Arithmetic CG Calculations

Question 1

What do we need to consider in order to eestablish the position of the CG?

Answer 1

• Empty aircraft

- Items of load and fuel

Question 2

What’s the moment of a mass?

Answer 2

Moment is the turning force created by the mass acting over a distance or lever arm

Question 3

What’s the formula of Moment? In what unit is it expressed?

Answer 3

• Moment = Mass x Distance, expressed in Kg force metres (kgm)

Question 4

What sign do moments aft of the datum have? what about the moments forward of the datum?

Answer 4

• Moments aft of the datum are positive

- Moments forward of the datum are negative

Question 5

Is the datum the CG? what is it?

Answer 5

• The datum is NOT the CG, it’s a reference point from which all the lever arm measurements are taken

Question 6

Where can we find the datum and moment arms for aircraft?

Answer 6

• Datum and moment arms for aircraft may be found in the W&B control and loading manual

Question 7

Where do manufacturers usually place the datum of an aircraft?

Answer 7

• Ahead of the aircraft
• On the nose
• Within the aircraft, perhaps on the engine bulkhead of a light aircraft
• Behind the aircraft

Question 8

Where is the datum “sometimes” located?

Answer 8

By measuring from a fixed point on the aircraft called the reference point

Question 9

What happends when we add mass to an aircraft where the CG was already calculated?

Answer 9

Once the CG is calculated, loads can be added or subtracted and the new CG found by re-working the total moment and total mass

Question 10

What’s the alternative method to calculate the change of CG?

• Formula when we want to find out much mass must be added or removed to bring the CG to a new position
• Formula to find out when the new total mass and the old CG position are known

Answer 10

Mass change change of CG
___________ = _________________________
Old total mass distance from mass to new CG

Mass change = change of CG
____________ _______________________
New total mass distance from mass to old CG

Question 11

What are body stations used for? In what units are they given?

Answer 11

Used to describe the location of equipment in large aircraft.
- Given as a body station (Stn) number
- Station datum is always Stn 0
- Stations forward of datum are negative, and stations aft of the datum are positibe