Larry the question is a little broad and hard to answer.  If using sheet fuselage sides like most all stunt airplanes there really isn't a need for stringers unless put at the very top and bottoms of the sides,  more to provide some meat for rounding than any strength requirements.  As far as formers or bulkheads that's more personal preference.  I set mine about 3-3 1/2" apart- no great reason-just seems right to me.  The sides and bulkheads provide very little resistance against twisting so that's where I use a light sheet 'webbing' top and bottom horizontally between formers and sides.  Boxing in if you like.

Dave

Built-up fuselage?  Flat sides and formers, with round deck and belly?

Stiff how?  Do you mean as an appliance to hold the tail in an exact relationship to the wing, or do you mean as something that'll paint up nice?

I don't think there's any one right answer, or even any one answer.

More formers means fewer "wows" in the fuselage sides -- but you can get the same effect with an all-molded fuselage and fewer formers, because the fuselage skin will serve to stiffen itself.

You'll lose a perfectly flat side and its pretty finish long before you'll lose wing-to-tail rigidity.

You could probably do a super-zoot three-layer epoxy-carbon/foam/epoxy-carbon layup, with no formers at all -- you can probably make that superbly stiff and light, at the cost of a lot of work.  (Don't tell Howard that -- we want him to have a plane for Poland).

If you're going to stick with a "box and turtle-deck" fuselage then lots of lightweight formers is probably a good idea.  Paul Walker's Impact that he published in 1991 in Flying Models has nine -- count 'em -- former stations along the length of its fuselage.  But that plane has tall flat sides -- had he made the sides convex, or had he made them out of thicker material, or had he been willing to put up with more waviness in the finish, he could have used fewer.

It'd be interesting to know how many formers Igor Burger used in his GeeBee, with its all-molded sides.

 I don't remember the year that the Stunt fire was published In Model Aviation, but that large model was built using stringers and looked to be well built.
 If memory serves Scott Bair built the model, and had one of his "home built" engines in it.
 
 Good luck and keep us posted with your results...

In the method described in Roberts post , should the X be visable from the top view or side. Or does it increase torsional rigidity and resistance to bending either way equally?

Spinner ,wing PUSHXROD   tail  From side view. Slot locked in the center will give the most for the weight. Run the grain from side to side not up and down. But please remember I didn't learn it in a book, its only with trial and error.

Now if you really want strong you could do XX and intersect them.

We're talking about stiff and light, not strong. Stiff structures usually have more strength up to the point of failure and then pretty much explode. Crash resistance is poor. No repairing required, a plus!  Steve

This is the stiffest and lightest method bar none. Easy to do. Nothing stiffer.