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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
129
The
above
method
is
rather
futile,
as
it
does
not
test
for
the
balance
of
centrifugal
force,
which
comes
into
play
as
soon
as
the
propeller
revolves.
It
can
be
tested
as
follows:
The
propeller
must
be
in
a
horizontal
position,
and
then
weighed
at
fixed
points,
such
as
A,
B,
C,
D,
E,
and
F,
and
the
weights
noted.
The
points
A,
B,
and
C
must,
of
course,
be
at
the
same
fixed
distances
from
the
centre
of
the
propeller
as
the
points
D,
E,
and
F.
Now
reverse
the
propeller
and
weigh
at
each
point
again.
Note
the
results.
The
first
series
of
weights
should
correspond
to
the
second
series,
thus:
Weight
A
should
equal
weight
F.
Weight
B
should
equal
weight
E.
Weight
C
should
equal
weight
D.
There
is
no
standard
practice
as
to
the
degree
of
error
permissible,
but
if
there
are
any
appreciable
differences
the
propeller
is
unfit
for
use.
5.
SURFACE
AREA.--The
surface
area
of
the
blades
should
be
equal.
Test
with
calipers
thus:
The
distance
A--B
should
equal
K--L.
The
distance
C--D
should
equal
I--J.
The
distance
E--F
should
equal
G--H.
The
points
between
which
the
distances
are
taken
must,
of
course,
be
at
the
same
distance
from
the
centre
in
the
case
of
each
blade.
There
is
no
standard
practice
as
to
the
degree
of
error
permissible.
If,
however,
there
is
an
error
of
over
1/8
inch,
the
propeller
is
really
unfit
for
use.
6.
CAMBER.--The
camber
(curvature)
of
the
blades
should
be
(1)
equal,
(2)
decrease
evenly
towards
the
tips
of
the
blades,
and
(3)
the
greatest
depth
of
the
curve
should,
at
any
point
of
the
blade,
be
approximately
at
the
same
percentage
of
the
chord
from
the
leading
edge
as
at
other
points.
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