An airplane is of course a three-dimensional shape, therefore it has
three axis:
Longitudinal Axis
Lateral Axis
Normal Axis
It is important to remember that these axis are defined relative to the
airplane, NOT the earth, the air, or the direction of flight.
When the engineers design the airplane they give it a floor that they
generally intend to be approximately level when the airplane is in cruise.
Think of the floor as the basic reference for understanding the three
axis. The longitudinal and lateral axis lie parallel to the floor. The
Normal axis is perpendicular to the floor, like a post sticking out of
the ground.
Center of gravity as Axis Reference Point
All the axis run through the center of gravity, by definition. As we learned
in the physics review, if a force acts through the center of gravity
it causes acceleration, but NO rotation. Therefore the C of G is
a handy reference point to use in defining our axis system. This
way we know that if a force acts along any of the three axis is
will not result in a rotation moment, or conversely if a force acts
anywhere other than along an axis it WILL cause a rotation moment.
You are probably more familiar with the earth's axis, which is
also usually called the poles. The center of gravity of the earth
is right at the center, where the molten core is.
In the simulation to the left drag the top half of the earth up
to reveal the c of g. Notice also that the pole is really just one
axis that passes through the c of g.
IMPORTANT: Visualize that when you break the earth into
two hemispheres, as to the left, the dividing line (at the equator)
is a flat plane, and the c of g lies at the center of that
plane.
Now consider the airplane to the left. It has a normal axis which corresponds
to the earth's poles.
Drag the top half of the airplane up to reveal the c of g. Try
to visualize that the airplane could be cut exactly in half like
this such that the top half weighs the same as the bottom half.
(If the division doesn't seem right to you, remember that the bottom
half has the wings, and they contain all the fuel.) Note that the
division into top and bottom half is based on weight, NOT size.
In the case of the earth the two halves are the same size, shape,
and weight. In the case of the airplane it has heavy and light parts,
so the top half is larger, but it weights the same as the bottom
half.
You should visualize that the point of division between the two
parts is a flat plane. This plane lies parallel to the floor
of the airplane by definition. The center of gravity is a point
on that plane. It is not at the center, it is at the balance point.
I.E. if you took the top half and balanced it on a point it would
balance right at the c of g point - ditto the bottom half, if you
turned it upside down. (Note: the c of g is so far forward because
the engine is so much heavier than the airframe.)
In the simulation to the left you can drag the front half of
the airplane forward, and the aft part rearward along the longitudinal
axis. The results is a front and back half that, as above, are of
the same weight.
The front half is physically smaller because the engine is so much
heavier than the thin aluminum that makes up the fuselage. But both
halves weigh the same. Obviously the exact division point will change
with the number of people onboard, the amount of fuel in the tanks,
and the baggage.
The place where the division occurs is the c of g and there is
a flat plane that divides the two halves. The normal axis lies in
that plane and the c of g is a point on the plane.
Both the normal and lateral axis lie in this plane.
The Plane of Symmetry
The airplane to the left can be split into left and right halves
- try it to see.
Unlike the other two axis this split results in two sides that
are mirror images of each other (like the hemispheres of the earth
shown above.) Consequently the flat plane that divides the two sides
is called the plane of symmetry.
The picture to the left shows a rear view of the airplane. It can also be split
into left and right symmetrical sides.
You can see that both the normal and and longitudinal axis lie
in the plane of symmetry, while the lateral axis is perpendicular
to the plane of symmetry. When we look at a side view of the airplane,
such as the ones above you can think of the paper, or computer screen,
as representing the plane of symmetry.
The plane of symmetry is important in aerodynamics because it contains
the lift vector, and usually also the thrust and drag vectors. Now
that we know about the three axis and the plane of symmetry we will
next discuss the forces acting on the airplane and how they relate
to these axis.
