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Pilot's Handbook of Aeronautical Knowledge
Weight and Balance
Balance, Stability, and Center of Gravity

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Principles of Weight and Balance Computations
It might be advantageous at this point to review and
discuss some of the basic principles of weight and balance
determination. The following method of computation can be
applied to any object or vehicle for which weight and balance
information is essential.

By determining the weight of the empty aircraft and adding
the weight of everything loaded on the aircraft, a total weight
can be determined—a simple concept. A greater problem,
particularly if the basic principles of weight and balance are
not understood, is distributing this weight in such a manner
that the entire mass of the loaded aircraft is balanced around a
point (CG) that must be located within specified limits.

The point at which an aircraft balances can be determined by
locating the CG, which is, as stated in the definitions of terms,
the imaginary point at which all the weight is concentrated. To
provide the necessary balance between longitudinal stability
and elevator control, the CG is usually located slightly
forward of the center of lift. This loading condition causes
a nose-down tendency in flight, which is desirable during
flight at a high AOA and slow speeds.

As mentioned earlier, a safe zone within which the balance
point (CG) must fall is called the CG range. The extremities
of the range are called the forward CG limits and aft CG
limits. These limits are usually specified in inches, along the
longitudinal axis of the airplane, measured from a reference
point called a datum reference. The datum is an arbitrary
point, established by aircraft designers, which may vary in
location between different aircraft. [Figure 9-2]

Weight and balance.
Figure 9-2. Weight and balance.

The distance from the datum to any component part or any
object loaded on the aircraft, is called the arm. When the
object or component is located aft of the datum, it is measured
in positive inches; if located forward of the datum, it is
measured as negative inches, or minus inches. The location
of the object or part is often referred to as the station. If
the weight of any object or component is multiplied by the
distance from the datum (arm), the product is the moment.
The moment is the measurement of the gravitational force
that causes a tendency of the weight to rotate about a point
or axis and is expressed in inch-pounds (in-lb).

To illustrate, assume a weight of 50 pounds is placed on
the board at a station or point 100 inches from the datum.
The downward force of the weight can be determined by
multiplying 50 pounds by 100 inches, which produces a
moment of 5,000 in-lb. [Figure 9-3]

Determining moment.
Figure 9-3. Determining moment.

To establish a balance, a total of 5,000 in-lb must be applied
to the other end of the board. Any combination of weight
and distance which, when multiplied, produces a 5,000 in-lb
moment will balance the board. For example (illustrated in
Figure 9-4), if a 100-pound weight is placed at a point (station)
25 inches from the datum, and another 50-pound weight is
placed at a point (station) 50 inches from the datum, the sum
of the product of the two weights and their distances will total
a moment of 5,000 in-lb, which will balance the board.

Establishing a balance.
Figure 9-4. Establishing a balance.