Frank van der Hulst wrote:

dean van winkle wrote:
Ref John Slade's comments on the Lyc at 310 lbs and the 13B at 260 lbs installed weight.

Note that John's 260lbs was NOT installed weight. I figure there's
another 46 lb or so (oil filter, cooler, pan, radiators, intake & exhaust
manifolds, ignition) to add, so the weight isn't very different from an
O-360 (I estimate 290 lbs for that).

Use the firewall as your fixed datum point rather than the aircraft CG which is
a floating point.

If you must use a fixed point, I suggest one that is close to where the
aircraft CG ought to be... perhaps 1/3 chord for a tractor aircraft. I
don't where the CG normally is in a pusher/canard configuration.

Any discrepancy between the fixed point and the actual CG will result in
a calculation error.

 For purposes of comparison assume the same prop weight and location, same cowl, and same > engine mount weights.  Also assume that the Lyc with all accessories weighs 310 lbs with the
engine CG 24 inches (arm)  forward of the firewall for a total moment of 7440 inch lbs. To
give the same moment, the CG of the 13B with all accessories and redrive would fall
7440/260 = 28.61 inches (arm) forward of the firewall.  To be more precise, the weights and
arms of the water and oil radiators with fluids should be summed into the 7440 inch lb moment
which would result in the engine moving back slightly closer to the firewall.

Using your figures and adding: Assume the firewall is 12 inches forward
of the aircraft's nominal CG position (1/3 chord).

The actual moment of the O360 = 310*36 = 11160inch lb
To give the same moment, the 260lb 13B CG needs to be 11160/260 = 42"
forward of the 1/3 chord
i.e. 42 = 12 = 30" forward of the firewall.

Using your technique, the aircraft's CG would be a little (maybe 1/3",
depending on airframe weights) further aft of its desired position. Not
a big deal, and probably correctible by moving the battery forward or
similar.

Nevertheless, it is an error which could be avoided. And I'd hate to see
someone end up adding a lump of lead to the nose if it wasn't necessary.

Frank.

The formula is the sum of all moments both positive and negative divided
by the total weight regardless of what reference point is chosen.

Usually a reference point is chosen at the tip of the tractor spinner so all the 
moments are of the same sign. The CG position is then the distance aft from
the tip of the spinner. As I recall this is the way it is taught in PP ground 
school.

Paul Lamar

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