Sorry londonblue, JT is correct.

When weighing an aircraft - like pb84's Maule for example - to calculate the empty CG location, you measure the weight on the mainwheels and the weight on the tailwheel (with the fuselage levelled as per the factory instructions).

You then calculate the mainwheel moment - mainwheel weight x mainwheel arm (from datum) and the tailwheel moment - tailwheel weight x tailwheel arm.

You sum the two weights to get the empty weight. You then sum the two moments and divide by the empty weight, to get the empty CG arm.

There are important details in this process, for example, you must do the weighing in a closed hangar to avoid any wind-induced lift affecting the scale readings and with a taildragger, the scale used for the tailwheel must be more accurate than the one used for the mainwheels, because the weight is significantly less and so any error would be a bigger percentage.

Interestingly, the location of the datum does not matter. As long as you pay attention to the signs of the arms, the empty CG location will come out correctly with any datum. For example, you could use the mainwheel axle as the datum, in which case the mainwheel moment is zero and only the tailwheel moment would be used in the calculation.

Cheers India Four Two. It seems that the empty weight is also a sum of other moments.

That makes more sense...

londonblue,

An important correction to your statement. It's the empty weight moment that is the sum of other moments. The empty weight is just the sum of the scale readings.

Another sideline point re datum position -

(a) the position of the datum is entirely arbitrary .. ie it doesn't matter where you choose to hang the end of your tape measure as the end result will be the same (different numbers but the same significance). Usually, the OEM will choose to place the fuse station datum somewhere out toward the nose of the aircraft, solely for convenience.

(b) however, once you choose a datum, then ALL calculations relating to each other must be with reference to that datum otherwise it all turns to worms.

(c) be wary when your aircraft uses trimsheets for loading systems. It is typical for the weight control person to use the OEM datum for the weighing stuff but then choose a different datum (usually around or at the aft limit of the CG envelope) to minimise trimsheet execution errors.

I had one occasion, many years ago, where a mate (who also was a weight control engineer) redid some configuration stuff on a cabin class twin, reissued the empty weight data (good boy) but then, without thinking what he was doing .. reissued the loading system (trimsheet) entry data using the OEM datum .. which made it incompatible with my trimsheet .. fortunately the chief pilot was the next to fly and, being a mate, gave me a call to sort out what was going on ...

Weight control is straight forward and really only requires good housekeeping .. but you do need to know what you are on about ..