So far it's looking pretty good. You've got 1188 sq inches of wing area but it's not THAT large a model. If you can keep the weight closer to the 7 lb mark or even less then I think you'll have a nice slow flying model that 3 30's will fly in fine form. The engines don't have much moment arm to help balance the model but then you've probably already figured out that it'll be VERY important to keep the tail light.
I thought there used to be an online Tail Volume Calculator out there but I can't find it. But I found this hand and calculator method.
The Tail Volume Coefficient is a very handy tool for understanding why a model acts like it does, and for determining what can be done to help it act more like what is desired. TVo will help decide just how big the stabilizer should be and provide a starting point for the Center of Gravity location.
Tail Volume = (Tail Area/Wing Area} X ( Tail Arm/Wing Avg. Chord)
where:
Tail Area = area of the horizontal stabilizer
Wing Area = area of the wing
[both areas include that encased by/covered by the fuselage]
Tail Arm = distance from 25% chord of wing to 25% chord of stab [for untapered surfaces; for tapered, use 1/4 chords at average chord]
Wing Avg. Chord = area / wing span (or use MAC)
Looking at the formula, one can see [other parts being the same] that a larger tail area and/or a longer tail arm will produce a larger tail volume.
Here are some sample TVo numbers:
AMA gas models 1.0 to 2.0
Mulvihill rubber 1.5 to 2.2
Wakefield rubber 1.4 to 1.7
Indoor rubber duration 1.0 to 1.5
Hand launched glider .6 to 1.1
Full size 1913 Moraine-Saulnier, Type 'L' .16
OK, so what do we do with the TVo number?
We can find a good starting point for the center of gravity location.
CG [in % back from the wing's LE]
=
16 + (36 X Tail Volume)
An example: if Tail Volume is .50, then CG is:
16 + 36 X .5 = 34%
In practice, one should experiment around this recommended number, to see if duration could be improved.
This CG calculation is really handy for those old timer gas models that have no balance point marked on the plans!
Portions of this page have been taken from William F. McCombs "Making Scale Model Airplanes Fly". See ads in Flying Models and the NFFS Digest for information on how to buy this book - which has many very helpful ideas for competition models as well as scale models.
I've edited part of the above to include the idea of the reference points being the 1/4 chord points as seen elsewhere. This version actually used the leading edges which is wrong by other accounts.
That gives you a TVC = (112/1188)x(20.5/5.5) = (.094)x(3.73) = .35
This is actually a LOT better than I thought it would be. I think a large part of this has to do with the fact that the average wing chord is so short. A ratio of wing to stab area similar to what you have is USUALLY the kiss of death. But the short effective chord as a result of the stacked wings works in your favour in this case. .35 is right in the ball park for "normal" airplanes.
Subbing in to find the CG placement you get 16 + (36 X .35) = 28.6% back from the leading edge. A slightly forward CG but given the small stabilizer quite respectable. To keep it safe for your first flights I would recomend 25 to 26%. One thing for sure, if you try the "usual" 1/3 back the model WILL bite you. There is no way of avoiding the fact that it has a very small tail area for all that wing. Knowing that the CG will be this far forward you'll want to take extra efforts to keep the tail super light.
This means your hinging system for the verticals will need to be done with things like thin wall carbon tubing and aluminium tube slip bearings. No steel wire or brass tubing for you ! ! ! And since there are three of them I would consider things like strip laminated outlines with ribs. And pull-pull controls made from Spectra line or similar will save you a lot of weight over pushrod systems. As complex as the controls would be in this case I really don't think you want to try putting servos in the tail for this one. Putting the elevator servo just behind the engine on one outboard boom and the rudder servo just behind the engine of the other boom would be best IMHO.
Even if you are careful with your design and control system work I suspect you'll still be adding a lump of ballast to that short nose pod.
Doing some quick figuring and knowing how I like my old timer RC assist models to fly I get a few numbers for you to consider....
You've got 8.25 sq feet of area total. At a light loading that will fly like an old timer of 12oz/sqft you want the model to only weigh just a bit over 6 lbs. At 16 oz, which IMHO will still fly slowly but not look like it's floating, you need to keep it to 8.5 lbs total. At wing loadings over 16 oz/sq ft you'll loose a lot of the "look" in the air since it'll tend to fly faster than a WW1 model of this sort should. Of course all this is subjective but in my case I like my WW1 stuff to fly slowly for a scale look.
Best of luck with the rest of the build. As you can see from all my figuring and time to type this above you got me all enthused with this one. A truly unique project and I wish you the best of luck with it's outcome.
PS: In thinking about your power situation and recognizing that this is not actually that large a model I wonder if you could not get by with just the two outboard engines and just put a free wheel prop on the center rear engine position. That would certainly help to control the overall weight in general and the nose ballast issue in particular. To power a lightly loaded 600 sq inch model with one Saito 30 is not unresonable at all and that is what you would basically have with only two engines in this. Loosing a pound or more worth of engine and fuel tank and the perhaps another pound or more of pod ballast to balance these rear mounted items would go a long way towards helping with the flying speed and overall weight issues. The resulting lighter model should easily be able to fly with the 2 engines.