The moment is a measure of the tendency
of an airplane to rotate around the center of gravity. It can rotate
on a yaw, pitch, or roll axis.
Moment is a term used in aerodynamics to describe the tendency
of a force to rotate a body under a given point-in the case of an
airplane, usually the center of gravity.
Imagine resting your arm on a table, with your palm up and with
your elbow on the table. Imagine a book resting in the palm of your
hand. Now lift up the book slightly above the table (let's say the
book weighs 4 pounds), pivoting your arm slightly above the table,
but keeping your elbow on the table. Let's say the length of your
arm from your elbow to your hand is 1.5 feet long. By definition,
the book is exerting a moment about your elbow equal to the force
(the weight of the book) times the distance to the pivot point (your
elbow). In this case, (force) x (distance)=4 x 1.5=6 foot-pounds.
The moment exerted by the book about your elbow is 6 foot-pounds.
In the above example the length of your arm, the 1.5 feet, is the
Another example is the lift, L, acting through a point on an airplane.
If this point is a distance d from the center of gravity of the
airplane, then the lift is exerting a moment about the center of
gravity equal to the product Ld. That is, the lift is trying to
rotate the airplane about the center of gravity. So a moment is
indeed a measure of the tendency for a force to rotate a body around
a given point. In the elbow example above, the weight of the book
is tending to rotate your arm back to the table.
Torque is essentially a moment, but it is usually a term used when
a force is being used to intentionally rotate something. Imagine
that you are trying to unscrew the top of a jelly jar. The force
exerted by your hand on the rim of the lid, times the distance from
the rim to the center of the lid, is by definition the torque you
are exerting on the lid. The torque is, like moments, equal to the
force times the distance to the point of rotation. In aeronautical
use, when we are considering the aerodynamic forces acting on a
flight vehicle, we usually use the term "moments" rather