CG, datum, arm, moment and %MAC

The seesaw

Mass was half the job. Now where it sits.

Stand the aeroplane on a wedge, and it behaves like what it is: a very long seesaw.

Put more weight behind the wedge, and the tail goes down.

You already know the rule. On a seesaw, a small child far out balances a big adult close in.

Half the weight at twice the distance: level.

So weight alone never says which way it tips. What counts is mass × distance. That is a moment.

Moment = mass × arm

Move the child closer and the balance is lost, with the masses unchanged.

To measure an arm you need one fixed line. It is called the datum.

The manufacturer picks it. There is no rule where: on the nose, on the wing, or in the air ahead of the aeroplane.

Behind the datum an arm is positive. In front of it, negative. Moments can be negative too.

Many manufacturers put the datum ahead of the aeroplane, so that every arm stays positive and no one slips on a sign.

Finding the CG

How do you find the balance point of a real aeroplane? Stand it on three scales.

One under each leg, on a level floor, in a closed hangar so no wind disturbs it.

Each wheel’s reading is a mass. Each wheel has an arm from the datum. Each gives a moment.

Add the masses. Add the moments. Divide.

CG = total moment ÷ total mass. Never the other way up.

CG = total moment / total mass

The law says when this must be done. These numbers are India’s.

After weighing, a licensed AME Category B or an approved person writes the weight schedule. The Regional Airworthiness Office approves it. A copy is displayed in the aeroplane.

Per cent of MAC

This lesson is part of the Navigation plan. See plans

Moving the load

This lesson is part of the Navigation plan. See plans

The floor

This lesson is part of the Navigation plan. See plans