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enough to support the aircraft.
The C of G is an imaginary point
around which the aircraft is
balanced, and is normally forward of
the C of P anyway. If it is too far
forward, the couple will be long
enough to produce a large nose
down pitch from the lift/weight
vectors. There will then be a longer
distance between the C of G and the
elevator, which will tend to make the
machine longitudinally overstable
(see Stability, below), meaning that
you will need more control input to
pull the column back on landing, and
you may run out of range.
A wing always stalls at the same
angle of attack, but stall speed varies
in proportion to aircraft weight.
To start off with, a wing is placed at
an angle on the airframe called the
angle of incidence, which is purely a
figure out of the designer's head,
although there are advantages in
having it as small as possible, in that
you can improve visibility and
reduce drag in the cruise because the
nose will not be so high (in practice,
it is set at the best lift/drag ratio, or
the point when you get the most lift
for the least drag). This angle may
vary throughout the length of the
wing, being maximum at the root
and minimum at the end, in a
process called washout (or washin if
you go the other way. The difference
is that the former decreases lift and
the latter increases it). The angle of
incidence changes this way because
the outer edges of the wing (or
propeller, which acts on the same
principle) will be moving faster than
the rest in some manoeuvres (a turn,
for example), creating more lift and
stress. In addition, washout allows
the outer parts of the wing to still be
creating lift at slower speeds when
the inner edges are stalled, as they
might be when landing. You can get
a similar effect by changing the shape
of the wing from root to tip.
Later, in flight, the wing will make
another angle with the relative airflow,
or relative wind, which will be the angle
of attack. Te relative airflow is just the
direction of the air that keeps the
aircraft up, which goes the opposite
way to the flight path. In other
words, it is the direction of the air
relative to the wing, irrespective of
whether the wing is pushed through
the air or vice versa, or a combination.
Relative wind has nothing to do with
the real wind.
The angle of attack is the angle at
which the wing meets the air, or,
more technically, at which the chord
158 Canadian Professional Pilot Studies
line (that joins the leading edge with
the trailing edge) meets the relative
airflow or the flight path – do not
confuse it with the angle of incidence,
mentioned above. You can either fly
at a high speed with a small angle of
attack, or a slow speed with a high
one, up to the accepted maximum of
around 15° - as the angle of attack
increases, there is more frontage to
the airflow, increasing drag markedly.
The optimum angle of attack is 3-4°,
for the best lift/drag ratio (see below),
which will increase from zero to that
point (after that, it decreases).
Put simply, the airflow will hit the
underside of the wing, to be forced
downwards, forcing the wing up.
This is a bit of a brute force solution,
so the wing will also be shaped to
help things along with the venturi
effect, discovered by Bernoulli, a
principle made use of in
carburettors, described in the
Airframes, Engines & Systems chapter,
and air-driven instruments (see
Instruments). Bernoulli found that the
pressure of a fluid decreases where
its speed increases or, in other
words, in the streamline flow of an
ideal fluid, the quantity of energy
remains constant - there is a given
amount of energy involving speed
and pressure, and each affects the
other directly.
If you take a tube with a smaller
diameter at its centre than at either
end, and blow air through it, the
pressure in the centre is less because
speed and pressure interact with
each other, in that, if you increase
one, the other decreases. In this case,
the air being forced around the
obstructions in the middle has to
increase speed to keep up with the
rest because it is taking a longer
path.
Of course, the same molecules of air
don't meet up at the other end –
those taking the longer route may be
up to 30% of the distance away,
depending on the angle of attack.
Since speed is increased, pressure is
　
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