Weight and Balance 2 Flashcards
Centre of Gravity Calculations
● To determine the CG location, add up all the weights and all of the moments.
● Next, divide the sum of the moments by the sum of the weights
CG Calculations
● All moments aft of the datum are positive numbers.
● All moments forward of the datum are negative.
● Typically oil and/or a baggage compartment in the nose will be the only negative moments since they would be forward of the DATUM.
● Oil is calculated at the weight of 7.5 pounds per gallon.
● Fuel (AVGAS) is calculated at 6 pounds per gallon.
To calculate the CG use the following steps.
1. Add up all of the weights, including the basic empty weight of the aircraft.
2. Multiply each weight by its corresponding moment arm in inches to get the moment for that item. WEIGHT x ARM = MOMENT
3. Add up all the moments.
4. Divide the sum of the moments by the total weight to get the CG.
SUM OF MOMENTS ÷ TOTAL WEIGHT = CG
Loading Graph
● Frequently, the manufacturer will provide a graphical method for determining the moments during an aircraft weight and balance calculation.
● This method will simply show a graph that “automatically” multiplies the Weight and the Arm for us
➢ Repeat the process for the fuel (blue line), the rear passengers (green Line), and the baggage (black line). Be sure to record each moment.
➢ Add up all the weights, and the Load Moments.
➢ We can now use the Moment envelope chart to check the loading.
Moment Loading Envelope
● Once the total Weight and the total Moment is found, we can then check the Moment envelope to see that the aircraft is properly loaded
CG Envelope
● If we divided the total Moment of the aircraft by the total weight, we would then come up with a Centre of Gravity Position in terms of inches aft of the Datum line.
● We could then use this number to find the position in relation to the CG envelope
Load Adjustment
● Every aircraft has a maximum allowance for a forward CG position, as well as a maximum rearward CG position
- c of p moves forward with increase in aoa. c of g ahead of it. if centre of pressure is ahead of cg (too much aft loading), if nose up and stall, it causes cg to pull back nose even more
Aircraft Balance
● There is a balance point in the middle (called a fulcrum), with weight on both sides of the fulcrum.
● Recall that the fulcrum of an aircraft in flight is located at the Centre of Lift.
● In most aircraft designs, the CG is forward of the Centre of Lift. This causes the aircraft to naturally want to “nose down.”
● With the elevator located at the aft end of the aircraft, it provides a counterbalancing force to the CG, and providing a level attitude in normal flight
● You can readily see that loading of the aircraft, which affects the CG, is a critical consideration in properly balancing the aircraft and allowing for positive control
- as the fuel is consumed when flying swept wing airplanes, C of G shifts forward or backwards depending on the what fuel tanks are used
Effects of a Tail Heavy CG on Performance
● If an aircraft is tail heavy:
➢ The airplane will require nose down trim. (Less tail pressure!)
➢ Produces very light control forces, this makes it easy for the pilot to inadvertently overstress an aircraft; and
➢ It will cruise faster.
➢ However, this results in poor longitudinal stability; and
➢ Reduces the capability to recover from stalls and spins
● These effects are all due to less tail pressure on the stabilizer.
● Recovering from a stalled condition in an aircraft loaded in this manner will likely be difficult, if not impossible
Effects of a Nose Heavy CG on Performance
● If an aircraft is nose heavy:
➢ The aircraft will need nose up trim; (More tail pressure!)
➢ It will cruise slower.
➢ Generally it will be more stable
● This is due to to the fact that there is more pressure and drag from the stabilizer or tail
Utility Category
● In this case, the Weight and Balance constraints meet the certification standards for an aircraft expected to be operated under training conditions.
● Allows for more advanced manoeuvres that place greater stress on the aircraft.
● Generally, spins are approved only for the Utility category
Mean Aerodynamic Chord
● Advanced aircraft are going to give their Weight and Balance Calculations in terms of a Mean Aerodynamic Chord
● The Mean Aerodynamic Chord is the average chord and position of a non-rectangular wing.
● The Leading Edge Mean Aerodynamic Chord (LEMAC) is the position that this imaginary leading edge would be placed at.
● The Trailing Edge Mean Aerodynamic Chord (TEMAC) is the position that the imaginary trailing edge would be placed at
● Most aircraft will have an acceptable CG range within 10 to 30% of MAC
