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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
125
THRUST-DRIFT
RATIO.--The
proportion
of
thrust
to
drift
is
of
paramount
importance,
for
it
expresses
the
efficiency
of
the
propeller.
It
is
affected
by
the
following
factors:
Speed
of
Revolution.--The
greater
the
speed,
the
greater
the
proportion
of
drift
to
thrust.
This
is
due
to
the
increase
with
speed
of
the
passive
drift,
which
carries
with
it
no
increase
in
thrust.
For
this
reason
propellers
are
often
geared
down
to
revolve
at
a
lower
speed
than
that
of
the
engine.
Angle
of
Incidence.--The
same
reasons
as
in
the
case
of
the
aeroplane
surface.
Aspect
Ratio.--Ditto.
Camber.--Ditto.
In
addition
to
the
above
factors
there
are,
when
it
comes
to
actually
designing
a
propeller,
mechanical
difficulties
to
consider.
For
instance,
the
blades
must
be
of
a
certain
strength
and
consequent
thickness.
That,
in
itself,
limits
the
aspect
ratio,
for
it
will
necessitate
a
chord
long
enough
in
proportion
to
the
thickness
to
make
a
good
camber
possible.
Again,
the
diameter
of
the
propeller
must
be
limited,
having
regard
to
the
fact
that
greater
diameters
than
those
used
to-day
would
not
only
result
in
excessive
weight
of
construction,
but
would
also
necessitate
a
very
high
undercarriage
to
keep
the
propeller
off
the
ground,
and
such
undercarriage
would
not
only
produce
excessive
drift,
but
would
also
tend
to
make
the
aeroplane
stand
on
its
nose
when
alighting.
The
latter
difficulty
cannot
be
overcome
by
mounting
the
propeller
higher,
as
the
centre
of
its
thrust
must
be
approximately
coincident
with
the
centre
of
aeroplane
drift.
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