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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
126
MAINTENANCE
OF
EFFICIENCY.
The
following
conditions
must
be
observed:
1.
PITCH
ANGLE.--The
angle,
at
any
given
point
on
the
propeller,
at
which
the
blade
is
set
is
known
as
the
pitch
angle,
and
it
must
be
correct
to
half
a
degree
if
reasonable
efficiency
is
to
be
maintained.
This
angle
secures
the
"pitch,"
which
is
the
distance
the
propeller
advances
during
one
revolution,
supposing
the
air
to
be
solid.
The
air,
as
a
matter
of
fact,
gives
back
to
the
thrust
of
the
blades
just
as
the
pebbles
slip
back
as
one
ascends
a
shingle
beach.
Such
"give-back"
is
known
as
Slip.
If
a
propeller
has
a
pitch
of,
say,
10
feet,
but
actually
advances,
say,
only
8
feet
owing
to
slip,
then
it
will
be
said
to
possess
20
per
cent.
slip.
Thus,
the
pitch
must
equal
the
flying
speed
of
the
aeroplane
plus
the
slip
of
the
propeller.
For
example,
let
us
find
the
pitch
of
a
propeller,
given
the
following
conditions:
Flying
speed
...
70
miles
per
hour.
Propeller
revolutions
...
1,200
per
minute.
Slip
...
15
per
cent.
First
find
the
distance
in
feet
the
aeroplane
will
travel
forward
in
one
minute.
That
is--
369,600
feet
(70
miles)
-----------------------
=
6,160
feet
per
minute.
60
"
(minutes)
Now
divide
the
feet
per
minute
by
the
propeller
revolutions
per
minute,
add
15
per
cent.
for
the
slip,
and
the
result
will
be
the
propeller
pitch:
6,160
-----
+
15
per
cent.
=
5.903
feet.
1,200
In
order
to
secure
a
constant
pitch
from
root
to
tip
of
blade,
the
pitch
angle
decreases
towards
the
tip.
This
is
necessary,
since
the
end
of
the
blade
travels
faster
than
its
root,
and
yet
must
advance
forward
at
the
same
speed
as
the
rest
of
the
propeller.
For
example,
two
men
ascending
a
hill.
One
prefers
to
walk
fast
and
the
other
slowly,
but
they
wish
to
arrive
at
the
top
of
the
hill
simultaneously.
Then
the
fast
walker
must
travel
a
farther
distance
than
the
slow
one,
and
his
angle
of
path
(pitch
angle)
must
then
be
smaller
than
the
angle
of
path
taken
by
the
slow
walker.
Their
pitch
angles
are
different,
but
their
pitch
(in
this
case
altitude
reached
in
a
given
time)
is
the
same.
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