Moment and Moment Arm in Aircraft | Flight Study

Moment and moment arm are fundamental concepts that explain how forces cause an aircraft to rotate about its center of gravity (CG). Understanding how these principles affect balance, stability, and control is essential for pilots when managing aircraft loading and flight performance.

A study of physics shows that a body free to rotate will always turn about its center of gravity (CG). In aeronautical terms, the mathematical measure of an aircraft’s tendency to rotate about its CG is called a moment. A moment is defined as the product of a force and the distance from the CG at which that force is applied. This distance is known as the moment arm, measured from a datum (reference point or line) to the point where the force acts. In aircraft weight and balance calculations, moments are expressed as the product of weight and arm, typically in inch-pounds.

Aircraft designers position the fore and aft CG as close as possible to approximately 20 percent of the mean aerodynamic chord (MAC). If the thrust line passes horizontally through the CG, changes in engine power will not create a pitching moment. However, it is not always possible to align drag forces so that they act directly through the CG. Designers therefore rely heavily on the size and placement of the tail to balance these effects. The goal is to minimize moments caused by thrust, drag, and lift, while using the tail to maintain longitudinal balance under all flight conditions.

The pilot does not have direct control over the location of aerodynamic forces acting on the aircraft, except by influencing the center of lift through changes in angle of attack (AOA). However, the pilot can control the magnitude of these forces. Any change in one force will affect others. For example, a change in airspeed alters lift, drag, and the forces acting on the tail. External influences such as turbulence and gusts can disturb the aircraft, requiring the pilot to apply corrective control inputs to maintain stability.

In some aircraft, the CG location changes with variations in load. Trimming devices, such as elevator trim tabs and adjustable horizontal stabilizers, are used to counteract the resulting moments caused by fuel burn, passenger movement, or cargo loading and unloading.

Frequently Asked Questions

What is a "moment" in aircraft weight and balance?

A moment is a measure of the tendency of a weight to cause rotation around a specific point, usually the Center of Gravity (CG). It is calculated by multiplying the weight of an item by its horizontal distance (arm) from a reference datum.

What is the "moment arm"?

The moment arm is the distance from a fixed reference line (the datum) to the point where a force or weight is applied. In aircraft documentation, this is typically measured in inches.

How does engine power affect pitching moments?

If the engine's thrust line does not pass directly through the Center of Gravity, changes in power will create a moment that causes the nose to pitch up or down. Designers use the tail surfaces to help balance these inherent moments.

How can a pilot counteract changes in moments during flight?

Pilots use trimming devices, such as elevator trim tabs, to counteract moments caused by changes in airspeed, fuel burn, or the movement of passengers and cargo, allowing the aircraft to maintain a desired attitude without constant control pressure.

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