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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
127
In
order
to
test
the
pitch
angle,
the
propeller
must
be
mounted
upon
a
shaft
at
right
angles
to
a
beam
the
face
of
which
must
be
perfectly
level,
thus:
First
select
a
point
on
the
blade
at
some
distance
(say
about
2
feet)
from
the
centre
of
the
propeller.
At
that
point
find,
by
means
of
a
protractor,
the
angle
a
projection
of
the
chord
makes
with
the
face
of
the
beam.
That
angle
is
the
pitch
angle
of
the
blade
at
that
point.
Now
lay
out
the
angle
on
paper,
thus:
The
line
above
and
parallel
to
the
circumference
line
must
be
placed
in
a
position
making
the
distance
between
the
two
lines
equal
to
the
specified
pitch,
which
is,
or
should
be,
marked
upon
the
boss
of
the
propeller.
Now
find
the
circumference
of
the
propeller
where
the
pitch
angle
is
being
tested.
For
example,
if
that
place
is
2
feet
radius
from
the
centre,
then
the
circumference
will
be
2
feet
x
2
=
4
feet
diameter,
which,
if
multiplied
by
3.1416
=
15.56
feet
circumference.
Now
mark
off
the
circumference
distance,
which
is
represented
above
by
A--B,
and
reduce
it
in
scale
for
convenience.
The
distance
a
vertical
line
makes
between
B
and
the
chord
line
is
the
pitch
at
the
point
where
the
angle
is
being
tested,
and
it
should
coincide
with
the
specified
pitch.
You
will
note,
from
the
above
illustration,
that
the
actual
pitch
line
should
meet
the
junction
of
the
chord
line
and
top
line.
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