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BOB TAIT'S AVIATION THEORY SCHOOL CPL AERODYNAMICS
THE BASICS
The Oxford Dictionary defines dynamics as the study of forces and motion. Aerodynamics applies this study
specifically to aircraft and considers the forces acting and the motion that results as the aeroplane manoeuvres in
flight. The manoeuvres of an aeroplane are the result of forces which act to initiate or prevent motion. Since the
pilot is responsible for the control of those manoeuvres, the pilot's task can be thought of in terms of management
of the forces which act on the aeroplane.
Because we cannot hope to manage forces [or anything else for that matter] unless we investigate the fundamentals,
our study of aerodynamics begins at the very beginning with the definition of the terms used so frequently, yet so
often not clearly understood. The exact meaning of terms such as force, pressure, work, thrust and power must be
clear in your mind, especially when preparing for a multi-choice examination where so much depends upon com-
prehending the question. If my idea of pressure is different to your idea of pressure, we can go on arguing until the
proverbial 'cows come home' about the answer to a particular question. Until we both agree on the exact meaning
of the
term
, the argument will never be resolved. Here we go!
STATE OF MOTION
Since aerodynamics is all about motion, let's begin by asking what motion is. It may be better to approach from the
other side and ask what we mean when we use the word 'stationary'. As you sit and read this page, are you station-
ary? No, you are on the surface of a planet which is rotating on its axis and moving through space in its orbit about
the sun. The sun is just one star on the spiral arm of a galaxy which is itself rotating and hurtling through space
in a gravitational dance with other galaxies. The word stationary in this context has no meaning. We humans are
not aware of motion itself, we are aware of
relative
motion i.e. the difference between our motion and that of other
objects. Most importantly we are aware of
change
of motion. If you have ever stood in a crowded bus you have
had no trouble detecting every change in its motion i.e. speed or direction. A Jumbo Jet cruising in level flight at a
constant speed and the Sydney Opera House are in very different states of motion, however they have one important
characteristic in common - there is no
change
in their state of motion.
FORCE
Ask a physics teacher to define force and he/she will probably say that force is mass multiplied by acceleration -
which leaves most people, especially pilots, just as confused as ever. A simple definition of force is:
A force is a
push or a pull
which attempts to
change
the state of motion of a body.
Whenever you see the word force used in a sentence [or multi choice question], you can replace it with 'push or
pull' and still preserve the exact meaning. It is important to note that all forces
attempt
to change the state of mo-
tion of a body. No one said that they have to
succeed
in producing a change in motion. For an aircraft in straight
and level flight at constant speed, the force of thrust is attempting to change its speed to a higher speed, while the
force of drag is attempting to change its speed to a lower speed.
Because these two forces are equal and opposite, neither
succeeds
in producing any change and the state of motion
remains unaltered. That doesn't mean that the forces are not present, it means they are not succeeding in producing
a change of motion. Likewise the force of lift is attempting to produce motion vertically upwards, while the force
of weight is attempting to produce motion vertically downwards. Because these forces are equal and opposite,
neither succeeds in producing any change.
EQUILIBRIUM
Whenever a body has no change in its state of motion it is said to be in equilibrium. It follows that if there is no
change in motion there must be a balance between all the forces acting so that no one force succeeds in changing
motion [equilibrium means 'equal balance']. A simple test for equilibrium is to ask if motion is changing. You
have not fully described the motion of a body until you have defined
both
its speed and direction. If neither speed
nor direction is changing, the body is in equilibrium. Ask yourself these two questions to decide whether a body
is in equilibrium:
Is the speed changing?
Is the direction changing?
If the answer to both of these questions is no, you can be sure the body is in equilibrium.
In which of the following situations is an aircraft in equilibrium?