Tall Paul,
Nice wee aeroplane. A homebuild I assume?

We can find the Aerodynamic Centre of a wing or pair of wings but no way can we discuss CG without knowing details about the tail. I know nothing about this aircraft's tail, so how about adding a tail and discussing the CG of my hypothetical 100% staggered biplane which you also drew and I have tried to upload here.[img][/img]

To sumarise, we have two identical wings, each 4" chord, section does not matter but I drew approx Clark Y, with 4" normal stagger, and a tail whose area is 15% of the total wing area and whose c/4 is 15" aft of the datum (LE of top wing). Let's say for completeness that the wingspan is 26".

Where would the CG go according to the method proposed and used by Tall Paul and others?

Too far aft I would suspect, unless a second error compensates for the first.

Alasdair
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Alasdair
I noted the position of the c.g on the Hatz biplane at the Bratz site where the range is calculated is given first in the usual manner... percentage back from the leading edge of the upper wing.
And the "usual manner" numbers are "..from 18% forward to 35% rearward".
Thru a certain amount of legerdermain, Brooks then locates the virtual mac aft of the leading edge of the upper wing. Then he uses the percentages to find the delta forward and aft of this m.a.c. for the c.gs. relative to the position of the virtual m.a.c. Percentages of a percentage.
The method used at the Palo site (and Jean Claude Etienble, and DJ Aerotech, Martin Simons, Appendix 1) OTOH considers the plan view area, and computes the m.a.c. graphically for a single bay wing, or using moments for multi-panel wings, based on that. The c.g. is then found.
When the percentage numbers of 18 and 35 are related to the plan view area of the wing as given by Brooks and with the Palo r/c method, the positions are identical between the method used by Brooks and the Palo method, which tells me the process "normally" used, based on plan view area, is correct.
Tail areas and moments are NOT involved because "general knowledge" of where the c.g. must be for stability is related to the wing shape only, IOW, for a normal wing-tail configuration.
25% m.a.c. for a stable not-too-manuverable plane, aft of 35% m.a.c. for an "exciting" plane.
In the neutral point and static margin computations (Martin Simons, Appendix 1) the horizontal tail efficiency contribution is a constant variable. "This must be estimated."
Again, IOW, if you have a plane which flies stably, by definition the static margin is "good". And it is aft of the c.g.. How far aft is a subject of what "good" is relative to the degree of stability of the airplane.
And it's a very subjective thing. Pilot skill and experience AND static margin contribute to stable flight.
For other shapes of airplanes, there's other positions for the c.g. based on what experience has found works.
Jiggling the constant variable then affirms the correctness of the position, if the plane flies.
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The example plane above, a 30% c.g. would be at 2.4" aft of the upper wing leading edge.
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(Later on, that same day, a plane appeared.... )
Toss testing found the c.g.s as marked by the pins.
Flies quite well with the lower wing back that far!