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familiarize themselves with all the operating limitations
of each glider they fly. [Figure 5-16]
TERMS AND DEFINITIONS
The pilot should be familiar with terms used in
working the problems related to weight and balance.
The following list of terms and their definitions is
well standardized, and knowledge of these terms will
aid the pilot to better understand weight and balance
calculations of any glider.
• Arm (moment arm)—is the horizontal distance in
inches from the reference datum line to the center
of gravity of an item. The algebraic sign is plus
(+) if measured aft of the datum, and minus (–) if
measured forward of the datum.
• Ballast—is a removable weight installed to meet
minimum balance conditions to comply with center
of gravity limitations. Ballast may also be in
the form of water used to enhance the performance
of the glider.
• Center of gravity (CG)—is the point about
which a glider would balance if it were possible
to suspend it at that point. It is the mass center
of the glider, or the theoretical point at which
the entire weight of the glider is assumed to be
concentrated. It may be expressed in inches
from the reference datum, or in percent of mean
aerodynamic chord (MAC).
• Center-of-gravity limits—are the specified forward
and aft points within which the CG must be
located during flight. These limits are indicated
on pertinent glider specifications.
• Center-of-gravity range—is the distance between
the forward and aft CG limits indicated on pertinent
glider specifications.
• Datum (reference datum)—is an imaginary vertical
plane or line from which all measurements of
arm are taken. The manufacturer establishes the
datum. Once the datum has been selected, all
moment arms and the location of CG range are
measured from this point.
• Empty weight—is the weight as established by
the manufacturer, and which may be modified by
addition or deletion of equipment.
• Fuel load—is the expendable part of the load of
the self-launch glider. It includes only usable
Figure 5-16. VG Diagram.
5-11
fuel, not fuel required to fill the lines or that
which remains trapped in the tank sumps.
• Maximum gross weight—is a weight limitation
established by the manufacturer that must not be
exceeded. Some gliders may have two maximum
gross weights, one with ballast and one without.
• Mean aerodynamic chord (MAC)—is the average
distance from the leading edge to the trailing
edge of the wing. Some GFM/POHs present the
acceptable CG range as a percent of the Mean
Aerodynamic Chord (MAC).
• Moment—is the product of the weight of an item
multiplied by its arm. Moments are expressed in
pound-inches (lb.-in). Total moment is the weight
of the airplane multiplied by the distance
between the datum and the CG.
• Moment index (or index)—is a moment divided
by a constant, such as 100, 1,000, or 10,000. The
purpose of using a moment index is to simplify
weight and balance computations of gliders
where heavy items and long arms result in large,
unmanageable numbers.
• Standard weights—have been established for
numerous items involved in weight and balance
computations. These weights should not be used
if actual weights are available. Some of the standard
weights are:
Gasoline.............................................6 lb./US gal
Oil...................................................7.5 lb./US gal
Water.............................................8.35 lb./US gal
• Station—is a location in the airplane that is identified
by a number designating its distance in
inches from the datum. The datum is, therefore,
identified as station zero. An item located at station
+50 would have an arm of 50 inches.
• Useful load—is the weight of the pilot, passengers,
baggage, ballast, usable fuel, and drainable
oil. It is the empty weight subtracted from the
maximum allowable gross weight.
CENTER OF GRAVITY
Longitudinal balance affects the stability of the longitudinal
axis of the glider. To achieve satisfactory pitch
attitude handling in a glider, the CG of the properly
loaded glider is forward of the Center of Pressure (CP).
When a glider is produced the manufacturer provides
glider center-of-gravity limitations, which must be
complied with. These limitations are generally found
in the GFM/POH and may also be found in the glider
airframe logbook. Addition or removal of equipment,
　
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