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Airplane Weight & Balance

Airplane Weight & Balance

The pilot should always be aware of the consequences of overloading. An overloaded aircraft may not be able to leave the ground, or if it does become airborne, it may exhibit unexpected and unusually poor flight characteristics. If not properly loaded, the initial indication of poor performance usually takes place during takeoff.
There are many factors that effect stability of an airplane to perform under certain conditions one of them is a proper loading passengers, cargo and fuel. You as a pilot need to learn how to determent if your airplane is in Weight and Balance limits which are established by manufacturers
Each airplane is design to be operated at or bellow specified maximum weights and certain center of gravity range. Stability of an airplane is important because improper loading can adversely effect aircraft performance in flight
Lateral and longitudinal unbalance

Although the aircraft was weighed during the certification process, this data is not valid indefinitely. Equipment changes or modifications affect the weight and balance data.
Weight and balance computations should be part of every preflight briefing. Never assume three passengers are always of equal weight. Instead, do a full computation of all items to be loaded on the aircraft, including baggage, as well as the pilot and passenger.
Pre flight planning should include a check of performance charts to determine if the aircraft’s weight may contribute to hazardous flight operations

Let’s look at a various aircraft weights you should be familiar

Basic Empty Wight
The starting point for any calculations is the Basic Empty Wight which includes airplane, optional equipment, unusable fuel and other operating fluids including engine oil.

Payloads includes weight of the passengers, any cargo or baggage. Overloading also effects stability. An aircraft that is stable and controllable when loaded normally may have very different flight characteristics when overloaded.

Useful Load
Useful Load = Payload + Flight Crew + Usable Fuel

Ramp Weight
Some of the Basic Empty Weight + Useful Load is called Ramp Weight which is the weight of an aircraft dooring ground operations.

Takeoff Weight
You may adjust Ramp Weight by subtracting - Fuel Used For Start, Taxi and Run-up in order to get a Takeoff Weight. This weight should not exceed Maximum Take-off Weight which you can find in POH (pilot operating handbook) or W&B records of an airplane you are flying
The excess weight can overstress the aircraft and degrade the performance. Pilots should never overload an aircraft because overloading causes structural damage and failures. Even a minor overload may make it impossible for the aircraft to clear an obstacle that normally would not be a problem during takeoff under more favorable conditions.

Overloading Aircraft

Excessive weight reduces the flight performance in almost every respect. For example, the most important performance deficiencies of an overloaded aircraft are:
- Higher takeoff speed
- Longer takeoff run
- Reduced rate and angle of climb
- Lower maximum altitude
- Shorter range
- Reduced cruising speed
- Reduced maneuverability
- Higher stalling speed
- Higher approach and landing speed
- Longer landing roll
- Excessive weight on the nose wheel or tail wheel
- Overloading has an adverse effect on all climb and cruise performance which leads to overheating during climbs, added wear on engine parts, increased fuel consumption, slower cruising speeds, and reduced range

Landing Weight
Airplane also have a maximum Landing Weight, this weight could be determent by subtracting Fuel Used in Flight from Take-off Weight before the flight. The maximum Landing Weight could also be found in airplane POH. If you operate your airplane above the maximum Landing Weight specified by manufacturer the shock absorbed by landing can overstress the landing gear of an airplane

Effect of Load Distribution
The effect of the position of the CG on the load imposed on an aircraft’s wing in flight is significant to climb and cruising performance.

Forward Loading
An aircraft with forward loading is “heavier” and consequently, slower than the same aircraft with the CG further aft. With forward loading, “nose-up” trim is required in most aircraft to maintain level cruising flight. Nose-up trim involves setting the tail surfaces to produce a greater down load on the aft portion of the fuselage, which adds to the wing loading and the total lift required from the wing if altitude is to be maintained. This requires a higher AOA (angle of attack)of the wing, which results in more drag and, in turn, produces a higher stalling speed.

- Steering difficulties on the ground may occur in nose-wheel-type aircraft, particularly during the landing roll and takeoff. To summarize the effects of load distribution:
-The CG position influences the lift and AOA (angle of attack) of the wing, the amount and direction of force on the tail, and the degree of deflection of the stabilizer needed to supply the proper tail force for equilibrium. The latter is very important because of its relationship to elevator control force.
- The aircraft stalls at a higher speed with a forward CG location. This is because the stalling AOA (angle of attack) is reached at a higher speed due to increased wing loading.
- Higher elevator control forces normally exist with a forward CG location due to the increased stabilizer deflection required to balance the aircraft.
- A forward CG location increases the need for greater back elevator pressure. The elevator may no longer be able to oppose any increase in nose-down pitching. Adequate elevator control is needed to control the aircraft throughout the airspeed range down to the stall.

