I thought of that too but then I thought that if the total wing area was kept at the max then it would better represent the worst case scenario. If the "equivalent" wing area was used at the wing area variable then the TVC would have been larger and suggested a more rearward CG location. So

Alasdair, reducing the TVC arbitrailiy by 1/3 AFTER calculating it would represent a much more harsh safety factor than the poor wing area to stab area would indicate it seems to me. I know that you're considering the downwash stuff but I'm not sure that needs to apply the way you're doing it. The idea of the TVC, I thought, was to keep the Cmo of the wing in control. And the Cmo effect of the wing is related to the leverage provided by the wing chord moment. So in effect the TVC calculation IS taking into account the downwash, or Cmo, effect. That is also why I considered this model to have an equivalent "one wing" chord of 5.5 inches but an 18 foot span wing. This would keep the effective forces of the Cmo the same but still relate it to the overall wing area. This was made easier because the wing stack is pretty much centered on the thrust line. If there are stacking effects that change this I am hoping that by using the total wing area rather than a fudged effective area the results are still conservative enough for safe flight. If there are other factors for multi winged models I can't seem to find them on the 'net.

However in amongst all this concern for jwc's model I think there is a sound message that the balance point should be much further forward at least for the first flights than standard doctrine indicates. It's a crap shoot with my original finding suggesting 28% and your fudged factor suggesting as forward as 16%. A nose heavy model can always lumber around the circuit and land for re- adjustment but a tail heavy/unstable model often doesn't make it to the first turn. So in light of that a 20 to 23% initial balance sounds safer but I'll be the first to admit that this is an educated guess.

I know it's common to consider equivalent wing areas for biplanes in wing loading considerations I'm not sure that this applies to TVC calculations. If it does then where does this fudging take place? Like I said before, if you do the fudging in the first equation it produces a more rearward CG which tells us we are being safer than need be. If you use it on the TVC value itself then it indicates we need a more forward CG. If we just use the numbers as is we end up in the middle ground. Granted I'm guessing on this but in doing a quick search for info on biplane and tail volume coefficients using a few different terms I turned up a big goose egg on the net.

So I toss my own coin and suggest that a starting balance of 23%.

Once flying an evaluation of hands off dive recovery will indicate how much to the rear you can move the CG. A more rearward CG that does not compromise stability will enhance the lower speed flying as the wings will be supporting the weight more efficiently and the power on to power off pitch up effect will be more tame. A nice profile to shoot for on a model of this type is a mid throttle 45 degree dive that recovers to level (and then noses up again) within 100 to 150 feet of altitude loss. Any signs of a speed related nose tuck (requireing up elevator to recover naturally ) or an initial flat and steady dive recovery that sharpens up as the speed bleeds off is a sign that the CG is probably as far back as you want. The recovery path should be a fairly smooth arc once the controls are neutralized if the balance point is at a safe spot. Keep retrimming the balance back and elevators for level mid throttle flight and then do the dive test again until you notice some of the bad things creeping into play. At that point you're as far back as you want to be.

Fortunetley this will be a slow flying model and slow flying models don't respond in a super harsh way to CG's being back a smidge too far. Many fun fly models are actually flying with CG's behind the neutral point which is usually a no-no in any higher speed models. The pilots need to be constantly on the controls to fly these but if it was a faster model it would probably be uncontrollable. So there's a safety factor in the slow speed as well.

Having said all this the narrow chord and long span will be sensitive to tip stalling in tighter turns at low speed. JCW, you WILL want to have a fair amount of washout in those wing tips. I'd say, given the short chord, that 1/8 to 3/16 trailing edge up at the tips would be good. Even then practice your slow speed close to stall tight turning about 4 mistakes high at first.