The empty weight and empty weight center of gravity (EWCG) of a helicopter are determined in the same way as for an airplane. See section, 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 1, with the following data specific to the aircraft.
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| Figure 1. Determining the longitudinal CG and the lateral offset moment |
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
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| Figure 2. Typical helicopter CG envelopes |
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.