How Center Of Gravity Shifts With Fuel Burn

The center of gravity (CG) of an aircraft can change with fuel burn, depending on the location of the fuel tank. If the fuel tank is located exactly on the CG, the CG will remain unchanged as the fuel is burned. However, if the fuel tank is located forward or aft of the CG, the CG will shift in the opposite direction as the fuel is consumed. This shift can have an impact on the stability and handling of the aircraft, with an aft CG generally being more desirable for reduced trim drag. Some aircraft, such as the A320, have a CG that moves backward with fuel consumption, while others, like the 737, may have a CG that moves in both forward and backward directions depending on which fuel tanks are used. Calculating the CG position during flight is important for pilots to ensure the aircraft remains within safe operating limits.

The CG moves forward as fuel is burned

The movement of an aircraft's centre of gravity (CG) as fuel is burned depends on the location of the fuel tank(s) in relation to the CG. If the fuel tank is located exactly on the CG, the CG position will not change as fuel is consumed. However, if the tank is located in front of or behind the CG, the CG will shift forward or backward, respectively, as fuel is burned.

In most small aircraft, the fuel tanks are positioned forward of the CG, resulting in a forward shift of the CG as fuel is burned. This forward movement of the CG is by design to ensure the aircraft remains within CG limits throughout the flight. It also improves pitch stability, making the aircraft safer to operate.

The CG shift due to fuel burn can be calculated using moment equations, taking into account the weight of the aircraft, the initial CG position, the weight and location of the fuel burned, and the final weight of the aircraft. This calculation can be performed before and after each flight to ensure the CG remains within the safe range.

It is worth noting that not all aircraft exhibit a forward CG shift with fuel burn. Some larger aircraft, such as the A320 and 747, have complex fuel systems and tank configurations that result in the CG moving backward or in different directions at different stages of the flight. These movements are managed by fuel transfer valves and fuel management systems to maintain the CG within the desired range.

The CG is calculated with a datum line

The center of gravity (CG) of an aircraft is the point over which the aircraft would balance. Its position is calculated after supporting the aircraft on at least two sets of weighing scales or load cells and noting the weight shown on each set of scales or load cells. The reference datum is a reference plane that allows accurate, and uniform, measurements to any point on the aircraft. The location of the reference datum is established by the manufacturer and is defined in the aircraft flight manual. The horizontal reference datum is an imaginary vertical plane or point, placed along the longitudinal axis of the aircraft, from which all horizontal distances are measured for weight and balance purposes. The arm is the horizontal distance from the reference datum to the CG of an item. The algebraic sign is plus (+) if measured aft of the datum or to the right side of the center line when considering a lateral calculation. The algebraic sign is minus (−) if measured forward of the datum or the left side of the center line when considering a lateral calculation.

To calculate the CG, you must first determine the weights and arms for all mass within the aircraft. Next, multiply the weights by the arms for all mass to calculate moments. Then, add all moments and weights together. Finally, divide the total moment by the total weight to give the overall arm. The arm that results from this calculation must be within the CG limits dictated by the aircraft manufacturer. If it is not, weight in the aircraft must be removed, added, or redistributed until the CG falls within the prescribed limits.

The CG depends on the tank configuration, fuel load and aircraft load

The Center of Gravity (CG) of an aircraft can change with fuel burn, depending on the tank configuration, fuel load, and aircraft load. If the fuel tank is located exactly on the CG, the CG position will remain unchanged as fuel is consumed. However, if the CG position is calculated relative to a different reference point, such as the leading edge of the wing, the CG will appear to change as fuel is burned.

In most small aircraft, the fuel tanks are not located exactly on the CG, and the CG does change as fuel is consumed. This change in CG can have a significant impact on the handling and stability of the aircraft. Therefore, it is important for pilots to calculate the weight and balance of the aircraft at both the beginning and end of the flight to ensure the CG remains within safe limits.

The CG can move forward or aft as fuel is burned, depending on the configuration of the fuel tanks. In some aircraft, such as the A320, the CG moves backward as fuel is consumed. In other aircraft, such as the 737, the CG moves forward when fuel from the main (wing) tanks is used and backward when fuel from the center tank is used.

The CG can also be affected by the loading of the aircraft, including the weight and distribution of passengers, cargo, and baggage. Therefore, it is crucial to consider both the fuel load and the aircraft load when determining how the CG will change during a flight.

Overall, while the CG may change with fuel burn in some aircraft, it is not a universal phenomenon and depends on the specific tank configuration, fuel load, and aircraft load.

The CG can be calculated by finding the spot where the sum of all moments (mass x distance) = zero

The center of gravity (CG) is the average location of the weight of an object. It is a crucial factor in determining the stability, performance, and safety of an aircraft. The CG can be calculated using the formula: CG = (total moment) / (total weight).

To calculate the CG, you need to first identify the weight of each separate object or component in the aircraft. This includes items such as fuel, oil, passengers, baggage, and cargo. Once you have the weights, choose a reference point, often the nose of the aircraft, and measure the distance of each object or component from this reference point. These measurements represent the "arm" of each item.

The next step is to calculate the moment of each item by multiplying its weight by its respective arm. Ensure that you use consistent units for weight and arm measurements, such as pounds and inches, or kilograms and meters. Calculate the total moment and total weight by summing up all the individual moments and weights, respectively.

Finally, to find the CG location, divide the total moment by the total weight. This calculation will give you the distance from the reference point to the center of gravity.

It is important to note that the CG can change as fuel is burned during a flight. This change in CG can impact the aircraft's handling and stability. Therefore, pilots need to consider fuel burn when calculating the CG before and during flights to ensure the aircraft remains within safe operating limits.

The CG changes with fuel burn in some aircraft but not others

The center of gravity (CG) of an aircraft can change with fuel burn, but this depends on the placement of the fuel tanks in relation to the aircraft's CG. If the tanks are precisely centered on the aircraft's CG, then the CG will not change as fuel is burned. However, since the CG already moves with different loadings, it is unlikely that the tanks are precisely centered.

In most cases, the fuel tanks are located forward of the CG, which means that the CG will move forward as fuel is burned. This can be advantageous as having a forward CG can help to prevent stalls and spins. Additionally, as weight decreases, the forward CG limit typically increases, further reducing the likelihood of exceeding the forward CG limit due to fuel burn.

On the other hand, some aircraft, such as the A320, have fuel tanks located aft of the CG, which causes the CG to move backward as fuel is burned. This can be managed by burning fuel from the forward tanks first to maintain the CG within the desired range.

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