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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
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.
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
Principles of Flight 7
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 reduced. This
system also pulls fuel into a
carburettor, and it's also the reason
why a door closes by itself if left
slightly ajar – there is less pressure in
the gap between it and the door
frame as the air moves through it.
If you take the top half of the tube
away, the phenomenon still works
on the remaining half, which looks
like the top surface of an aerofoil.
You can see this yourself by taking a
large piece of paper and folding it
back over the top of your hand,
keeping hold of it with your fingers.
If you blow across the top, you will
see the paper rise. Used sideways,
this is how yachts make use of the
wind to get along.
The aerofoil will therefore have a
natural tendency to go up or, looked
at another way, to pull air down, to
the low pressure area on the top and
help the brute force effect. Around
two thirds of the total lift comes
from the reduced pressure effect
across the top, not forgetting the
higher pressure underneath.
The wing loading is the average weight
lifted by each square foot of the
wing. It works in a similar way to
loadspreading, in that a larger area
has a lesser loading.
Aeroplanes
Aeroplanes are either low wing, like
most Piper aircraft (see overleaf), or
　
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