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Definitions
· Inertia - the tendency of a body
to remain at rest, or at least
carry on with what it's doing - in
other words, not to change its
present state.
· Momentum – the quantity of
motion in a body, or a tendency
to keep right on going.
· Equilibrium - a state of balance
between forces.
· Centrifugal Force acts outwards
along a radius of a curve.
· Centripetal Force acts inwards
along a radius of a curve.
· Acceleration – the rate of change
of motion in speed and/or
direction.
· Nose – the front part of the
aircraft, where the cockpit is.
· Tail – the rearmost part, with
the rudder assembly and
horizontal stabiliser. Sometimes
known as the empennage.
· Wings – the lift producing
surfaces, traditionally forward of
the tailplane, but a canard has
them the other way round. In a
helicopter, the rotor blades are
the equivalent.
· Ailerons – moveable surfaces on
the rear of the wings, at the
outer end, that alter the way the
wing produces lift, in order to
make it go up or down, most
useful when turning.
· Stall – a condition of flight
where the wings stop producing
lift and the aircraft is no longer
able to stay airborne, when the
156 Canadian Professional Pilot Studies
machine is not going fast
enough for the conditions.
· Angle of Attack – the angle
formed between the wing and
its path through the air.
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), a principle to remember
during mountain flying. It is also,
therefore, compressible, and can
flow and change its shape. As it has
a mass, it can exert force, and do
work, and therefore transfer energy.
As a quantity, it is normally
expressed in terms of cubic feet.
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.
In any fluid, the internal pressure is
the same at any point in all
directions. If a body is immersed in
it when stationary, the pressure on
any point of the body acts at right
angles to the surface, regardless of its
shape or position.
The Aerofoil
This is the posh 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
Principles of Flight 157
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
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