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

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 moment arm.

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 than "torque."