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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
101
CONDITIONS
TO
BE
OBSERVED:
1.
All
the
spars
and
struts
must
be
perfectly
straight.
The
above
sketch
illustrates
a
section
through
an
interplane
strut.
If
the
strut
is
to
be
kept
straight,
i.e.,
prevented
from
bending,
then
the
stress
of
compression
must
be
equally
disposed
about
the
centre
of
strength.
If
it
is
not
straight,
then
there
will
be
more
compression
on
one
side
of
the
centre
of
strength
than
on
the
other
side.
That
is
a
step
towards
getting
compression
on
one
side
and
tension
on
the
other
side,
in
which
case
it
may
be
forced
to
take
a
bending
stress
for
which
it
is
not
designed.
Even
if
it
does
not
collapse
it
will,
in
effect,
become
shorter,
and
thus
throw
out
of
adjustment
the
gap
and
all
the
wires
attached
to
the
top
and
bottom
of
the
strut,
with
the
result
that
the
flight
efficiency
of
the
aeroplane
will
be
spoiled.
The
only
exception
to
the
above
condition
is
what
is
known
as
the
Arch.
For
instance,
in
the
case
of
the
Maurice
Farman,
the
spars
of
the
centre-section
plane,
which
have
to
take
the
weight
of
the
nacelle,
are
arched
upwards.
If
this
was
not
done,
it
is
possible
that
rough
landings
might
result
in
the
weight
causing
the
spars
to
become
slightly
distorted
downwards.
That
would
produce
a
dangerous
bending
stress,
but,
as
long
as
the
wood
is
arched,
or,
at
any
rate,
kept
from
bending
downwards,
it
will
remain
in
direct
compression
and
no
danger
can
result.
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