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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
103
6.
The
atmosphere
is
sometimes
much
damper
than
at
other
times,
and
this
causes
wood
to
expand
and
contract
appreciably.
This
would
not
matter
but
for
the
fact
that
it
does
not
expand
and
contract
uniformly,
but
becomes
unsymmetrical,
i.e.,
distorted.
I
have
already
explained
the
danger
of
that
in
condition
2.
This
should
be
minimized
by
well
varnishing
the
wood
to
keep
the
moisture
out
of
it.
FUNCTION
OF
INTERPLANE
STRUTS.--These
struts
have
to
keep
the
lifting
surfaces
or
"planes"
apart,
but
this
is
only
part
of
their
work.
They
must
keep
the
planes
apart,
so
that
the
latter
are
in
their
correct
attitude.
That
is
only
so
when
the
spars
of
the
bottom
plane
are
parallel
with
those
of
the
top
plane.
Also,
the
chord
of
the
top
plane
must
be
parallel
with
the
chord
of
the
bottom
plane.
If
that
is
not
so,
then
one
plane
will
not
have
the
same
angle
of
incidence
as
the
other
one.
At
first
sight
one
might
think
that
all
that
is
necessary
is
to
cut
all
the
struts
to
be
the
same
length,
but
that
is
not
the
case.
Sometimes,
as
illustrated
above,
the
rear
spar
is
not
so
thick
as
the
main
spar,
and
it
is
then
necessary
to
make
up
for
that
difference
by
making
the
rear
struts
correspondingly
longer.
If
that
is
not
done,
then
the
top
and
bottom
chords
will
not
be
parallel,
and
the
top
and
bottom
planes
will
have
different
angles
of
incidence.
Also,
the
sockets
or
fittings,
or
even
the
spars
upon
which
they
are
placed,
sometimes
vary
in
thickness
owing
to
faulty
manufacture.
This
must
be
offset
by
altering
the
length
of
the
struts.
The
best
way
to
proceed
is
to
measure
the
distance
between
the
top
and
bottom
spars
by
the
side
of
each
strut,
and
if
that
distance,
or
"gap"
as
it
is
called,
is
not
as
stated
in
the
aeroplane's
specifications,
then
make
it
correct
by
changing
the
length
of
the
strut.
This
applies
to
both
front
and
rear
interplane
struts.
When
measuring
the
gap,
always
be
careful
to
measure
from
the
centre
of
the
spar,
as
it
may
be
set
at
an
angle,
and
the
rear
of
it
may
be
considerably
lower
than
its
front.
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