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are added and their distance from the datum are used to
compute the moments at each weighing point. The total
moment is divided by the total weight to determine the
location of the CG in inches from the datum. The datum of
some helicopters is located at the center of the rotor mast,
but since this causes some arms to be positive (behind the
datum) and others negative (ahead of the datum), most
modern helicopters have the datum located ahead of the
aircraft as do most airplanes. When the datum is ahead of the
aircraft, all arms are positive.
The lateral CG is determined in the same way as the
longitudinal CG, except the distances between the scales and
butt line zero (BL 0) are used as the arms. Arms to the right
of BL 0 are positive and those to the left are negative.
Butt (or buttock) line zero: A line
through the symmetrical center of an
aircraft from nose to tail. It serves as
the datum for measuring the arms
used to find the lateral CG. Lateral
moments that cause the aircraft to
rotate clockwise are positive (+), and
those that cause it to rotate counterclockwise
are negative (–).
Figure 7-2. Typical helicopter CG envelopes.
Figure 7-3. Determining the longitudinal CG and the lateral offset moment.
Maximum permissible hoist load:
The maximum external load that is
permitted for a helicopter to carry.
This load is specified in the POH.
7– 3
Determine whether or not a helicopter with these specifica-tions
is within both longitudinal and lateral weight and balance limits
by constructing a chart like the one in Figure 7-3:
Empty weight ......................................................... 1,545 lbs
Empty-weight CG ....................... 101.4 in. aft of the datum
Lateral balance arm .............................. 0.2 in. right of BL 0
Maximum allowable gross weight ........................ 2,250 lbs
Pilot ........................................ 170 lbs @ 64 in. aft of datum
and 13.5 in. left of BL 0
Passenger ............................... 200 lbs @ 64 in. aft of datum
and 13.5 in. right of BL 0
Fuel 48 gal ............................. 288 lbs @ 96 in. aft of datum
and 8.4 in. left of BL 0
Check the helicopter CG envelopes in Figure 7-2 to determine
whether or not the CG is within limits both longitudinally
and laterally.
In the longitudinal CG envelope, draw a line vertically upward
from the CG of 94.4 inches aft of datum and a horizontal line
from the weight of 2,203 pounds gross weight. These lines
cross within the approved area.
Lateral offset moment: The
moment, in lb-in, of a force that
tends to rotate a helicopter about its
longitudinal axis. The lateral offset
moment is the product of the weight
of the object and its distance from
butt line zero. Lateral offset
moments that tend to rotate the
aircraft clockwise are positive, and
those that tend to rotate it
counterclockwise are negative.
Figure 7-4. Determining the longitudinal CG and the lateral offset moment for the second leg of the flight.
Figure 7-5. Determining the longitudinal CG and the lateral offset moment for the second leg of the flight with the pilot flying
from the right seat.
In the lateral offset moment envelope, draw a line vertically
upward from left, or –1,705 lb-in, and a line horizontally
from 2,203 pounds on the gross weight index. These lines
cross within the envelope, showing the lateral balance is also
within limits.
Effects of Offloading Passengers and Using Fuel
Consider the helicopter in Figure 7-3. The first leg of the flight
consumes 22 gallons of fuel, and at the end of this leg, the
passenger deplanes. Is the helicopter still within allowable
CG limits for takeoff?
To find out, make a new chart like the one in Figure 7-4 to
show the new loading conditions of the helicopter at the
beginning of the second leg of the flight.
Under these conditions, according to the helicopter CG
envelopes in Figure 7-2, the longitudinal CG is within limits.
However, the lateral offset moment is excessive since both
the pilot and the fuel are on the left side of the aircraft. If the
POH allows it, the pilot may fly the aircraft on its second leg
from the right-hand seat. According to Figures 7-5 and 7-2,
this will bring the lateral balance into limits.
7– 4
8– 1
Chapter 8
Use of Computers for Weight
and Balance Computations
Almost all weight and balance problems involve only simple
math. This allows slide rules and hand-held electronic
calculators to relieve us of much of the tedium involved with
　
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