FINALS
AMT 629 AIRCRAFT WEIGHT AND BALACE
EMPTY WEIGHT CENTER OF GRAVITY
There are four possible conditions and their formulas that relate the
location of CG to the datum. Notice that the formula for each
condition first determines the moment of the nosewheel or tailwheel
and then divides it by the total weight of the airplane. The arm thus
determined is then added to or subtracted from the distance between
the main wheels and the datum, distance D.
EWCG Formulas
Datum Forward of the Airplane
Nosewheel Landing Gear
The datum of the airplane in Figure 8 is 100 inches forward of the leading edge of
the wing root or 128 inches forward of the main-wheel weighing points. This is
distance (D). The weight of the nosewheel (F) is 340 pounds, and the distance
between main wheels and nosewheel (L) is 78 inches. The total weight of the
airplane (W) is 2,006 pounds. Determine the CG by using the formula in Figure 9. .
The CG is 114.8 inches aft of the datum. This is 13.2 inches forward
of the main-wheel weighing points, which proves the location of
the datum has no effect on the location of the CG if all
measurements are made from the same location
Datum Aft of the Main Wheels
Nosewheel Landing Gear
The datum of some aircraft may be located aft of the main wheels. The airplane
in this example is the same one just discussed, but the datum is at the
intersection of the trailing edge of the wing with the fuselage. The distance (D)
between the datum of the airplane in Figure 10 and the main-wheel weighing
points is 75 inches, the weight of the nosewheel (F) is 340 pounds, and the
distance between main wheels and nosewheel (L) is 78 inches. The total net
weight of the airplane (W) is 2,006 pounds.
The CG location is a negative value, which means it is 88.2 inches forward
of the datum. This places it 13.2 inches forward of the main wheels, exactly
the same location as when it was measured from other datum locations.
Datum Forward of the Main Wheels
Tailwheel Landing Gear
Locating the CG of a tailwheel airplane is done in the same way as locating it
for a nosewheel airplane except the formula is
The distance (D) between the datum of the airplane in Figure 12 and
the main-gear weighing points is 7.5 inches, the weight of the
tailwheel (R) is 67 pounds, and the distance (L) between the main-
wheel and the tailwheel weighing points is 222 inches. The total
weight of the airplane (W) is 1,218 pounds. Determine the CG by using
the formula in Figure 13.
The CG is 19.7 inches behind the datum.
Datum Aft of the Main Wheels
Tailwheel Landing Gear
This places the arm of the main gear (D) at –80 inches. The net weight of the
tailwheel (R) is 67 pounds, the distance between the main wheels and the tailwheel
(L) is 222 inches, and the total net weight (W) of the airplane is 1,218 pounds.
Since the datum is aft of the main wheels, use the formula found in Figure 15.
The CG is 67.8 inches forward of the datum or 12.2 inches aft of the main-
gear weighing points. The CG is in exactly the same location relative to
the main wheels, regardless of where the datum is located.
