Ok, here goes. I have a complete in good shape wing from a Model Tech "Joss Stick". It is a sheeted foam core with a 67" span, 14 1/2 cord, & about 877 sq ". I am planning a relative layout about the same as a full scale P-38, but with all the "Ugly Stick" outlines . Booms will be 24" apart, and about 45" long(prop to rudder tip). Center pod will be about 25 1/2" long, and stab span will be about 29" with a cord (inc elevator)of 6" I am planning on 2 .40's for power. What I really need to know is where is the best point for the thrust line and stab C/L in relation the wing C/L. I am not refering to incedence, as it will be 0-0 on wings & stab, and about 2 down on motors. I am more interested in distance above or below the wing. I am thinking the stab should be just slightly above the wing, and the engines just about 1"-1 1/2" below wing C/L. Wing has about 1 1/2" dihedral at each tip and has strip ailerons.

Most important factor in this design will be "KISS".
So lets have some opinions and ideas.

TLH101:

As luck would have it, for the most balanced aerobatic performance everything should be at zero. Including the engine thrust line and the dihedral.

The wing you're using already has dihedral, so, keeping it simple, don't change it. And with a "Stick" fuselage design the engine thrust line is going to be below the wing centerline.

While some down thrust will allow you to avoid trimming the elevator for power changes, when you go inverted all those trims, and the down thrust, will be working against you.

For the best upright/inverted balance, the wing and stab are at zero incidence, directly in line one with the other, and the engine thrust line is also on the same center.

This last cannot be accomplished with the "Standard" stick fuselage, and having the prop centerline below the wing/stab centerline a very slight amount of downthrust can help. Amount to be determined by experiment, but if you start with one degree you'll probably be so close you wont change it later. two degrees? Maybe.

Bill.

PS: If you want to use some out thrust, that's OK. Twinman swears by out thrust, I don't use it. wr.

Bill,
You must have me confused with other person. I also do not use out thrust and center my engines. I did have to add down thrust on the Twin Ugly Stick, after, as Bill said, I flew it and found what was needed.
Whole project sounds cool. If you plan on aerobatics with this plane, increase the rudder size, put the engines on separate channels, and mix the engines to the rudders....above 50% throttle setting for safety. You cannot imaging how easy the knife edge is with differential thrust. Getting into and out of a flat spin, is merely reversing the rudders..ect ect!!!
OK, so I have lost it!!!!!
PS Also mix the ailerons to the elevator. More lift for manuvers and reduced unexpected snap rolls.

Twinman:

Sorry George, I must indeed have you confused. Haw.

I mean, of course, confused with someone else. Thought sure I had seen you recommending out thrust.

I used it only on my very first twin, and not since.

Bill.

I have one twin that has a heavy out thrust.. It is a Midwest Twin
Stix.. I have yet to see any help from it.. Now that I need to
make a new wing for it there will be no out thrust and no more
fuel system, yyeeahhh...
One ten oz tank behind each motor, zero out and about zero
down and it should be great.. Where it used to have one servo
to operate the throttles I am putting in HS81 on each motor..
And of course, more alumn to polish ..

Robby:

I have never built a twin with two throttle servos. That had engines smaller than 40s, I mean. 40 and larger everything has a servo for each engine.

t this moment I have two 15 size twins building, they will get my "Standard" cable operted throttles from one servo in the middle of the wing.

Haven't yet decided on one or two aileron servos. All a matter of weight.

But the Duellist 2/15 already has its Robart retracts. Wow.

Bill.