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stall can happen at any speed with
the wrong angle of attack) it may
well catch you by surprise if you use
the ailerons too abruptly, especially
when the lifting wing stalls and puts
you in a spin, which is just what you
don't want at 200 feet (remember the
aileron's purpose is to temporarily
increase the angle of attack). Tip:
use ailerons last out of a steep turn.
Put the control column forward and
use opposite rudder first,
remembering that the controls are
much less effective at slower speeds.
This can also be a problem when
taking off from a short strip, with
both wings at a high angle of attack.
Sharp movement one way or the
other will increase the angle on the
lifting wing and stall it the wrong
way. Again, modern design, with a
twist in the wing to make the tip ride
flatter, has improved matters, but try
it in a Cessna 152 (a long way off the
ground!) and see what I mean.
Helicopter
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
Airframes, Engines & Systems 197
together and, as it is impractical to
change the speed of the blades, or
their shape, the pitch is altered when
varying the thrust. Blade speed is
constant, and the minimum and
maximum speeds are close together,
because of engine limitations in
piston machines, and transmission
limits for turbines (the minimum
limits are for the coning angle).
The collective, to the left of the
pilot's seat, is called that because it
changes the pitch of all the rotor
blades at the same time, that is,
collectively, thus changing their
angles of attack. Unfortunately, this
also increases drag, which will tend
to decrease the engine and rotor
RPM, so some throttle needs to be
applied when the collective is moved
to keep them up (in fact, the throttle
in a piston machine should be
applied just before, so the engine
doesn't lag behind, called leading with
throttle). There is some sort of
automatic linkage between the
collective and throttle on most
machines, but with pistons, this is
rudimentary at best, and may not
exist at all, as on the Hiller 12E.
Traditionally, the throttle is mounted
on the end of the collective lever. Its
function is to regulate engine RPM
(exam question), and it is moved by
the left hand outwards (away from
the thumb) to increase power, and
the other way to reduce it. Where
(turbine) engine RPM is maintained
by a Fuel Control Unit (FCU) or
FADEC (Full Authority Digital
Electronic Control – see below), it isn't
moved at all, except in some
emergencies where it can be used to
control the directional attitude of the
fuselage. Because they are usually left
in one position, turbine helicopters
may also have the throttles mounted
in the roof or on the floor which, of
course, restricts their use when
problems occur.
The cyclic control is the equivalent
of the central column in a plane, and
only changes the pitch of one blade
at a time, to raise the rotor disk (or,
rather, the tip path plane) at that point
and tilt it in the direction you want
to go. In other words, it changes the
direction of the lifting force, and not
its magnitude, except in the one place
required to lift the blade.
Like the rudder in an aeroplane, the
tail rotor pedals are not used to turn
the ship (except in the hover), but to
stop it turning the wrong way when
you are turning, or to provide fine
tuning for trimming purposes. In
straight and level flight, or above
about 60 knots, they can, to all
intents and purposes, be ignored, as
the tail boom does all the work.
The normal tail rotor, as found on
AStars and JetRangers, requires a
large number of components and
sits in the dirty airflow from the
main rotors – it therefore lives a
stressful life. The fenestron, as used by
Eurocopter, is a different solution,
consisting of a series of very small
blades enclosed in a shroud:
The blades are not equally spaced, to
help with noise, and the shroud
prevents tip losses, for more
　
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本文鏈接地址：Canadian Professional Pilot Studies1(131)