Helicopter Weight and Balance
Most helicopters have a much more restricted CG range than do airplanes. In some cases, this range is less than 3". The exact location and length of the CG range is specified for each helicopter, and usually extends a short distance fore and aft of the main rotor mast or centered between the main rotors of a dual rotor system. Whereas airplanes have a center of gravity range only along the longitudinal axis, helicopters have both longitudinal and lateral center of gravity ranges. Because the wings extend outward from the center of gravity, airplanes tend to have a great deal of lateral stability. A helicopter, on the other hand, acts like a pendulum, with the weight of the helicopter hanging from the main rotor shaft.
Ideally, the helicopter should have such perfect balance that the fuselage remains horizontal while in a hover.
If the helicopter is too nose heavy or tail heavy while it is hovering, the cyclic pitch control will be used to keep the fuselage horizontal. If the CG location is too extreme, it may not be possible to keep the fuselage horizontal or maintain control of the helicopter.
When a helicopter is being weighed, the location of both longitudinal and lateral weighing points must be known to determine its empty weight and empty weight CG. This is because helicopters have longitudinal and lateral CG limits. As with the airplane, the longitudinal arms are measured from the datum, with locations behind the datum being positive arms and locations in front of the datum being negative arms. Laterally, the arms are measured from the butt line, which is a line from the nose to the tail running through the middle of the helicopter. When facing forward, arms to the right of the butt line are positive; to the left they are negative.
Before a helicopter is weighed, it must be leveled longitudinally and laterally. This can be done with a spirit level, but more often than not it is done with a plumb bob. For example, the Bell JetRanger has a location inside the aft cabin where a plumb can be attached, and allowed to hang down to the cabin floor. On the cabin floor is a plate bearing cross hairs, with the cross hairs corresponding to the horizontal and lateral axis of the helicopter. When the point of the plumb bob falls in the middle of the cross hairs, the helicopter is level along both axes. If the tip of the plumb bob falls forward of this point, the nose of the helicopter is too low; if it falls to the left of this point, the left side of the helicopter is too low. In other words, the tip of the plumb bob will always move toward the low point.
A Bell JetRanger helicopter is shown in Figure 4-35, with the leveling plate depicted on the bottom right of the figure. The helicopter has three jack pads, two at the front and one in the back. To weigh this helicopter, three jacks would be placed on floor scales, and the helicopter would be raised off the hangar floor. To level the helicopter, the jacks would be adjusted until the plumb bob point falls exactly in the middle of the cross hairs.
As an example of weighing a helicopter, consider the Bell JetRanger in Figure 4-35, and the following specifications and weighing data.
Using six column charts for the calculations, the empty weight and the longitudinal and lateral center of gravity for the helicopter would be as shown in Figure 4-36.
Based on the calculations in Figure 4-36, it has been determined that the empty weight of the helicopter is 1,985 lb, the longitudinal CG is at +108.73", and the lateral CG is at –0.31".
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