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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, the difference
between what 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.
Principles of Flight 27
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.
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.
The Helicopter
This is just a flying machine that has
its wings going round instead of
remaining still, cynically referred to
by some as 50,000 rivets in loose
formation – this means that the liftproducing
surfaces (i.e. rotors) are
separate from the body.
Another difference is that an
aeroplane engine is directly related to
the forward movement of the
aircraft, whereas the engine on a
helicopter isn't – its function is to
drive the rotor system which is really
what makes the machine move
through the air.
In fact, the rotors provide lift, thrust
and directional control in one go –
all three being separate functions on
an aeroplane. In some ways, this is
beneficial, since loss of an engine on
a twin does not need the strength of
a gorilla to keep it straight, but it
does make flying more demanding.
A helicopter's fuselage will have the
tailboom and main rotors attached
28 JAR Private Pilot Studies
to it. The most common landing
gear is skids:
But you may find wheels, too:
As you probably already know, a
helicopter can move or turn in any
direction, including up and down.
The flight controls are the cyclic and
collective controls, the throttle and the
tail rotor pedals, which all have much
the same effect as they do in
aeroplanes (once out of the hover),
except for the collective, which isn't
used in them at all. As mentioned
before, the main rotors provide lift,
thrust and directional control and,
because it is impractical to change
　
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