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165
The Aeroplane Speaks. Fifth Edition
·
H. Barber
124
INEFFICIENT
CONTROL
is
probably
due
to
(1)
wrong
setting
of
control
surfaces,
(2)
distortion
of
control
surfaces,
or
(3)
control
cables
being
badly
tensioned.
WILL
NOT
"TAXI"
STRAIGHT.--If
the
aeroplane
is
uncontrollable
on
the
ground,
it
is
probably
due
to
(1)
alignment
of
undercarriage
being
wrong,
or
(2)
unequal
tension
of
shock
absorbers.
CHAPTER
IV
THE
PROPELLER,
OR
"AIR-SCREW"
The
sole
object
of
the
propeller
is
to
translate
the
power
of
the
engine
into
thrust.
The
propeller
screws
through
the
air,
and
its
blades,
being
set
at
an
angle
inclined
to
the
direction
of
motion,
secure
a
reaction,
as
in
the
case
of
the
aeroplane's
lifting
surface.
This
reaction
may
be
conveniently
divided
into
two
component
parts
or
values,
namely,
Thrust
and
Drift
(see
illustration
overleaf).
The
Thrust
is
opposed
to
the
Drift
of
the
aeroplane,
and
must
be
equal
and
opposite
to
it
at
flying
speed.
If
it
falls
off
in
power,
then
the
flying
speed
must
decrease
to
a
velocity,
at
which
the
aeroplane
drift
equals
the
decreased
thrust.
The
Drift
of
the
propeller
may
be
conveniently
divided
into
the
following
component
values:
Active
Drift,
produced
by
the
useful
thrusting
part
of
the
propeller.
Passive
Drift,
produced
by
all
the
rest
of
the
propeller,
i.e.,
by
its
detrimental
surface.
Skin-Friction,
produced
by
the
friction
of
the
air
with
roughness
of
surface.
Eddies
attending
the
movement
of
the
air
caused
by
the
action
of
the
propeller.
Cavitation
(very
marked
at
excessive
speed
of
revolution).
A
tendency
of
the
propeller
to
produce
a
cavity
or
semi-vacuum
in
which
it
revolves,
the
thrust
decreasing
with
increase
of
speed
and
cavitation.
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