# Balance ## **Center of Gravity** Balance is of primary importance to aircraft stability. The CG is the point about which an aircraft would balance if it were possible to support the aircraft at that point. An aircraft should never be flown if the aviator is not satisfied with its loading and balance condition. *** ## **Center of Gravity** The CG is defined as the theoretical point where all the aircraft’s weight is considered to be concentrated. If an aircraft is suspended by a cable attached to the CG point, it balances like a teeter-totter. For aircraft with a single main rotor, the CG is usually close to the main rotor mast. The CG is not necessarily a fixed point; its location depends on distribution of items loaded in the aircraft. As variable load items are shifted or expended, there is a resultant shift in CG location. If mass center of an aircraft is displaced too far forward on the longitudinal axis, a nose heavy condition results. Conversely, if mass center is displaced too far aft on the longitudinal axis, a tail heavy condition results. An unfavorable location of the CG could possibly produce such an unstable condition that the pilot could lose control of the aircraft. *** ## **Lateral Balance** Location of the CG with reference to the lateral axis is important. The design of an aircraft is such that symmetry is assumed to exist about a vertical plane through the longitudinal axis. This means for each item of weight existing to the left of the fuselage centerline there is generally an equal weight existing at a corresponding location on the right. Lateral mass symmetry, however, may be easily upset due to unbalanced lateral loading. Location of the lateral CG is not only important from the aspect of loading rotary-wing aircraft, but is also extremely important when considering FW exterior drop loads. The position of the lateral CG is not computed, but the crew must be aware adverse effects arise as a result of a laterally unbalanced condition. *** ## **Balance Definitions** Definitions of the more important terms pertaining to balance and its relationship to aircraft weight distribution are as follows. *** ### **Arm** Arm is the distance of the center of gravity of an item from a reference datum. Another term used interchangeably with arm is station. If the component or object is located rear of the datum, it is measured as a positive number and usually referred to as inches aft the datum. Conversely, if the component or object is located forward of the datum, it is indicated as a negative number and usually referred to as inches forward the datum. *** ### **Moment** Moment is a measure of the rotational tendency of a weight about a point. The moment of an item product is the weight of an item multiplied by its arm. Moments are expressed in pound-inches (lb-in). Total moment is the weight of the airplane multiplied by the distance between the datum and the CG. Moment is also referred to as tendency of an object to rotate or pivot about a point. The farther an object is from a pivotal point, the greater its force. *** ### **Reference Datum** Reference datum is a zero reference point from which distances are measured. If a negative arm is encountered, the corresponding moment will also be negative. Simplified moment is one which has been reduced in magnitude through division by a constant. For example, 3,201 inch pounds/1,000 is the simplified expression of 3,200,893 divided by 1,000 and rounded to the nearest whole number. *** ### **Aircraft Station** Aircraft station is location in the airplane that is identified by a number designating its distance in inches from the datum. The datum is, therefore, identified as station zero. An item located at station +50 would have an arm of 50 inches. The number designation of this station signifies its distance from the reference datum. A station forward of the reference datum is negative, while a station aft of the reference datum is positive. Figure 2-1 illustrates location of stations. ***
### **Gross Weight Moment** Gross weight moment is the sum of moments of all items making up the aircraft in the gross weight condition. Gross weight moment is the product of gross weight multiplied by gross weight arm. *** ### **Basic Arm** Basic arm is the distance from the reference datum to the aircraft Basic Weight center of gravity. Basic arm is determined by dividing the aircraft basic moment by the aircraft Basic Weight. *** ### **Gross Weight Arm** Gross weight arm is the distance from reference datum to the aircraft CG in gross weight condition.
*** ## Principles of Moments To understand balance, a working knowledge of the principle of moments is necessary. To calculate a moment, force (or weight) and distance must be known. The distance is measured from some desired known point (reference point or reference datum) to the point through which the force acts. A moment is meaningless unless the reference point about which the moment was calculated is specified. For the purpose of illustration, an aircraft may be compared to a seesaw with the sum of the moments on each side of the balance point or fulcrum equal in magnitude (figure 2-2). The moment produced about the fulcrum by the 200-pound weight is 200 pounds x -50 inches = -10,000 inch-pounds counterclockwise. The moment produced about the same reference point by the 100-pound weight is 100 pounds x 100 inches = 10,000 inch-pounds clockwise. In this case, the clockwise moment counterbalances the counterclockwise moment, and the system is in equilibrium. This example illustrates the principle of moments: for a system to be in static equilibrium, the sum of the moments about any point must equal zero. The clockwise moment is arbitrarily given a positive sign, while the counterclockwise moment is given a negative sign. In determining the balance of an aircraft, the fulcrum or CG is the unknown and must be determined.
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