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produces a slot (i.e. a gap between it
and the rear of the wing) to smooth
out the airflow over it:
Large aircraft may have multiple
ailerons, with each set disabled at
certain speeds (the 767’s second set
on each wing is locked at 240 kts).
Airframes, Engines & Systems 185
This allows control over the whole
range, since smaller ailerons are
needed at high speeds.
Roll spoilers are also used on large
aircraft in harmony with the ailerons.
They are flat panels on the upper
wing that come up on the down
wing to spoil the lift and give the
down aileron a helping hand, since it
is the least effective in a turn.
On some aircraft, such as the MU-2,
spoilers are used instead of ailerons,
so you need new crosswind landing
skills. Ailerons and spoilers usually
have separate control columns.
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
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,
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, which
helps when you have to fly with one
engine out.
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.
A servo tab, or control tab, is used on
large aeroplanes, such as the
McDonnell Douglas series, and
works in a similar way, except that
only the tab is moved by controls,
and the force of the airflow over it
makes the primary surface move. Its
function is therefore to assist the
pilot in moving large control
surfaces, rather than holding them in
one position.
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
186 Canadian Professional Pilot Studies
aircraft, especially where controls
have a low aerodynamic loading.
Mass Balance
At high speeds, control surfaces may
flutter because of buffeting,
especially if the wings are flexible
with a high aspect ratio. 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
　
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