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determined is then added to or subtracted from the distance
between the main wheels and the datum, distance D.
Nose wheel airplanes with the datum forward of the main wheels
Figure 3-4. Locating the CG of an airplane relative to the datum which is located at the firewall. See Figure 3-3.
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.
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
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.
Determining the Location of the CG
1. Determine the location of the CG relative to the main-wheel weighing
points. This is the or part of the formula.
2. Convert the location of the CG measured from the main-wheel
weighing points to the location measured from the datum or the
LEMAC.
3–6
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.
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 net weight of the airplane (W)
is 1,218 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.
Figure 3-7. The datum of this tail wheel airplane is 7.5 inches
forward of the wing root leading edge.
Determine the CG by using this formula:
The CG is 19.7 inches behind the datum.
Location of Datum
It makes no difference where the datum is located as long as all
measurements are made from the same location.
3–7
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.
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.
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 this 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
AMTs are primarily concerned with the location of the CG
relative to the datum, an identifiable physical location from
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