Aircraft Arms, Weight, and Moments

Aircraft Arms, Weight, and Moments

The term arm, usually measured in inches, refers to the
distance between the center of gravity of an item or object
and the datum. Arms ahead of, or to the left of the datum
are negative(-), and those behind, or to the right of the
datum are positive(+). When the datum is ahead of the
aircraft, all of the arms are positive and computational
errors are minimized. Weight is normally measured in
pounds. When weight is removed from an aircraft, it is
negative(-), and when added, it is positive (+).

The manufacturer establishes the maximum weight and
range allowed for the CG, as measured in inches from the
reference plane called the datum. Some manufacturers
specify this range as measured in percentage of the mean
aerodynamic chord (MAC), the leading edge of which is
located a specified distance from the datum.

The datum may be located anywhere the manufacturer
chooses; it is often the leading edge of the wing or some
specific distance from an easily identified location. One
popular location for the datum is a specified distance
forward of the aircraft, measured in inches from some
point, such as the nose of the aircraft, or the leading edge
of the wing, or the engine firewall.
The datum of some helicopters is the center of the rotor
mast, but this location causes some arms to be positive
and others negative. To simplify weight and balance
computations, most modern helicopters, like airplanes,
have the datum located at the nose of the aircraft or a
specified distance ahead of it.

A moment is a force that tries to cause rotation, and is the
product of the arm, in inches, and the weight, in pounds.
Moments are generally expressed in pound-inches (lb-in)
and may be either positive or negative. Figure 2-1 shows
the way the algebraic sign of a moment is derived. Positive
moments cause an airplane to nose up, while negative
moments cause it to nose down.