Determining the Aircraft Center of Gravity

Determining the Aircraft  Center of Gravity

When the aircraft is in its level flight attitude, drop a
plumb line from the datum and make a mark on the hangar
floor below the tip of the bob. Draw a chalk line through
this point parallel to the longitudinal axis of the aircraft.
Then draw lateral lines between the actual weighting
points for the main wheels, and make a mark along the
longitudinal line at the weighing point for the nose wheel
or the tail wheel. These lines and marks on the floor allow
you to make accurate measurements between the datum
and the weighting points to determine their arms.

Figure 3-3. The datum is located at the firewall.
Determine the CG by adding the weight and moment of
each weighing point to determine the total weight and total
moment. Then divide the total moment by the total weight
to determine the CG relative to the datum.

As an example of locating the CG with respect to the
datum, which in this case is the firewall, consider the
tricycle landing gear airplane in Figures 3-3 and 3-4.
When the airplane is on the scales with the parking brakes
off, place chocks around the wheels to keep the airplane
from rolling. Subtract the weight of the chocks, called
tare weight, from the scale reading to determine the net
weight at each weighing point. Multiply each net weight
by its arm to determine its moment, and then determine the
total weight and total moment. The CG is determined by
dividing the total moment by the total weight.

The airplane in Figures 3-3 and 3-4 has a net weight of
2,006 pounds, and its CG is 32.8 inches behind the datum.

Two Ways to Express CG Location

The location of the CG may be expressed in terms of
inches from a datum specified by the aircraft manufacturer,
or as a percentage of the MAC. The location of the leading
edge of the MAC, the leading edge mean aerodynamic
cord (LEMAC), is a specified number of inches from the
datum.