longitudinal stability to the aircraft during flight, and they are of
semimonocoque construction.
PART B:
METAL QUALITIES AND STRESSES
GENERAL
Each of the structural parts discussed is designed to meet
requirements for that particular part.
One of the important
decisions to make in manufacturing, maintaining, and repairing the
parts is selecting the metal to be used.
Each metal or alloy has
properties and characteristics that make it desirable for a
particular use. However, if the metal has undesirable qualities, it
is the metallurgist's job to build up the desirable qualities and
tone down the undesirable ones. This is done by alloying (combining)
metals and by various metal-working processes.
It is not necessary
for the airframe repairman to be a metallurgist. But, because it is
advantageous to understand some metallurgical terms, this section
explains the terms used to describe metal qualities or properties.
The
quality
that
permits
relative
to
abrasion,
penetration, cutting action, or permanent distortion is hardness.
This property can be increased by working the metal and, in the case
of steel and certain aluminum alloys, by heat treatment and/or cold
working. Structural parts are often formed from metals in their soft
state and are then heat treated to harden them enough to develop the
strength necessary to retain their finished shape.
Hardness and
strength are closely associated metal properties.
The quality in a metal that permits breaking or cracking when the
metal is stressed is brittleness.
Brittle metal can break or crack
without changing shape. Because structural metals are often heavily
stressed, brittleness is an undesirable property.
Cast iron, cast
aluminum, and very hard steels are examples of brittle metals.
The property in a metal that permits it to be hammered, rolled, or
pressed into various shapes without the metal cracking or breaking is
malleability. This property is required in sheet metal to be worked
into curved shapes such as cowlings, fairings, and contoured skin.
Copper is an example of a malleable metal.
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