Determining the Loaded CG of a
Helicopter
The empty weight and empty weight center of gravity
(EWCG) of a helicopter are determined in the same way
as for an airplane. See Chapter 5, Single-Engine Aircraft
Weight and Balance Computations. The weights recorded
on the scales supporting the helicopter are added and their
distances 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 modern airplanes.
When the datum is ahead of the aircraft, all longitudinal
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. The
butt line zero (or sometimes referred to as the buttock) is 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 roll clockwise are positive (+), and those that cause it to
roll counterclockwise are negative (–).
To determine whether or not a helicopter is within both
longitudinal and lateral weight and balance limits, construct
a table like the one in Figure 8-4, with the following data
specific to the aircraft
Empty weight ..................................... 1,545 lb
EWCG ................................................ 101.4 inches aft of
the datum
Lateral balance ................................... arm 0.2 inches right
of BL 0
Maximum allowable gross weight ..... 2,250 lb
Pilot .................................................... 200 lb @ 64 inches
aft of datum and
13.5 inches right of
BL 0
Passenger ........................................... 170 lb @ 64 inches
aft of datum and
–13.5 in left of BL 0
Fuel (48 gal) ....................................... 288 lb @ 96 inches
aft of datum and
–8.4 inches left of
BL 0
Check the helicopter CG envelopes in Figure 8-3 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.
In the lateral offset moment envelope, draw a line vertically
upward from the –1,705 lb-in point (on the left side of the
horizontal axis) 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 an
Using Fuel
Consider the helicopter in Figure 8-4. The first leg of the
flight consumes 26 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 8-5 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 8-3, both the longitudinal CG and the
lateral offset moment fall outside of the approved area of
the envelope. The aircraft longitudinal CG is too far aft and
the potential for excessive tail-low attitudes is very high.
Under these conditions, it is possible that there will not be
enough forward cyclic authority to maintain level flight
The helicopter’s lateral offset moment is too far right and
may lead to control issues, as well as an increased hazard of
dynamic rollover. One possible option to bring the aircraft
loading conditions within the approved envelope is to load
either ballast or a passenger, as computed in Figure 8-6 and
plotted in Figure 8-3.
Figure 8-6. Determining the longitudinal CG and the lateral offset moment for the second leg of the flight with ballast and/or a different
passenger.
Item Weight WeightLongitude
Arm
Latitude
Arm
Longitude
Moment
Longitude
CG
Lateral Offset
Moment
95.0
Helicopter empty weight
Pilot
Ballast/Passenger
Fuel (22 gallons)
156,663
12,800
9,600
14,976
194,039
1,545
200
150
132
2,027
101.4
64.0
64.0
96.0
309
+2,700
−2,025
−1,310
−731
1,545
200
150
132
2,027
+0.2
+13.5
−13.5
−8.4
xx = =
Figure 8-5. Determining the longitudinal CG and the lateral offset moment for the second leg of the flight.
Item Weight WeightLongitude
Arm
Latitude
Arm
Longitude
Moment
Longitude
CG
Lateral Offset
Moment
98.2
Helicopter empty weight
Pilot
Fuel (22 gallons)
156,663
12,800
14,976
184,439
1,545
200
132
1,877
101.4
64.0
96.0
309
+2,700
−1,310
+1,699
1,545
200
132
1,877
+0.2
+13.5
−8.4
xx = =
Figure 8-4. Determining the longitudinal CG and the lateral offset moment.
Item Weight WeightLongitude
Arm
Latitude
Arm
Longitude
Moment
Longitude
CG
Lateral Offset
Moment
94.4
Helicopter empty weight
Pilot
Passenger
Fuel (48 gallons)
156,663
12,800
10,880
27,648
207,991
1,545
200
170
288
2,203
101.4
64.0
64.0
96.0
309
+2,700
−2,295
−2,419
−1,705
1,545
200
170
288
2,203
+0.2
+13.5
−13.5
−8.4
xx = =