as a very strong structure.
The airframe of a fixed-wing aircraft consists of the
following five major units:
1. Fuselage
2. Wings
3. Stabilizers
4. Flight controls surfaces
5. Landing gear
A rotary-wing aircraft consists of the following
four major units:
1. Fuselage
2. Landing gear
3. Main rotor assembly
4. Tail rotor assembly
You need to be familiar with the terms used for
aircraft construction to work in an aviation rating.
STRUCTURAL STRESS
LEARNING OBJECTIVE: Identify the five
basic stresses acting on an aircraft.
The primary factors to consider in aircraft
structures are strength, weight, and reliability. These
factors determine the requirements to be met by any
material used to construct or repair the aircraft.
Airframes must be strong and light in weight. An
aircraft built so heavy that it couldn't support more than
a few hundred pounds of additional weight would be
useless. All materials used to construct an aircraft must
be reliable. Reliability minimizes the possibility of
dangerous and unexpected failures.
Many forces and structural stresses act on an
aircraft when it is flying and when it is static. When it is
static, the force of gravity produces weight, which is
supported by the landing gear. The landing gear absorbs
the forces imposed on the aircraft by takeoffs and
landings.
During
flight,
any
maneuver
that
causes
acceleration or deceleration increases the forces and
stresses on the wings and fuselage.
Stresses on the wings, fuselage, and landing gear of
aircraft are tension, compression, shear, bending, and
torsion. These stresses are absorbed by each component
of the wing structure and transmitted to the fuselage
structure. The empennage (tail section) absorbs the
same stresses and transmits them to the fuselage. These
stresses are known as loads, and the study of loads is
called a stress analysis. Stresses are analyzed and
considered when an aircraft is designed. The stresses
acting on an aircraft are shown in figure 4-1.
TENSION
Tension (fig. 4-1, view A) is defined as pull. It is the
stress of stretching an object or pulling at its ends.
Tension is the resistance to pulling apart or stretching
produced by two forces pulling in opposite directions
along the same straight line. For example, an elevator
control cable is in additional tension when the pilot
moves the control column.
COMPRESSION
If forces acting on an aircraft move toward each
other to squeeze the material, the stress is called
compression. Compression (fig. 4-1, view B) is the
opposite of tension. Tension is pull, and compression is
push. Compression is the resistance to crushing
produced by two forces pushing toward each other in
the same straight line. For example, when an airplane is
on the ground, the landing gear struts are under a
constant compression stress.
4-1 SHEAR
Cutting a piece of paper with scissors is an example
of a shearing action. In an aircraft structure, shear (fig.
4-1, view D) is a stress exerted when two pieces of
fastened material tend to separate. Shear stress is the
outcome of sliding one part over the other in opposite
directions. The rivets and bolts of an aircraft experience
both shear and tension stresses.
BENDING
Bending (fig. 4-1, view E) is a combination of
tension and compression. For example, when bending a
piece of tubing, the upper portion stretches (tension)
and the lower portion crushes together (compression).
The wing spars of an aircraft in flight are subject to
bending stresses.
TORSION
Torsional (fig. 4-1, view C) stresses result from a
twisting force. When you wring out a chamois skin, you
are putting it under torsion. Torsion is produced in an
engine crankshaft while the engine is running. Forces
that produce torsional stress also produce torque.
VARYING STRESS
All structural members of an aircraft are subject to
one or more stresses. Sometimes a structural member
has alternate stresses; for example, it is under
compression one instant and under tension the next.
The strength of aircraft materials must be great enough
to withstand maximum force of varying stresses.
SPECIFIC ACTION OF STRESSES
You need to understand the stresses encountered on
the main parts of an aircraft. A knowledge of the basic
stresses on aircraft structures will help you understand
why aircraft are built the way they are. The fuselage of
an aircraft is subject the fives types of stresstorsion,
bending, tension, shear, and compression.
Torsional stress in a fuselage is created in several
ways. For example, torsional stress is encountered in
engine torque on turboprop aircraft. Engine torque
tends to rotate the aircraft in the direction opposite to
the direction the propeller is turning. This force creates
a torsional stress in the fuselage. Figure 4-2 shows the
effect of the rotating propellers. Also, torsional stress
on the fuselage is created by the action of the ailerons
when the aircraft is maneuvered.
