James I am not sure if you are just being casual when you mention multiplying mm by grams or if you were looking for recommendations on calculating the CG. I apologize now if I am overegging an answer because you are fundamentally correct , one adds up the moments and divided by the total weights. I give an example below.
I will assume some typical figures for an F1M, (cg (lower case) refers to a component cg))
Motor stick, 15.5" long, 0.65g, cg half way along, Front of motor stick is taken as the Datum
Tail Boom, 17" long, 0.37g, cg half way along. (cg is 24" from the Datum)
Wing, 8" chord, 0.81g, cg at mid chord, LE is 2" behind the Datum. (cg is 6" from the datum)
Tail, 5" chord, 0.40g, cg at mid chord. (cg is 30" from the Datum)
Propeller, 0.70g, ( cg assumed 1" in front of the Datum),
The total moments are: ((7.75x0.65)+(24x0.37)+(6x 0.81)+(30x0.40)+(-1x0.70)) = 30.0775 inch gram.
The total weights are: (0.65+0.37+0.81+0.40+0.70) = 2.93g.
The CG is (30.0775/2.93) = 10.265 inches behind the Datum.
This is just behind the trailing edge of this theoretical model but I have not yet added in the moment from the rubber motor (or the 0.07g ballast to bring the weight up to 3g!). You could, of course, run through all the calcs again but including the motor weight of 1.5g as allowed by the rules but it is usually more convenient to start with the airframe weight and CG that has just been calculated and add in the rubber. The rubber will have its cg at half the motor stick length behind the Datum so the new total moment is: (10.265"x2.93g) + (7.75"x1.5g) = 41.7 inch gram and the new Total weight is: (2.93+1.5)=4.43g. So the CG with rubber is: (41.7/4.43) = 9.413" behind the Datum. At about 93% of the wing chord. Just for interest; if the 0.07g ballast needed to bring the model up to weight is added on the extreme end of the tail boom them the CG will move to 101% of the wing chord.
Obviously your weights and dimensions will be somewhat different but I hope this is of some use. It is not easy to tabulate calculations neatly on mailing lists.
John Barker - England
Received on Mon Apr 22 2013 - 13:50:15 CEST
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