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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.
Tail wheel airplanes with the datum aft of the main wheels.
Datum Forward of the Airplane -
Nose Wheel Landing Gear
The datum of the airplane in Figure 3-5 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 3-5. The datum is 100 inches forward of the wing root leading
edge.
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.
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.
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.
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.
3–
Location of Datum
It makes no difference where the datum is located as long
as all measurements are made from the same location.
Datum Forward of the Main Wheels-
Tail Wheel Landing Gear
Locating the CG of a tail wheel airplane is done in the
same way as locating it for a nose wheel airplane except
the formulas use rather than .
The distance (D) between the datum of the airplane in
Figure 3-7 and the main-gear weighing points is 7.5
inches, the weight of the tail wheel (R) is 67 pounds, and
the distance (L) between the main-wheel and the tail wheel
weighing points is 222 inches. The total weight of the
airplane (W) is 1,218 pounds.
Figure 3-7. The datum of this tail wheel airplane is the wing root
leading edge.
Determine the CG by using this formula:
The CG is 19.7 inches behind the datum.
Datum Aft of the Main Wheels -
Tail Wheel Landing Gear
The datum of the airplane in Figure 3-8 is located at the
intersection of the wing root trailing edge and the fuselage.
This places the arm of the main gear (D) at -80 inches.
The net weight of the tail wheel (R) is 67 pounds, the
distance between the main wheels and the tail wheel (L) is
222 inches, and the total net weight (W) of the airplane is
1,218 pounds.
Figure 3-8. The datum is aft of the main wheels, at the intersection of
the wing trailing edge and the fuselage.
Since the datum is aft of the main wheels, use the formula:
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.
Location with Respect to the Mean Aerodynamic
Chord
The aircraft mechanic or repairman is primarily concerned
with the location of the CG relative to the datum, an
identifiable physical location from which measurements
can be made. But because the physical chord of a wing that
does not have a strictly rectangular plan form is difficult
to measure, wings such as tapered wings express the
allowable CG range in percentage of mean aerodynamic
chord (MAC). The allowable CG range is expressed in
percentages of the MAC. The MAC, as seen in Figure 3-
9, is the chord of an imaginary airfoil that has all of the
aerodynamic characteristics of the actual airfoil. It can also
be thought of as the chord drawn through the geographic
center of the plan area of the wing.
3–
Figure 3-9. The MAC is the chord drawn through the geographic
center of the plan area of the wing.
The relative positions of the CG and the aerodynamic
　
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