Aft Loading
With aft loading and “nose-down” trim, the tail surfaces exert less down load, relieving the wing of that much wing loading and lift required to maintain altitude. The required AOA (angle of attack) of the wing is less, so the drag is less, allowing for a faster cruise speed. Generally, an aircraft becomes less controllable, especially at slow flight speeds, as the CG is moved further aft. An aircraft which cleanly recovers from a prolonged spin with the CG at one position may fail completely to respond to normal recovery attempts when the CG is moved aft by one or two inches.
- The recovery from a stall in any aircraft becomes progressively more difficult as its CG moves aft. This is particularly important in spin recovery, as there is a point in rearward loading of any aircraft at which a “flat” spin develops
- The aircraft cruises faster with an aft CG location because of reduced drag. The drag is reduced because a smaller AOA (angle of attack) and less downward deflection of the stabilizer are required to support the aircraft and overcome the nose-down pitching tendency.
- The aircraft becomes less stable as the CG is moved rearward. This is because when the CG is moved rearward it causes an increase in the AOA (angle of attack). Therefore, the wing contribution to the aircraft’s stability is now decreased, while the tail contribution is still stabilizing. When the point is reached that the wing and tail contributions balance, then neutral stability exists. Any CG movement further aft results in an unstable aircraft

Weight Changes
The operating weight of an aircraft can be changed by simply altering the fuel load. Gasoline has considerable weight—6 pounds per gallon. During flight, fuel burn is normally the only weight
change that takes place. As fuel is used, an aircraft becomes lighter and performance is improved.

The proper loading techniques of an aircraft and good understanding of weight and balance are very important for any pilot, ether he/she is a student, recreational, private or commercial pilot
Let’s review some of the principles that you as a pilot can use to make sure you know the proper way to figure weight and balance for an aircraft.

The basic formula that everyone use is WEIGHT x ARM = MOMENT

DATUM - is an imaginary vertical line from which all measurements of arm are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of CG range are measured from this location point

WEIGHT for W&B - is useful load is the weight of the pilot, copilot, passengers, baggage, usable fuel, and drainable oil. It is the basic empty weight subtracted from the maximum allowable gross weight. This term applies
to general aviation (GA) aircraft only

ARM - is the horizontal distance in inches from the reference datum line to the CG of an item. The sign is plus (+) if measured aft of the datum, and minus (–) if measured forward of the datum.

CG - Center of gravity is the point about which an aircraft would balance if it were possible to suspend it at that point. It is the mass center of the aircraft, or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated. It may be expressed in inches from the reference datum, or in percent of MAC. The CG is a three-dimensional point with longitudinal, lateral, and vertical positioning in the aircraft. CG computes along LONGITUDINAL AXIS of the aircraft, and LONGITUDINAL AXIS runs from the nose to the tail of the aircraft, CG moves forward and aft depending on how you load your aircraft

MOMENT - is the product of the weight of an item multiplied by its arm. Moments are expressed in pound-inches (in-lb). Total moment is the weight of the airplane multiplied by the distance between the datum and the CG.

To find out the WEIGHT you need to add or subtract from specific location you should use this formula:

If you know the MOMENT and the WEIGHT and you trying to figure out the ARM you should use this formula:

When you load an aircraft you will get all the WEIGHTS and after multiplying by ARM you will get all the MOMENTS and what you want to know at the end is the BALANCE point of the aircraft it’s called CG (center of gravity) or the aircraft.
To find this CG point or a balance of your aircraft you need to use this formula:

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Comments (2) Comments
Comment By Alexander - Wed, Jan 25th, 2012 6:43 PM
Hello Jeff This subject got my interest and attention as well. Basically, there is no measurement for a vertical CG-only laterally and longitudinally. The reason being is that equipment contained within the fuselage can’t really be moved far enough up, down, left or right to affect anything. Fore and aft, or course, is whole different animal! It’s general procedure to put everything REALLY heavy down low. Engines come to mind, there are exceptions to that, such as the Boeing YC-14 and the Antonov An-72, which had the engines basically mounted above the wings. On our cargo airplanes, we try to distribute the load evenly between upper and lower decks, but it doesn’t always work out that way. The MACs and CGs are always figured on fore and aft loading and fuel distribution in the wing tanks. Good Question!
Comment By Jeff.Liu - Tue, Jan 24th, 2012 4:20 AM
How to measure the vertical center of gravity a aircraft? I need know, please tell me.
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