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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
128
The
propeller
should
be
tested
at
several
points,
about
a
foot
apart,
on
each
blade;
and
the
diagram,
provided
the
propeller
is
not
faulty,
will
then
look
like
this:
At
each
point
tested
the
actual
pitch
coincides
with
the
specified
pitch:
a
satisfactory
condition.
A
faulty
propeller
will
produce
a
diagram
something
like
this:
At
every
point
tested
the
pitch
angle
is
wrong,
for
nowhere
does
the
actual
pitch
coincide
with
the
specified
pitch.
Angles
A,
C,
and
D,
are
too
large,
and
B
is
too
small.
The
angle
should
be
correct
to
half
a
degree
if
reasonable
efficiency
is
to
be
maintained.
A
fault
in
the
pitch
angle
may
be
due
to
(1)
faulty
manufacture,
(2)
distortion,
or
(3)
the
shaft
hole
through
the
boss
being
out
of
position.
2.
STRAIGHTNESS.--To
test
for
straightness
the
propeller
must
be
mounted
upon
a
shaft.
Now
bring
the
tip
of
one
blade
round
to
graze
some
fixed
object.
Mark
the
point
it
grazes.
Now
bring
the
other
tip
round,
and
it
should
come
within
1/8
inch
of
the
mark.
If
it
does
not
do
so,
it
is
due
to
(1)
faulty
manufacture,
(2)
distortion,
or
(3)
to
the
hole
through
the
boss
being
out
of
position.
3.
LENGTH.--The
blades
should
be
of
equal
length
to
1/16
inch.
4.
BALANCE.--The
usual
method
of
testing
a
propeller
for
balance
is
as
follows:
Mount
it
upon
a
shaft,
which
must
be
on
ball-bearings.
Place
the
propeller
in
a
horizontal
position,
and
it
should
remain
in
that
position.
If
a
weight
of
a
trifle
over
an
ounce
placed
in
a
bolt-hole
on
one
side
of
the
boss
fails
to
disturb
the
balance,
then
the
propeller
is
usually
regarded
as
unfit
for
use.
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