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Flight Controls
When airflow over them is high, the
controls have a positive feel. They
are less responsive at slow speeds,
which is a point to remember when
flying low and slow (also, because of
the nose high attitude, the control
surfaces may not have any airflow
over them at all, which is why some
aircraft have the tail at the top of the
fin). In fact, the effectiveness of any
control depends on its distance from
the Centre of Gravity, the size of the
control surface, its speed through
the air and the degree of movement.
Control surfaces in small aircraft are
usually activated by cables and
pulleys, or rods and tubes.
The elevator, rudder and ailerons are
attached by hinges to the tailplane
(or horizontal stabiliser), fin and
wing trailing edges, respectively (a
Canard is a horizontal stabiliser
mounted on the front, with the
advantage of a longer moment arm,
so they can be smaller).
The elevator controls pitching by
increasing the angle of attack above
or below the tailplane, according to
whether it is raised up or forced
down by movement of the control
column in the cockpit (if the column
is pushed forward, the elevator is
Principles of Flight 9
forced down into the airflow
underneath the stabiliser, the angle
of attack is increased, the tail rises
because more lift is created and the
nose goes down, the opposite if
pulled back). Sometimes, there is no
elevator, but the whole stabiliser is
moved, in which case it is a stabilator.
The rudder does much the same
thing, only sideways, making the
nose yaw, or move left and right. It is
controlled by the foot pedals –
whichever one goes forward moves
the rudder to that side, where more
lift is created and the fin is forced
sideways in the opposite direction, to
produce a flat turn with a skid (you
don't use the rudder to turn, but to
fine tune one initiated by the
ailerons, or stop it going the wrong
way – see Low and Slow, below).
The ailerons make the aircraft roll
around the nose. If you move the
control column to the left, the right
aileron goes down, increasing the
angle of attack on that side, and the
left one goes up, decreasing it,
causing a roll in the same direction.
To counteract Aileron Drag, which
comes from the downgoing aileron,
you might see frise or differential
ailerons used. With the former, the
downgoing aileron is streamlined.
The latter moves the down aileron
through a smaller angle.
In fact, the frise aileron's hinge is
offset, so a portion of the leading
edge of the downgoing aileron sticks
out into the airflow, to create a little
drag to reduce adverse yaw. It also
produces a slot (i.e. a gap between it
and the rear of the wing) to smooth
out the airflow over it:
The controls will move the aircraft
in one of three axes – pitching (nose
up or down), rolling (wings up or
down) or yawing (nose left or right).
They do not move in isolation,
however – an adjustment in one
causes a secondary effect in another
and must be allowed for, as we shall
see in the discussions below. For
example, an uncontrolled yaw
eventually results in a roll, because
one wing will be moving faster and
will generate more lift on that side.
Your instructor will need to
demonstrate certain manoeuvres,
and there is a well established system
of establishing who has control of
the aircraft at any given time. The
person handing over the controls
will say "You have control", while
the person receiving them replies
with "I have control".
Trim
Depending on the net result of
power and control positions, it may
take more physical force to keep the
aircraft in a particular attitude. That
is to say, for any combination of
power and control position, they will
move freely with a certain range, but
take a lot of force to go outside of it.
These extra forces can be trimmed out
with a wheel or similar device which
operates a very small control surface
in the elevator (for example), so you
10 JAR Private Pilot Studies
have a control surface within a
control surface. The wheel moves
the surface up or down in the
airflow, which moves the elevator
the opposite way and does the work
you would otherwise have to do to
keep it there. If the trim wheel is
moved forward, it forces the trim
surface upwards, which creates more
lift between it and the elevator,
　
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