VARYING STRESS
straight line. While an airplane is on the ground, the
landing gear struts are under a constant compression
All materials are somewhat elastic. A rubberband is
stress.
extremely elastic, whereas a piece of metal is not very
elastic. All the structural members of an aircraft
Shear
experience one or more stresses. Sometimes a
Cutting a piece of paper with a pair of scissors is an
structural member has alternate stresses. It is under
example of shearing action. Shear in an aircraft
compression one instant of time and under tension the
structure is a stress exerted when two pieces of fastened
next. The strength of aircraft materials must be great
material tend to separate. Shear stress is the outcome of
enough to withstand maximum force of varying
sliding one part over the other in opposite directions.
stresses.
The rivets and bolts in an aircraft experience both shear
SPECIFIC ACTION OF STRESSES
and tension stresses.
You should understand the stresses encountered on
Bending
the main parts of an aircraft. A knowledge of the basic
Bending is a combination of tension and
stresses on aircraft structures helps you understand why
compression. Consider the bending of an object such as
aircraft are built the way they are. The fuselage of the
a piece of tubing. The upper portion stretches (tension)
aircraft encounters the five types of stress--torsion,
and the lower portion crushes together (compression).
bending, tension, shear, and compression.
The wing spars of an aircraft in flight undergo bending
Torsional stress in a fuselage is created in several
stresses.
ways. An example of this stress is encountered in
engine torque on turboprop aircraft. Engine torque
Torsion
tends to rotate the aircraft in the direction opposite to
that in which the propeller is turning. This force creates
Torsional stresses are the result of a twisting force.
a torsional stress in the fuselage. Figure 2-23 shows the
When you wring out a chamois skin, you are putting it
effect of the rotating propellers. Another example of
under torsion. Torsion is produced in an engine
torsional stress is the twisting force in the fuselage due
crankshaft while the engine is running. Forces that
to the action of the ailerons when the aircraft is
cause torsional stresses produce torque.
maneuvered.
Figure 2-23.--Engine torque creates torsional stress in aircraft fuselages.
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