What you describe will work.  The bellcrank can be located anywhere.  However, there are more practical locations to use considering structure and the amount of flex that the leadouts might require at the leadouts.  What is importand is the leadout position relative to the CG.

Now, one problem that you might have if you are using flaps is that there will be some degree of asymmetrical twist from the inboard flap to the outboard flap.  The asymmetry will be a function of how stiff your flaps are and how stiff the interconnecting wires/horns to the flaps (assuming full span flaps).  If only flaps are used in the center section of the wing (between the booms) and the flap is one piece, there will still be some asymmetrical twist from one flap tip to the other.  Any asymmetry in flap deflection due to airload will result in an airplane that will be difficult to trim and flay satisfactorily.  The solution to this is to have a transfer bellcrank to the outboar flap with similar connections to the outboard flap. 

I am not trying to talk down to anyone who has only a single bellcrank in the inboard boom, but for intermediate fliers, this will not make much difference.  However, there should be no reason to build an airplane that will have almost unsolvable trim problems regardless of skill level.  Flaps that twist more on from one side to the other causes trim problems that cannot be easily corrected.  Even a flexible flap horn in the center section can cause problems.  Flaps and flap linkages cannot be too stiff.  (This is a quote from a world famous and world class Super Semi-Scale Stunt Ship (SSSSS) builder/flier/former competitor.)

Keith

Guys,
I'm working on a new ship that will have twin booms and want to have all the push rods enclosed. To do this I was thinking about running the push rods up the inside boom. I read that the bellcrank position is not important and it is the location of the leadouts that count. To that end I was thinking about mounting the bellcrank in the inboard wing in line with the inside boom. The leadouts would be in the normal position. Anyone have any experience with this type of bellcrank installation?

Best,         DennisT

Dennis, in a twin boom design like a P38 Lightening you will always have a centre section on the trailing edge, and an inboard  and an outboard thus having three discreet trailing 'edges.'
Now, in your design do all three trailing edges need control supplied to them?

For example, if you only need control supplied to the centre edge then supplying the drive from either boom will make no difference if the centre flap is really stiff - and this should be very easily accomplished as this is the thickest part of the wing and should support a very thick flap.

Of course if the control is needed equally for in and outboard edges then the above is sage advice.

But when viewed horizontally the lead-outs 'within' any wing should point directly from the lines that enter the wing in order to avoid wear and drag on the guides.
When viewed vertically it seems that you only need an attachment  point of mechanical strength and an outboard boom in my opinion would be a stronger place than an inboard one, and of course a better place to place the weight.

Cheers.

Hi Bill,
After reading your suggestion I just couldn't get the RC method of centrally located servo's out of my head.

One centrally located  bell crank, then actuating rods connected left and right to those 90º nylon 'L' shaped cranks and then onto each control horn.

Your idea seems more plausible but instead of a full second outboard bell crank perhaps one of the above L shaped ones would have sufficed?