUSS.
See Figure 1-32, in architecture and engineering, a
supporting structure or framework composed of beams, girders, or rods commonly
of steel or wood lying in a single plane. A truss usually takes the form of a
triangle or combination of triangles, since this design ensures the greatest
rigidity. Trusses are used for large spans and heavy loads, especially in
bridges and roofs. Their open construction is lighter than, yet just as strong
as, a beam with a solid web between upper and lower lines. The members are
known as tie-beams, posts, rafters, and struts; the distance over which the
truss extends is called the span. The upper and lower lines or beams are
connected by web members.
Figure
1-32, Fuselage Construction
AIRCRAFT MONOCOQUE CONSTRUCTION.
The monocoque type of fuselage construction is like a
shell in which the skin of the fuselage carries the primary stresses. Rings and
bulkheads are used to shape the fuselage. Since no longitudinal bracing members
are present, the skin must have sufficient thickness and strength to keep the
fuselage rigid. Thus, weight becomes a problem. Most early aircraft featured
this type of construction.
SEMI-MONOCOQUE
CONSTRUCTION.
The semi-monocoque type of construction uses aluminum
alloy and is used in nearly all present-day aircraft. The fuselage members are
arranged so that each member carries part of the load.
Fuselage Structural Components
AIRCRAFT
LONGERONS.
The longerons are the longest and strongest structural
members of the fuselage and are used primarily to obtain strength. They run
from the nose of the fuselage to the empennage. In the total construction the
longerons hold the rings, formers, and bulkheads together. The rings, formers,
and bulkheads hold the stringers.
AIRCRAFT
STRINGERS.
Stringers are small, lightweight members running
lengthwise from the nose of the fuselage to the empennage. Stringers furnish
some strength but are mainly used for attachment of the skin. The skin of the
aircraft is fastened to the stringers by rivets, screws, and other fasteners.
AIRCRAFT
BULKHEADS.
Bulkheads give cross-sectional strength to the
fuselage and divide the aircraft into compartments bulkheads are the strongest
support members, set at a 90° angle to the longitudinal axis. Bulkheads that
separate pressurized compartments have doors with seals to prevent pressure
loss. Bulkheads also have openings for conduits, hoses, lines, and control
cables to pass through.
AIRCRAFT EMPENNAGE.
The empennage is the stabilizing group of surfaces
located at the aft section of the fuselage, and is designed to balance and
stabilize the aircraft during flight. The empennage has two main parts, the
horizontal stabilizer and the vertical stabilizer.
AIRCRAFT Horizontal stabilizer.
The horizontal
stabilizer is the horizontal tail surface which keeps the aircraft stable about
the lateral axis. Attached by hinges to its trailing edge are the elevators,
which control the up and down movement, or pitch, of the aircraft. On some
aircraft, the entire surface moves. This type of unit is called a stabilator
and is commonly found on fighter aircraft.
Figure 1-33, Empennage
AIRCRAFT
Vertical Stabilizer.
The vertical stabilizer, figure 1-33, gives the
aircraft positive stability about the vertical axis. The rudder is the hinged
flight control surface attached to the trailing edge of the vertical
stabilizer. It controls the side-to-side movement, or yaw, of the aircraft
(fishtailing effect).
Tags:
Aircraft
Construction, TRUSS, Fuselage Construction, MONOCOQUE CONSTRUCTION, LONGERONS,
STRINGERS, BULKHEADS, EMPENNAGE, Horizontal stabilizer, Vertical Stabilizer