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clockwise, so there is more of a
turning moment if the left engine
fails, as the thrust from the
downgoing blade of the right
propeller is further away from the
longitudinal axis. The left engine in
this case is called the critical engine,
because its loss creates the most
adverse conditions. Later aircraft
Principles of Flight 165
(such as the Navajo Chieftain) have
contra-rotating propellers to deal
with this (that is, the right engine
rotates to the left). Watch for the
designation of parts for particular
engines with L or R so they go in the
right place. The other thing to note
is that the slipstream going back
over the flight controls will also be
asymmetric, and parts of some
controls will be more responsive
than others.
Slipstream results from rotating air
going round and round the fuselage
until it eventually hits the tail fin,
forcing it one way or the other (thus
causing yaw), depending on which
way round the propeller is going.
It can be reduced by offsetting the
fin, as most of an aeroplane's life is
spent in the cruise. Outside of that,
simply use rudder.
Remember that, although torque and
slipstream produce roll and yaw,
they ultimately produce yaw and roll
as secondary effects.
Propeller efficiency is the ratio of thrust
to brake horsepower, or the
difference between what power is
available from the engine to what is
actually used.
Geometric pitch is how far a propeller
should move forwards in one
rotation – the effective pitch is how far
it actually moves. The difference
between the two is propeller slip, and
is a measure of the efficiency or
otherwise of the process.
The pitch angle is that between the
blade's chord line and its plane of
rotation. The helix angle is between
the resultant airflow and the plane of
rotation. The surfaces of a propeller
are the thrust face on the rear, and
pressure face on the front.
Constant Speed Propeller
Otherwise known as a Variable Pitch
Propeller, this performs pretty much
the same function as the gearbox
does in a car, in that it "maintains
engine RPM over varying conditions
of road", or flight, in this case. The
gearbox (or constant speed prop) is
there because engines work best
within a certain range of RPM –
going too fast or too slow is not
good for them. In other words, a
constant speed propeller can have its
pitch adjusted for varying
conditions, to maintain a constant
angle of attack. Most are
hydraulically operated with a
centrifugal governor operating a control
valve that lets oil in to make the
pitch coarser or releases it for fine
pitch (best for takeoff). Coarse pitch
is used for the cruise because the
propeller blades move a longer
distance per rotation due to the
higher angle of attack.
The centrifugal twisting moment is a
component of centrifugal force that
tends turn the blades into fine pitch.
The aerodynamic twisting moment is the
opposite, tending to coarse pitch
(the blade's CP is in front of the
pitch change axis). They both act in
the same direction in a dive.
166 Canadian Professional Pilot Studies
If the engine fails, and feathering is
not available, select fully coarse.
Counterweights
These can be used to force the
blades into increasing their angle of
attack. The angle is reduced by oil
pressure, so when oil pressure is lost,
the blade angle will increase,
although there may be an automatic
feathering system.
With counterweights, therefore, loss
of oil pressure feathers the blades.
Otherwise, they go into full fine
pitch (exam question).
Now you have more choice – you
can operate at high RPM and low
manifold pressure, and vice versa.
Using lower RPM in the cruise (with
higher manifold pressure) helps fuel
consumption, since the engine is
going round fewer times per minute,
and the losses due to friction are less
than at high RPM. However, using
too much MP against low RPM will
damage the engine (check the flight
manual) and risk detonation.
Oil Control
Turboprops use oil controlled
systems – the power levers control
the torque and the prop levers
control pitch and RPM, very similar
to a piston. Higher oil pressure
creates flatter pitch and higher RPM,
so if an engine fails, and you lose oil
pressure, the blades will feather,
which is where you want them to be
　
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