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Empty-Weight Center of Gravity
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Empty-Weight Center of Gravity Formulas
Datum Aft of the Main Wheels - Nose Wheel 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.
There are four possible formulas that relate the location of CG to the datum.
The distance (D) between the datum of the airplane in Figure 3-6 and the main-wheel weighing points is 75 inches, the weight of the nose wheel (F) is 340 pounds, and the distance between main wheels and nose wheel (L) is 78 inches. The total net weight of the airplane (W) is 2,006 pounds.
Datum Forward of the Airplane - Nose Wheel Landing Gear
Notice that the formula for each condition first determines the moment of the nose ((FxL)/W) wheel or tail ((RxL)/W) wheel 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.
Figure 3-5. The datum is 100 inches forward of the wing root leading edge.
-The datum 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 nose wheel (F) is 340 pounds, and the distance between main wheels and nose wheel (L) is 78 inches. The total weight of the airplane (W) is 2,006 pounds.
Figure 3-6. The datum is aft of the main wheels at the wing trailing edge.
The location of the CG may be determined by using this formula:
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 it was when it was measured from other datum locations.
Nose wheel airplanes with datum forward of the main wheels.
Nose wheel airplanes with the datum aft of the main wheels.
Tail wheel airplanes with the datum forward of the main wheels.
Figure 3-5. The datum is 100 inches forward of the wing root leading edge.
Tail wheel airplanes with the datum aft of the main wheels.
Determine the CG by using this formula:
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 so long as all measurements are made from the same location.