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between 69 to 74 inches from the datum.
b. To work a CG problem, figure the EMPTY
WEIGHT CG first. On a piece of paper draw four
blocks. Title each block from left to right as shown
in figure 3.
(1) Under the block titled item, vertically
list ‘‘left wheel,’’ ‘‘right wheel,’’ and ‘‘nose/tail
wheel.’’
(2) Place a calibrated scale under each
wheel and record the weight on each gear, in pounds,
in the weight block along side the appropriate wheel.
This process is done with an empty fuel tank.
(3) Measure in inches the distance from the
datum line, or imaginary point identified by the
manufacturer (e.g., nose of the aircraft), to the center
line (C/L) of the three wheels. Record the distance
of each wheel and place it in the moment arm block
beside the appropriate wheel (see figure 2).
(4) Multiply the number of inches (arm)
by the weight on each wheel to get the moment (inchpounds)
for each wheel. Add the weight on the three
gears and the three moments in inch pounds and
divide the total weight into the total moment. The
sum is the ‘‘EMPTY WEIGHT CENTER OF
GRAVITY’’ in inches. In the sample case, the empty
weight CG is 80.8.
NOTE: All calculations should be carried
out to two decimal places.
16
AC 90-89A 5/24/95
ITEMS WEIGHT (LBS) ARM (INCHES) MOMENT (IN-LBS)
A/C
Pilot
Fuel
TOTALS
242
170
30
442
80.8
65
70
74
19560
11050
2100
32710
TOTAL MOMENT = Takeoff CG or 32710 = 74
TOTAL WEIGHT 442
FIGURE 3. TAKE-OFF CG
3. DETERMINING TAKE-OFF WEIGHT CG.
a. Since the aircraft’s empty weight and empty
weight CG are fixed numbers, the only way an aircraft’s
CG can be changed is by adding weight in
other locations.
b. For example, in figure 3, the aircraft’s
empty weight has been written in the appropriate
blocks. The pilot weighs 170 pounds and fuel (5 gallons)
weighs 30 pounds.
c. Again, all measurements are made from the
datum to the center line of the object that has been
added. Weight multiplied by inches from the datum
equals moment. Add the weights and moments to
find the take-off CG for that particular flight.
d. Loaded in this configuration, the aircraft
is within the CG flight envelope and is safe to fly.
17
5/24/95 AC 90-89A
ITEMS WEIGHT (LBS) ARM (INCHES) MOMENT (IN-LBS)
A/C
S/B
Pilot
Strobe
Fuel
Fuel
TOTALS
242
15
170
1.5
30
1.5
460
80.8
75
65
179
70
55
74.3
19560
1125
11050
268.5
2100
82.5
34186
TOTAL MOMENT = Alteration Takeoff Weight CG or 34186 = 74.3
TOTAL WEIGHT 460
FIGURE 4. ADDITIONAL EQUIPMENT ADDED
4. ADDING ADDITIONAL EQUIPMENT.
a. During flight testing, a strobe battery and
hand held radio are added. The battery/battery box
weight is 15 pounds and the location is 75 inches
aft of the datum; the strobe assembly weight is 1.5
pounds and is located 179 inches aft of the datum;
the radio’s weight is 1.5 pounds and is located 55
inches aft of the datum (see figure 4).
b. In the sample problem, the previous figures
for take-off weight and moment are still accurate,
hence those numbers have been listed in the appropriate
blocks.
(1) Add the battery, strobe, and radio numbers
in the appropriate locations and calculate the
totals. At 465 pounds, the aircraft is still 35 pounds
under its design gross weight limit of 500 pounds
but is out of balance because the CG has moved
.3 inches further aft (74.3 inches) than the allowable
rear CG limit of 74 inches.
(2) Since the aircraft is out of balance with
an aft CG, it is no longer 100 percent stable in pitch
and would be dangerous to fly. In most cases, it
is not the amount of weight added to the aircraft
that can cause a major safety problem but its location.
(3) To bring this aircraft back into the safe
CG range, the battery would have to be moved 9
inches forward (66 inches from the datum line).
Another alternative is to install 8 pounds of ballast
in the nose (20 inches from the datum).
(4) If the sample aircraft exceeded the
designer’s gross weight limit (e.g., 300 pound pilot)
instead of the CG limit, its climb, stall, and performance
capability would be poor and the possibility
for in-flight structural failure would be high.
NOTE: In the sample weight and balance,
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