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which therefore is forced down,
creating more lift underneath the tail
which lifts and forces the nose
down. The thing to remember for
exams is that the control column,
when moved forward, moves the
elevator down, whereas the trim
wheel moves its attached surface up.
Power affects trim tabs, as more
airflow varies the sensitivity of the
controls. Reducing power makes the
nose pitch down because the trim
tab has become less effective and
cannot hold the nose in position.
Trim surfaces may also be found on
rudders, depending on the
complexity of the machine.
You may occasionally see a fixed trim
tab, which is there to provide a fixed
amount of trim to make the machine
fly true (it may be one wing low, for
example, from the factory). It must
only be altered by an engineer. Fixed
tabs are used on helicopter rotor
blades to make them fly higher and
lower with respect to each other,
with the goal of making them fly in
line, to reduce bouncing.
An anti-balance tab moves in the same
sense as the main surface, and is
there to increase the force required to
move the control. The further it is
deflected, the greater the force (the
angle of attack increases at a greater
rate on the tab). This prevents overcontrolling
and overstressing the
aircraft, especially where controls
have a low aerodynamic loading.
Balance
At high speeds, control surfaces may
flutter because of buffeting. To
prevent this, a streamlined balancing
weight (usually lead) is fitted forward
of the control surface's hinge. It may
be inside the control surface itself, or
fitted externally (Mass Balance).
Sometimes, part of the control
surface is placed forward of the
hinge line, so that airflow hitting it
will help the pilot move the controls
(known as aerodynamic balance).
Flaps
These are hinged devices on the
trailing edges of wings, inboard of
the ailerons, that temporarily
increase the lift producing areas for
certain modes of flight, like landing,
and sometimes takeoff (not in the
PA 31, or aircraft without enough
power to overcome the extra drag
that reduces acceleration), where you
might be going very much slower
than normal and need a boost – in
fact, flaps produce the same lift at
lower speed by increasing the upper
camber (exam question), and the
negative pressure underneath
because the chord line moves further
down at the rear and changes the
angle of attack against the relative
airflow (pushing the nose down
restores the original angle).
Thus, the reason for using flaps (or
any other low speed lift-producing
device) is to change the shape of the
type of wing required for high speed
flight into one suitable for low speed
flight, otherwise you would need
several miles of runway to get
Principles of Flight 11
airborne. Or land, in which case you
would need sturdier (and heavier)
undercarriages.
However, there is a point beyond
which the extra surface structure in
the airflow produces more drag than
lift, which is made use of when
deliberately trying to bring the speed
down, as with a short field landing,
or increase the angle of approach
without much sacrifice in speed.
Sometimes, the ailerons are made to
move in sympathy with flaps.
Various flap designs create different
effects (all try to reduce drag), but
the Fowler Flap is generally
considered to be the most efficient –
they do not just drop down from the
wing, but slide out from the back:
There is a maximum flap extension
speed, as they are not designed for
high speed flight. Lowering flaps will
generally force the nose down, as
they make the centre of pressure
move backwards, but sometimes
they affect the airflow over the
tailplane enough to go nose-up.
Once lowered, flaps should not
normally be raised until actually on
the ground. On a missed approach,
they should be raised after power
has been applied, in easy stages.
Other Wing Devices
Slats are small aerofoils that open
forward of the main one to smooth
out the airflow - when the angle of
attack is high, they do this
automatically (low pressure just
behind the leading edge sucks them
out. They are pushed back in by air
pressure when the angle is low).
They are usually found near the wing
tips on the leading edge to help with
lateral control.
Slots are openings a little bit back of
the leading edge of a wing that allow
　
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