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¶ INTRODUCTION
The concept of weight and balance is critical in aviation industry.
Compliance with the known limits is very important relating to the
flight safety. Aircraft balance refers to the location of the center of
gravity (CG) of an aircraft. Operation with CG outside the approved
limits results in control difficulty.
Preflight planning should include a check of performance charts to
determine if the aircraft's weight may contribute to hazardous flight
operations.
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¶ CG Concept
The CG of a body is the theoretical point at which the entire weight of
that body is assumed to be concentrated. In a airplane, the CG is a
point at which the aircraft would balance if it were suspended atthat
point.
The primary concern in balancing an aircraft is the fore and aft
location of the CG along the longitudinal axis. The CG is not
necessarily a fixed point. Its location depends on the distribution of
weight in the aircraft. The distance between the forward and back limits
for the position of the center for gravity or CG range is certified for
an aircraft by the manufacturer.\
On small airplanes and on helicopters, the CG location is identified as
being a specific number of inches or millimeters from the datum and
the CG range is identified the same way. On larger airplanes, the center
of gravity and its range are typically identified in relation to the
width of the wing (usually mean aerodynamic chord is used).
¶ Balancing the aircraft
Each aircraft has a determed and precalculated weight and balance
references and numbers. Basically speaking it looks like this:
- Manufacturer defines:
- reference datum - an imaginary vertical plane or line from
which all measurements of arm are taken. - the arms (moment arms) - the horizontal distance in inches
from the reference datum line to the CG of an item. The
algebraic sign is plus (+) if measured aft of the datum and
minus (--) if measured forward of the datum. - the empty weight and the empty weight centre of gravity.
- reference datum - an imaginary vertical plane or line from
- All calculated data are published on pilot oparating manual and
weight and balance sheet.
The actual location of the CG can be altered by many variable factors
and is usually controlled by the pilot:
- Placement of baggage and cargo items determines the CG location.
- The assignment of seats to passengers can also be used as a means
of obtaining a favorable balance. If an aircraft is tail heavy, it
is logical to place heavy passengers in forward seats. - Fuel burn can also affect the CG based on the location of the fuel
tanks. For example, most small aircraft carry fuel in the wings very
near the CG and burning off fuel has little effect on the loaded
CG.\
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¶ Effects of adverse balance
Flying an aircraft that is out of balance can produce increased pilot
fatigue with obvious effects on the safety and efficiency of flight.
Adverse balance conditions affect flight characteristics in much the
same manner as those mentioned for an excess weight condition. Stability
and control are also affected by improper balance.
It is vital to comply with weight and balance limits established for all
aircraft.
¶ Stability
- Loading in a nose-heavy condition causes problems in controlling and
raising the nose, especially during takeoff and landing. - Loading in a tail heavy condition has a serious effect upon
longitudinal stability, and reduces the capability to recover from
stalls and spins. - During landing exceeding the forward CG limit may result in
excessive loads on the nosewheel, decreased performance, higher
stalling speeds, and higher control forces.
¶ Control
- CG location that is beyond the forward limit may result in nose
heaviness, making it difficult or impossible to flare for landing. - CG location aft of the allowable range may include extreme control
difficulty, violent stall characteristics, and very light control
forces which make it easy to overstress an aircraft inadvertently. - A restricted forward CG limit is also specified to assure that
sufficient elevator/control deflection is available at minimum
airspeed.
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¶ Practical example
Let's take a look onto standard Cessna 152 weight and balance sheet and
try to calculate weight and CG for different phases of flight.
Conditions:
Pilot weight: 160 lbs
Passenger weight: 140 lbs
Fuel required for flight: 15 gallons
Fuel consumption for flight: 7 gallons Baggage: 50 lbs.
Usually weight and balance sheet for general aviation consists of 2
parts: data table and CG graph (envelope). Our first step is to fill the
table with actual data.
1. Fill the table with weight data.
Use the conditions from above to fill the table and calculate total
weight.
- Remember, that gallon is a volume value, not weight. To get fuel
mass, multiple volume by 6 (standard 100LL fuel weight per 1
gallon). - As far as we have a limit of 40 lbs in the baggare area 2, the only
option is to put baggage into area 1. - Make sure total weight is lower then maximum certified weight (1675
lbs in this case).
2. Calculate the moments.
* Multiply each weight that was entered by its corresponding arm from
the "Arm" column and put the result in the "Moment" column.
- Devide total moment by total weight to get current CG.
3. Verify the numbers.
Verify that the total weight and CG for takeoff fall within the
envelope.
4. Check weight and CG for landing.
During flight, fuel burn is normally the only weight change that takes
place. As fuel is used, an aircraft becomes lighter. Furthermore, fuel
moment also changes and it for sure affects total moment and CG
position.
- Fuel weight at destionation will be 8 * 6 = 48 lbs.
- Resulted moment will be 48 * 40 = 1920 in-lbs.
- Landing weight will be 1567 lbs, landing total moments is 52580
in-lbs, therefore CG on landing is going to be 33.55. Let's check
the envelope.
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Summary: weight and CG is insight the limits during all phases of
flight, nor extremly forward or aft, so the flight could be safely
performed.
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See also
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Reference
- FAA Pilot's Handbook of Aeronautical Knowledge
- Skybrary
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Author
- 531824 - Creation