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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
100
SHEAR
STRESS
is
such
that,
when
material
collapses
under
it,
one
part
slides
over
the
other.
Example:
all
the
locking
pins.
Some
of
the
bolts
are
also
in
shear
or
"sideways"
stress,
owing
to
lugs
under
their
heads
and
from
which
wires
are
taken.
Such
a
wire,
exerting
a
sideways
pull
upon
a
bolt,
tries
to
break
it
in
such
a
way
as
to
make
one
piece
of
the
bolt
slide
over
the
other
piece.
TORSION.--This
is
a
twisting
stress
compounded
of
compression,
tension,
and
shear
stresses.
Example:
the
propeller
shaft.
NATURE
OF
WOOD
UNDER
STRESS.--Wood,
for
its
weight,
takes
the
stress
of
compression
far
better
than
any
other
stress.
For
instance:
a
walking-stick
of
less
than
1
lb.
in
weight
will,
if
kept
perfectly
straight,
probably
stand
up
to
a
compression
stress
of
a
ton
or
more
before
crushing;
whereas,
if
the
same
stick
is
put
under
a
bending
stress,
it
will
probably
collapse
to
a
stress
of
not
more
than
about
50
lb.
That
is
a
very
great
difference,
and,
since
weight
is
of
the
greatest
importance,
the
design
of
an
aeroplane
is
always
such
as
to,
as
far
as
possible,
keep
the
various
wooden
parts
of
its
construction
in
direct
compression.
Weight
being
of
such
vital
importance,
and
designers
all
trying
to
outdo
each
other
in
saving
weight,
it
follows
that
the
factor
of
safety
is
rather
low
in
an
aeroplane.
The
parts
in
direct
compression
will,
however,
take
the
stresses
safely
provided
the
following
conditions
are
carefully
observed.
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