Tim, are you redesigning something existing,, how about some more information so we can give an educated answer. things like tail moment, ( from cg to elevator hinge line) and some general configuration information.
whatcha trying to accomplish,,

How do you determine weight and length of nose with out using the size of the airplane either ?
As is an 8 .5 in nose on a cinder block vs an 8.5 inch noe on a block of foam ?

Tim-

Based on your aim to keep this sort of semi-scale and the fact that, as Ty says, you should take into account prop, tank, and other fixture weights, your problem dimishes some and can be diminished more with appropriate structure. I don't know whether you have included muffler and spinner, or what tank size you use, but just adding an ounce for a prop like an APC 10"x4" and 1.4 oz for a tank (3.5 oz) that's probably smaller than you use, the increase in nose length reduces to about 3". A spinner, screws, larger tank, etc. reduce this further.

Your Monnet design has a characteristically short appearing nose; so I recommend the hollowed trussed aft fuselage. Built from narrower strip wood, it's a lot cheaper, warp resistant, stiffer, and really doesn't take that long to make. I sort of settled on 1/16" plywood nose doublers mated to 1/16" balsa aft-fuselage doublers. If you want even greater stiffness, you can laminate CF or 45-degree-biased fiberglas (use minimal resin) under the doublers. My current fuselage is quite stiff torsionally with a similar set-up.

I think you'll still have to stretch it some, but not nearly 4".

Good luck.

SK

Don't forget the weight of the added fuselage.  At least as an estimate, use 1 oz. per in.  It makes a significant difference.

Serge:
So, are you saying you use 1/16" sides in the back?  What total fuselage thickness do you use?  

Tim-

Yes, 1/16" thick balsa from the aft edge of the nose ply to the end of the fuselage. I put it on a 1/2" core for a total thickness of 1/2" + 2 x 1/16" = 5/8". My original intent had been to use a beautiful piece of 1/2" balsa for a solid fuselage. So I did not take full advantage of the technique. Next one will be from strips of 1/2" thick balsa (Edit: probably more like 1/2" wide strips of 1/4" balsa? Whatever...). I'll post a picture of the fuselage, after I'd switched gears, making the cutout and inserting the diagonals. After a long pause, I started to finish it this week. So I'm laminating .56-oz glass, biased 45 degrees, with West systems epoxy. This thing is really stiff torsionally.

Progressive fuselage weights were recorded as follows - so far (weights +/- .03 oz scale error):

w/maple engine bearers: 3.85 oz
w/aft cut-out: 3.35 oz
w/nine balsa (10#/ft^3) diagonals epoxied in: 3.40 oz
w/left-side 1/16" ply nose doubler (epoxied) and hard point for fair lead: 4.10 oz
w/left-side 1/16" aft balsa laminate epoxied and trimmed: 4.40 oz
w/tank recess material removed: 4.30 oz
w/1/64" ply epoxied to back, front, and rear of tank recess: 4.40- oz
w/1/16" right-side ply nose doubler (epoxied): 4.90 oz
w/aft right-side balsa doubler plus recessed aluminum engine pads (.16 oz): 5.35 oz
w/l.g. tube carry-throughs and engine recess gussets: 5.40 oz
w/roughly contoured 9/16" balsa cheek tripler: 6.5 oz
After fully contouring fins, cheek, and fuselage: 5.60 oz
w/rubbed epoxy during paper-towel removal after aborted bias-glass application: 5.65 oz
w/.56-oz glass lamination followed by surface glassing of about 35%-40% of fuselage/fin surface: 5.90- oz
after wet sanding (#400 emory paper) with Prep-Sol: 5.80 oz

That's where it stands now. The left rear fuselage is satin smooth with glass weave ready to show. So that gives a projected pre-primer/color finished weight of about 6.2 - 6.4 oz. We'll see.

Incidentally, should you try the 45-degree weave bias, be prepared for a real mess, if you don't do it in sections. Norm Skuderin read my adventures in this month's club newsletter and suggested that I might have less trouble, if I thinned the epoxy 50% with alcohol (I rolled it out, following by a Speedwell roller over waxed paper, but had major troubles). The idea was to anchor the glass with a surface sheen of epoxy set to tackiness, followed by a second coat to fill the weave. The end result seems to justify that approach, IF one can actually position biased weave without creating the worst mess imaginable. I finally got the important part down with the weave biased 45 degrees, but covered the fins with thread directions vertical and horizontal. Sanding the indentations where the glass pieces meet is not fun. The fuselage picture is of the un-glassed side, but both sides look the same, with the left being very smooth.

SK

Tim-

It looks pretty good! You might even end up needing that tail weight. But that's the part that is least predictable, since weights of finish and extra nose structure in your change are less predictable. That's why I included all the progressive weight changes above.

I thought I'd add a note here that I actually did manage to apply the .56-oz glass to the right side in one piece without another set of hands and without the tape. I cut it with a bit more excess and rolled it gently over a the thinnest surface coat of resin possible (scraped with a plastic card), but before applying resin to the recesses and fin. Using a roller over waxed paper again worked fine and eliminated minor wrinkles. Weight seems to be following the previous effort or perhaps a bit lighter. So now without sanding, the total weight is about 6.05 oz. I think this means that you can make this aft structure just as light as dope, significantly more rigid and torque resistant. While Al and Ron haven't posted exactly how they finish their planes (I mean their techniques), I believe they have used the glass/epoxy surfaces. Final weight will be for a profile fuselage - with nose tripler - ready for color (or primer).

SK

Yes. Phil C. does that with great success.

SK

Chris:

That's an anchor point for the wire loop I'd use to keep a 3/32" wire control rod straight, if I were to use a wire control rod. I'm using CF tubing though, as intended. That's a good thing, since "CRS" syndrome set in again, and I sanded off the exterior locating mark.

The weight came out at about 6.1 oz, ready for color or primer, but I see now that I sanded through the glass near the edges on one side. 'not ideal. Next time, it's glass laminations inside the 1/16" balsa sides! That will eliminate much of the work at just a little sacrifice in potential torsional stiffness.

SK