18.64MB - View From The Trenches

Just why an airplane behaves the way that it does
may surprise many uninitiated in the mysteries of
aerodynamics. It certainly fooled me. I had care-
lessly supposed that one operated an airplane rather
like a car, except for the need of an additional
control to regulate diving and climbing. I was
absolutely wrong. Because an airplane must be able
to create a counter-force to gravity, something the
ground handles at no cost for a car, its controls give
responses very different from what one might ex-
pect. For example, what happens when a car driver
steps on the gas pedal? Our experience tells that the
car will speed up. But what happens when a pilot
pushes forward on his throttle, the airplane's equiva-
lent to the gas pedal? The airplane doesn't acceler-
ate; it climbs. This paradox can be easily explained
once the principles of flight control are understood;
and, because they were of great influence in my
design of XhlIGHTS OF THE AIR, let's look at these
principles.
THE BASICS
An airplane must obey the same laws of physics
that govern every body with mass in the universe.
Try to remember those rules and formulae about
energy and motion that you assidiously memorized
in school. Do you recall the law that described the
relationship between force and acceleration? It is
perhaps, next to Einstein's E=mc2, the most
famous equation in physics-force=mass times
acceleration (usually abbreviated as f=ma)-and it
serves as the foundation to the understanding of
motion. To translate, it says that in order to change
the speed or direction of a body with mass, a force
must be exerted upon it. It is important to note that
the term, "acceleration", is used here in its scien-
tific meaning of "change of velocity". This encom-
passes changes of direction as well as changes to
speed since velocity, because it is a vector, has a
measured speed and a measured direction.
When no force is exterted on a body, no acceler-
ation can occur and, ergo, the body must remain
PLANE FACTS
The Design of KNIGHTS OF THE AIR
By Mick Uhl
in a state of constant velocity. In other words, it
must continue to move at a uniform speed and in
a uniform direction until such time as another force
is exerted upon it. Physicists describe this condi-
tion as a state of inertia. An important corollary of
the f =ma equation states that, upon entering an in-
ertial state, a body continues to move at the same
speed and in the same direction it was travelling at
the moment the operating force was removed.
Controlling an airplane then becomes a matter of
creating a way to direct a force upon the airplane
when a change of speed or direction is wanted and
then removing or cancelling the force when a fixed
course is wanted. Studies show, however, that not
one but four different kinds of forces impinge upon
an airplane:
1. Thrust-a force produced by the airplane's en-
gine to propel it forward.
2. Drag-the resisting force of the air to the passage
of the airplane.
3. Lift-a part of the drag force redirected by the
wings to counteract gravity.
4. Gravity-an absolute force of nature.
How does an airplane respond to four distinct
forces? It simply treats them as if they were one
force with a magnitude and direction equal to their
sum. This accumulative effect is a consequence of
the vector nature of force. It's easy to visualize how
it works in simple situations. To give just one
example, when two equally strong forces press upon
a body from opposite sides, the body doesn't react
at all and continues unconcernedly along its current
course as if there are no forces acting upon it at all.
More complicated force patterns are illustrated in
the diagram.
Often the true reasons for a physical action are
at great variance with our intuitive ideas of the laws
of nature. Suppose, for instance, that an airplane
is in a steep dive at a high speed. Again, by referring
to f=ma, we know that if the speed isn't changing
and if the dive is in a straight line then no force can
be acting on the airplane. Yet someone will surely
protest that gravity must be pulling on the airplane;
otherwise why would it be diving? Admittedly,
gravity does play a role in the acceleration process.
However, once the acceleration ceases, the sum of
the four forces acting on the airplane must reach
a total of zero. What nullifies the force of gravity?
The drag must be blamed. It has been blithly in-
creasing right along with the airplane's speed. Even-
tually, it reaches a strength to neutralize the gravity
and thereby stop the acceleration process. The air-
plane continues to dive only because that was the
course it was travelling at the moment the acceler-
ation ceased.
I hope that you're beginning to see how force and
inertia affect flight. When an airplane is flying a
steady course, say 250 mph at 20000 feet, it need
only provide enough forward thrust to nullify the
amount of drag at that speed. Gravity is cancelled
by the portion of the drag the wing redirects to lift.
The four forces are in balance qnd their sum force
is zero. If either the thrust or drag suddenly changes,
so that one no longer matches the other, the forces
are out of balance and the airplane will accelerate
until thrust and drag are again equal.
THE CONTROLS
We've all seen airplanes taking off, performing
all sorts of intricate maneuvers, and then landing
safely. Somehow the pilot manages, through his
controls, to regulate enough of the thrust and drag
to permit all of these activities. What does he use?
Only two-a throttle which regulates the engine's
forward thrust; and a joystick (or control stick)
which, when moved forward and back regulates the
elevator vanes on the airplane's tail. The elevator
sets the angle of the wings to the flight path and,
thus, the amount of lift generated at any speed. The
same joystick when moved from side to side tilts
the airplane's wings; in this altitude, part of the
wing's lift force is brought into a horizontal direc-