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If a weight on either side is moved, or a weight is changed, the plank will no longer be in balance.
An aircraft in flight is very similar. The pivot point (fulcrum) is the located at the Center of Lift of the
wing. The load on the left is the total weight of the aircraft located at the Center of Gravity (CG) with a
counter-balancing force on the right provided by the elevators.
Note that if the location of the CG or the weight on the left changes, the elevator force must also change
in order to maintain the balance. Also note, if the fulcrum (center of wing lift) changes, the elevator force
must be changed to maintain a balanced condition. Such an event can occur when the angle of attack
and/or engine thrust is changed.
The Center of Gravity
As previously stated, the weight of an aircraft and its load is distributed throughout the aircraft as shown
below by the small downward arrows. All of the small individual weights can be resolved into one single
weight acting at the Center of Gravity and shown as the large arrow..
Weight and Balance
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From the analogy of the plank above, we can see that if we change either the weight of the aircraft, or the
center of gravity, this in turn changes the force (either up or down) that the elevator must produce.
For each aircraft design, the manufacturer specifies a maximum weight for operation of the aircraft, and
also a maximum forward and rearward location of the Center of Gravity (CG). This is called the CG
RANGE. For safe operation, the aircraft must be operated within these parameters.
In order to calculate where the center of gravity is located, the manufacturer specifies some point in the
aircraft as a reference point (DATUM). In many Cessna 172 type aircraft, the datum is located at the
lower firewall of the cabin, just ahead of the rudder pedals. You as a pilot do not need to know where this
is located in order to calculate weight and balance, as the manufactirer provided moment arm and/or
Weight and Balance
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moment in the weight and balance tables for the aircraft. An aircraft mechanic must know where whis
point is, however, if equipment change is made to the aircraft which changes either the aircraft CG or
Empty Weight.
An airplane is designed and certified to withstand specified loading on it’s structure. As long as the gross
weight and load factor are within limits, the aircraft can be operated safely. Continued operaton of an
aircraft in an overloaded condition can cause structural failures. Metal fatigue is hastened, and can lead to
stress failures even in normal operating modes.
Effect on Wing Loading
The location of the CG affects the total load which the wings must sustain. If the CG is at or near the
Center of Lift of the wing the elevators do not have to generate much (if any) downward force. If the CG
is aft of the center of lift, the elevators must produce an upward force. If the aircraft is nose heavy
(forward CG) the load on the wing and elevator surfaces will be greater.
An aft CG location causes the airplane to require more "nose down" elevator for stall recovery. A
forward CG enhances stall recovery as the aircraft will naturally want to "nose down".
Definitions
MOMENT ARM -- a horizontal distance of an object measured from a defined “datum” point to the CG
of the object, usually measured in inches. A (+) arm means the object is behind the datum. A (-) arm
indicates the object is forward off the datum point.
MOMENT -- the product of a moment arm and the associated weight. (Weight x Arm)
EMPTY WEIGHT-- the combined weight of the aircraft, and permanently mounted equipment. It
includes unusable fuel and hydraulic fluid. Most manufactures include the oil in the empty weight.
Center of Gravity -- the point at which the airplane will be in balance.
CG Limits -- the most forward and most rearward CG points specified by the manufacturer for safe
control.
CG Range -- the distance from the most forward and rearward CG points as specified for the given
aircraft.
DATUM -- a point in the aircraft from which all moment arms are measured.
FUEL LOAD -- the weight of the useable fuel. It does not include unusable fuel in the tanks and lines.
GROSS WEIGHT-- Total weight of aircraft, fuel, passengers and baggage.
Weight and Balance
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MAX LANDING WEIGHT - Maximum gross weight allowed for landing.
MAX RAMP WEIGHT -- Maximum gross weight prior to taxi and take-off.
MAX TAKEOFF WEIGHT -- the maximum allowable weight at start of takeoff run
　
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