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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
117
The
board
is
cut
to
the
same
angle
as
that
specified
for
the
upward
inclination
of
the
surface
towards
its
wing-tips.
It
is
placed
on
the
spar
as
indicated
above,
and
it
is
provided
with
two
short
legs
to
raise
it
above
the
flanges
of
the
ribs
(which
cross
over
the
spars),
as
they
may
vary
in
depth.
A
spirit-level
is
then
placed
on
the
board,
and
the
wires
must
be
adjusted
to
give
the
surface
such
an
inclination
as
to
result
in
the
bubble
being
in
the
centre
of
the
level.
This
operation
must
be
performed
in
respect
of
each
bay
both
front
and
rear.
The
bays
must
then
be
diagonally
measured
as
already
explained.
YET
ANOTHER
METHOD
of
finding
the
dihedral
angle,
and
at
the
same
time
the
angle
of
incidence,
is
as
follows:
A
horizontal
line
is
taken
from
underneath
the
butt
of
each
spar,
and
the
set
measurement
is
either
the
angle
it
makes
with
the
spar,
or
a
fixed
measurement
from
the
line
to
the
spar
taken
at
a
specified
distance
from
the
butt.
This
operation
must
be
performed
in
respect
of
both
main
and
rear
spars,
and
all
the
bays
must
be
measured
diagonally
afterwards.
Whichever
method
is
used,
be
sure
that
after
the
job
is
done
the
spars
are
perfectly
straight.
STAGGER.--The
stagger
is
the
distance
the
top
surface
is
in
advance
of
the
bottom
surface
when
the
aeroplane
is
in
flying
position.
The
set
measurement
is
obtained
as
follows:
Plumb-lines
must
be
dropped
over
the
leading
edge
of
the
top
surface
wherever
struts
occur,
and
also
near
the
fuselage.
The
set
measurement
is
taken
from
the
front
of
the
lower
leading
edge
to
the
plumb-lines.
It
makes
a
difference
whether
the
measurement
is
taken
along
a
horizontal
line
(which
can
be
found
by
using
a
straight-edge
and
a
spirit-level)
or
along
a
projection
of
the
chord.
The
line
along
which
the
measurement
should
be
taken
is
laid
down
in
the
aeroplane's
specifications.
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