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weight of the occupants.
Step 2—Enter the moment for each item listed.
Remember “weight x arm = moment.” To simplify calculations,
the moments may be divided by 100.
Step 3—Total the weight and moments.
Step 4—To determine the CG, divide the moments by
the weight.
NOTE: The weight and balance records for a particular
glider will provide the empty weight and moment as
well as the information on the arm distance. [Figure 5-
18]
In Figure 5-18, the weight of each pilot has been
entered into the correct block in the table. For the front
seat pilot, multiplying 180 pounds by +30 inches yields
a moment of +5400 inch/pounds. For the rear seat pilot,
multiplying 200 pounds by -5 inches yields a moment
of -1000 inch/pounds.
The next step is to find the sum of all weights, and
record it: 980 pounds. Then, find the sum of all
moments, and record it: +16,400 inch/pounds.
Now we can find the Arm (the CG position) of the
loaded glider. Divide the total moment by the total
weight to discover the CG of the loaded aircraftglider.
So, +16,400 divided by 980 = +16.73 inches from
datum. [Figure 5-20]
We now know the total weight (980 pounds) and the
CG location (+16.73 inches from datum) of the loaded
glider. The final step is to determine whether these two
values are within acceptable limits. The GFM/POH
lists the maximum gross weight as 1,100 pounds. The
operating weight of 980 pounds is less than 1,100
pounds maximum gross weight. The GFM/POH lists
the approved CG range as between +14.80 inches and
+18.60 inches from datum. The operating CG is
+16.73 inches from datum and is within these limits.
We have determined that the weight and balance are
within operating limits.
BALLAST WEIGHT
Ballast weight is non-structural weight that is added
to a glider. In gliding, ballast weight is used for two
purposes. Trim ballast is used to adjust the location
of the center of gravity of the glider so handling
characteristics remain within acceptable limits.
Performance ballast is loaded into the glider to
improve high-speed cruise performance.
Removable trim ballast weights are usually made of
metal and are bolted into a ballast receptacle incorporated
in the glider structure. The manufacturer
generally provides an attachment point well forward
in the glider cabin for trim ballast weights. These
weights are designed to compensate for a front seat
pilot who weighs less than the minimum permissible
front seat pilot weight. The ballast weight mounted
well forward in the glider cabin helps place the CG
within permissible limits.
Some trim ballast weights are in the form of seat cushions,
with sand or lead shot sewn into the unit to provide
additional weight. This type of ballast, which is
installed under the pilot’s seat cushion, is inferior to
bolted-in ballast because of the propensity to shift
position. Seat cushion ballast should never be used
during acrobatic or inverted flight.
EFFECTS OF WATER BALLAST
Sometimes trim ballast is water placed in a tail tank
in the vertical fin of the fuselage. The purpose of the
fin trim ballast tank is to adjust CG location after
water is added to, or drained from, the main wing
ballast tanks. Unless the main wing ballast tanks are
precisely centered on the center of gravity of the
loaded aircraftglider, CG location shifts when water is
added to the main ballast tanks. CG location shifts
Figure 5-18. Weight and balance: front and rear seat pilot weights and moments.
5-14
again when water is dumped from the main ballast
tanks. Adjusting the amount of water in the fin tank
compensates for CG shifts resulting from changes in
the amount of water ballast carried in the main wing
ballast tanks. Water weighs 8.35 pounds per gallon.
Because the tail tank is located far aft, it does not take
much water to have a considerable effect on CG location.
For this reason, tail tanks do not need to contain a
large volume of water. Tail tank maximum water
capacity is generally less than two gallons of water.
Although some older gliders employed bags of sand or
bolt-in lead weights as performance ballast, water is
used most commonly to enhance high-speed performance
in modern sailplanes. Increasing the operating
weight of the glider increases the optimum speed-to-fly
during wings-level cruising flight. The higher groundspeed
that result provide a very desirable advantage in
cross-country soaring and in sailplane racing.
Water ballast tanks are located in the main wing panels.
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Glider Flying Handbook(51)
