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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
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.
Airframes, Engines & Systems 187
Once lowered, flaps should not
normally be raised again until
actually on the ground. On missed
approach, they should be raised after
power is applied, in easy stages.
The aircraft becomes less stable
when flaps are extended, because the
lateral CP moves inboard, so it
becomes more manoeuvrable in roll.
Longitudinal stability is decreased.
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
high pressure air underneath the
wing to pass through them at high
angles of attack to the low pressure
area, to re-energise it and extend the
laminar flow by reducing eddies.
This reduces loss of lift and drag,
and increases the stalling angle of
attack. Slots may go across the whole
length of the wing, or just where the
ailerons are.
A Wing Fence is a small vertical fin a
couple of inches high running the
length of the chord on the upper
surface of a wing that stops air
moving towards the tips at high
angles of attack (swept wing) or
controls the airflow around the flaps
(straight wing). Either way, handling
is improved at slow speeds and help
is given with the stall.
Spoilers are there to spoil the airflow
on the upper or lower surface of a
wing, taking away the lift. They may
work with the ailerons and assist
with their control or may be linked
with the brakes to increase the
weight on the wheels and help with
the braking action. Sometimes,
spoilers may be used instead of
ailerons, as they are less complicated
and more effective anyway.
Speed brakes on high performance
aircraft allow you to keep the
engines running enough not to
shock them when power is reduced
(see Engine Handling, later), and to
produce decent descent rates, as
modern aircraft are very slick. They
create drag without altering the
shape of the wing, essentially
producing the same effect as
introducing a barn door into the
airflow, though not as big, of course.
Manoeuvres
Taxi
Before you get to take off, you have
to get from where you are to the
runway, and return when you land.
The "roads" that get you there are
called taxiways, and they are usually
identified with letters, such as
Taxiway Alpha, or Taxiway Bravo, etc.
You will be told the sequence to use
when you first get clearance from
ATC to start moving. The clearance
may involve crossing a runway or
two to get to the one you want – you
should still check to see that nobody
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Canadian Professional Pilot Studies1(124)
