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get airborne in the first place, the lift
must always be more than the weight
of the aircraft - in the cruise, of
course, they should be equal).
The complete force produced by any
aerofoil is called the total reaction,
which can be split into two forces,
called lift, which acts at right angles
to the airflow, and drag, which acts
parallel to it.
In the diagram below, the thrust and
lift vectors are longer than those for
their opposites, weight and drag, so
you will fly forwards and upwards:
You will also notice that the
lift/weight and thrust/drag vectors
are offset from each other. This is to
create couples around the lateral axis
to produce pitching moments when
lift and thrust are taken away (as
with an engine failure), placing the
machine in the correct attitude. They
will be balanced in normal flight by
forces produced by the tailplane.
The chord line is the straight line
joining the leading and trailing edges
of an aerofoil:
The Centre of Pressure is the point on
the chord line through which the
resultant of all forces (i.e. total
reaction) is said to act:
It moves forward steadily as the
angle of attack increases (see below),
until just before the stalling angle, when
it moves rapidly backwards, creating
such a long couple between it and
the Centre of Gravity that the nose
pitches forward. The range can be as
much as 25% of the chord length.
The stalling angle is that above which
the aerofoil stalls. It is where lift is at
its maximum. Although lift is still
being produced above it, it is not
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
6 JAR Private Pilot Studies
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.
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
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
　
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