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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
132
For
a
given
pitch,
the
gap
of
a
four-bladed
propeller
is
only
half
that
of
a
two-bladed
one.
Therefore
the
four-bladed
propeller
is
only
suitable
for
large
pitch,
as
such
pitch
produces
spirals
with
a
large
gap,
thus
offsetting
the
decrease
in
gap
caused
by
the
numerous
blades.
The
greater
the
speed
of
rotation,
the
less
the
pitch
for
a
given
aeroplane
speed.
Then,
in
order
to
secure
a
large
pitch
and
consequently
a
good
gap,
the
four-bladed
propeller
is
usually
geared
to
rotate
at
a
lower
speed
than
would
be
the
case
if
directly
attached
to
the
engine
crank-shaft.
[Footnote
18:
I
have
heard
of
temporary
ones
being
made
quickly
by
bending
strips
of
lead
over
the
convex
side
of
the
blade,
but
I
should
think
it
very
difficult
to
secure
a
sufficient
degree
of
accuracy
in
that
way.]
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