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aft, keeping the formers together,
spreading the load between them
and stiffening the structure in
general. Bulkheads are similar to
formers, but tend to be found at
both ends of the fuselage, at each
end of a compartment, or when
more strength is required.
A firewall is a fireproof partition that
separates the engine compartment
from the cabin. It is normally made
of stainless steel.
In a wing, ribs are the equivalent of
formers, and they are held in place
with spars, which perform a similar
function to longerons. Modern
wings will also contain fuel tanks,
which may or may not contribute to
the strength of the wing.
An egg is a good example of a
monocoque structure, which is
handy, as cocque is French for eggshell.
Aside from saving weight, the big
advantage of monocoque is that it
leaves more space inside. Older
flying boats, made of wood, were
among the earliest examples.
Airflow
Air is a liquid medium, meaning that
it behaves rather like water (as is
shown by submerged aircraft, which
will "fly" to the bottom of the sea,
miles away from where they splash
down). It is also compressible, and
can flow and change its shape.
Principles of Flight 5
The speed at which an object moves
through the air is called the airspeed,
and it doesn't matter whether the
wind flows over it, or the object
itself moves – the effects are the
same. Up to a certain critical
airspeed, airflow round a body is
quite well-behaved, after which it
breaks up to form vortices that may
interfere with any lifting action.
The Aerofoil (or Airfoil)
This is the official name for a wing,
or any other device that creates a lift
reaction out of thin air (in order to
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 Canadian 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.
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
　
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