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center of lift of the wing have critical effects on the flight
characteristics of the aircraft.
Consequently, relating the CG location to the chord of
the wing is convenient from a design and operations
standpoint. Normally, an aircraft will have acceptable
flight characteristics if the CG is located somewhere near
the 25 percent average chord point. This means the CG
is located one-fourth of the total distance back from the
leading edge of the wing section. Such a location will
place the CG forward of the aerodynamic center for most
airfoils.
In order to relate the percent MAC to the datum, all weight
and balance information includes two items: the length
of MAC in inches and the location of the leading edge of
MAC (LEMAC) in inches from the datum.
The weight and balance data of the airplane in Figure 3-10
states that the MAC is from stations 144 to 206 and the CG
is located at station 161.
MAC = 206" - 144" = 62" inches
LEMAC = station 144
CG is 17 inches behind LEMAC
(160 - 144 = 17.0 inches)
The location of the CG expressed in percentage of MAC is
determined using this formula:
The CG of the airplane is located at 27.4% MAC.
It is sometimes necessary to determine the location of the
CG in inches from the datum when its location in %MAC
is known.
The CG of the airplane is located at
27.4% MAC
MAC = 206 - 144 = 62
LEMAC = station 144
Determine the location of the CG in inches from the datum
by using this formula:
The CG of this airplane is located at station 160.9 inches
aft of the datum. It is important for longitudinal stability
that the CG be located ahead of the center of lift of a wing.
Since the center of lift is expressed as a percentage of the
MAC, the location of the CG is expressed in the same
terms.
Trailing Edge Mean
Aerodynamic Cord
Leading Edge Mean
Aerodynamic Cord
3–
Figure 3-10. Aircraft weight and balance calculation diagram.
4–
4 Chapter Small Fixed Wing Aircraft Operational
Weight and Balance Computations
Weight and balance data allows the pilot to determine the
loaded weight of the aircraft and determine whether or not
the loaded CG is within the allowable range for the weight.
See Figure 4-1 for an example of the data necessary for
these calculations.
Figure 4-1. Weight and balance data needed to determine proper
loading of a small fixed wing aircraft.
Determining the Loaded Weight and CG
An important part of preflight planning is to determine
that the aircraft is loaded so its weight and CG location
are within the allowable limits. [Figure 4-2] There are two
ways of doing this: by the computational method using
weight, arms, and moments; and by the loading graph
method, using weight and moment indexes.
Figure4-2. Airplane loading diagram.
Computational Method
The computational method uses weights, arms, and
moments. It relates the total weight and CG location to a
CG limits chart similar to those included in the TCDS and
the POH/AFM.
A worksheet such as the one in Figure 4-3 provides space
for all of the pertinent weight, CG, and moment along with
the arms of the seats, fuel, and baggage areas.
4–
When planning the flight, fill in the blanks in the
worksheet with the specific data for the flight. [Figure 4-4]
Pilot..............................................120 lbs
Front seat passenger.....................180 lbs
Rear seat passenger......................175 lbs
Fuel 88 gal...................................528 lbs
Baggage A...................................100 lbs
Baggage B......................................50 lbs
Determine the moment of each item by multiplying its
weight by its arm. Then determine the total weight and the
sum of the moments. Divide the total moment by the total
weight to determine the CG in inches from the datum. The
total weight is 3,027 pounds and the CG is 43.54 inches aft
of the datum.
To determine that the airplane is properly loaded for this
flight, use the CG limits envelope in Figure 4-5 (which
is typical of those found in the POH/AFM). Draw a line
vertically upward from the CG of 43.54 inches, and one
horizontally to the right from the loaded weight of 3,027
pounds. These lines cross inside the envelope, which
shows the airplane is properly loaded for takeoff, but 77
pounds overweight for landing.
Figure 4-4. Completed weight and balance worksheet.
Figure 4-3. Blank weight and balance worksheet.
Figure 4-5. Center of gravity limits chart from a typical POH.
4–
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Aircraft Weight and Balance Handbook(17)