When an aircraft is on the ground, there is a
bending force on the fuselage. This force occurs
because of the weight of the aircraft. Bending increases
when the aircraft makes a carrier landing. This bending
action creates a tension stress on the lower skin of the
fuselage and a compression stress on the top skin.
Bending action is shown in figure 4-3. These stresses
are transmitted to the fuselage when the aircraft is in
flight. Bending occurs because of the reaction of the
airflow against the wings and empennage. When the
4-2
Figure 4-1.Five stresses acting on an aircraft. aircraft is in flight, lift forces act upward against the
wings, tending to bend them upward. The wings are
prevented from folding over the fuselage by the
resisting strength of the wing structure. The bending
action creates a tension stress on the bottom of the
wings and a compression stress on the top of the wings.
Q4-1.
The resistance to pulling apart or stretching
produced by two forces pulling in opposite
directions along the same straight lines is
defined by what term?
Q4-2.
The resistance to crushing produced by two
forces pushing toward each other in the same
straight line is defined by what term?
Q4-3.
Define the term shear as it relates to an
aircraft structure.
Q4-4.
Define the term bending.
Q4-5.
Define the term torsion.
CONSTRUCTION MATERIALS
LEARNING OBJECTIVE:
Identify the
various types of metallic and nonmetallic
materials used in aircraft construction.
An aircraft must be constructed of materials that
are both light and strong. Early aircraft were made of
wood. Lightweight metal alloys with a strength greater
than wood were developed and used on later aircraft.
Materials currently used in aircraft construction are
classified as either metallic materials or nonmetallic
materials.
4-3
TORSIONAL
STRESS
PROPELLER
ROTATION
ANfO4O2
Figure 4-2.Engine torque creates torsion stress in aircraft fuselages.
COMPRESSION
TENSION
ANf0403
Figure 4-3.Bending action occurring during carrier landing. METALLIC MATERIALS
The most common metals used in aircraft
construction are aluminum, magnesium, titanium,
steel, and their alloys.
Alloys
An alloy is composed of two or more metals. The
metal present in the alloy in the largest amount is called
the base metal. All other metals added to the base metal
are called alloying elements. Adding the alloying
elements may result in a change in the properties of the
base metal. For example, pure aluminum is relatively
soft and weak. However, adding small amounts or
copper, manganese, and magnesium will increase
aluminum's strength many times. Heat treatment can
increase or decrease an alloy's strength and hardness.
Alloys are important to the aircraft industry. They
provide materials with properties that pure metals do
not possess.
Aluminum
Aluminum alloys are widely used in modern
aircraft construction. Aluminum alloys are valuable
because they have a high strength-to-weight ratio.
Aluminum
alloys
are
corrosion
resistant
and
comparatively easy to fabricate. The outstanding
characteristic of aluminum is its lightweight.
Magnesium
Magnesium is the world's lightest structural metal.
It is a silvery-white material that weighs two-thirds as
much as aluminum. Magnesium is used to make
helicopters. Magnesium's low resistance to corrosion
has limited its use in conventional aircraft.
Titanium
Titanium is a lightweight, strong, corrosion-
resistant metal. Recent developments make titanium
ideal for applications where aluminum alloys are too
weak and stainless steel is too heavy. Additionally,
titanium is unaffected by long exposure to seawater and
marine atmosphere.
Steel Alloys
Alloy steels used in aircraft construction have great
strength, more so than other fields of engineering
would require. These materials must withstand the
forces that occur on today's modern aircraft. These
steels contain small percentages of carbon, nickel,
chromium, vanadium, and molybdenum. High-tensile
steels will stand stress of 50 to 150 tons per square inch
without failing. Such steels are made into tubes, rods,
and wires.
Another type of steel used extensively is stainless
steel. Stainless steel resists corrosion and is particularly
valuable for use in or near water.
NONMETALLIC MATERIALS
In addition to metals, various types of plastic
materials are found in aircraft construction. Some of
these plastics include transparent plastic, reinforced
plastic, composite, and carbon-fiber materials.
Transparent Plastic
Transparent
plastic
is
used
in
canopies,
windshields, and other transparent enclosures. You
need to handle transparent plastic surfaces carefully
because they are re