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REMOVE a changed item, and to ADD the new or shifted item into the new location.
Weight Change
Example 1:
An airplane takes off with a Gross Weight of 6230 lb., and a CG of 79.0. The CG of the fuel is at 87.00
aft of datum. What is the New CG location after 50 gallons of fuel is burned?
Procedure:
Subtract the Weight and Moment of the burned fuel from the initial values to arrive at a new set of
values. At 6 pounds per gallon, the burned fuel weight is 300 pounds.
WEIGHT CHANGE PROBLEM
Weight and Balance
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WEIGHT CG MOMENT
Initial Weight 6230 79.00 492,170
Burned Fuel -300 87.00 -26,100
New Weight 5930 New CG 466,070
NEW CG (after Fuel Burned) = 466,070 / 5930 = 78.59
Weight Shift
Example 2:
The gross weight of the aircraft is 3,000 lbs. with a CG of 60 in. Since takeoff, 25 gallons of fuel has
been used. The fuel cell CG is 65 in. aft of Datum (Station 65).
Also, a 200 pound passenger moves from Station 50 to Station 90. (Note: Some problems will state the
CG location as "Station". The 50 and 90 are CG location in inches aft of datum respectively).
Find New CG.
Procedure:
1. Subtract burned Fuel
2. Subtract Passenger who moves from the old location.
3. Add passenger who moves to the new location.
WEIGHT SHIFT PROBLEM
Weight CG Station Moment
Initial Loading 3000 60.00 180,000
Fuel Burned -150 65.00 9,750
Passenger Off -200 50.00 -10,000
Passenger On +200 90.00 +18,000
New Totals 2850 New CG 178,250
New Cg = 178,250 / 2850 = Station 62.54
Back to Home Back to Table of Conents To Aircraft Performance
Weight and Balance
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Aircraft Performance
Aircraft Performance
Factors Affecting Performance
Performance of the aircraft depends on the density of the air in which it flies.
Factors affecting air density are:
1. Barometric pressure
2. Altitude
3. Temperature
4. Humidity
Standard Atmosphere Definition
The International Standards Association (ISA) has defined a Standard Atmosphere as:
· Sea Level Barometric Pressure of 29.92 inches of Mercury (in. Hg)
· Sea Level Temperature of 15° Celsius (15° C or 59° F)
· Relative humidity of 0 %
· Standard temperature lapse rate of 2° C per 1000 feet altitude
· Standard pressure lapse rate of 1 in. Hg per 1000 feet altitude
· A standard decrease in density as altitude increases
The standard atmosphere definition provides a means for instrument and aircraft manufacturers to
specify the performance of their products in a uniform way. This definition was arrived at by studying
the average sea level pressure and temperature over a number of years, seasons, and locations around the
world.
Seldom will an aircraft be in standard atmosphere conditions. In order to define peformance of an
instrument or an aircraft in a non-standard atmosphere, conversions must be applied to adjust the
readings or performance numbers to agree with the standard atmosphere. This adjustment is called
Density Altitude, and will be more fully defined later in this chapter.
Effects of Non-standard Air Density
Air Density decreases:
· With Air Temperature Increase
· With Altitude Increase
· With Humidity Increase
· With Barometric Pressure Decrease
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With lower air density:
· The engine develops less power.
· The propeller produces less thrust.
· The wings produce less lift.
This results in:
· Longer takeoff run
· Poorer climb performance
· Longer landing distance
Density Altitude
Density altitude is a way of relating the density of the air you are in compared to the standard
atmosphere. Three atmospheres are illustrated. The Standard Atmosphere (29.92 in. Hg and 15 degrees
Celsius) in middle shown in gray. A less dense atmosphere (A ) (lower pressure and/or Higher
Temperature) is shown on the right in red. A more dense atmosphere (B) (higher pressure and/or Colder
Temperature) is illustrated on the left in blue.
Aircraft Performance
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Aircraft Performance
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If you are at an actual (true) altitude at location A in atmosphere (A) (the red atmosphere on the right),
you will have to go to altitude (A') in the Standard Atmosphere to find the same air density. This altitude
　
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