I'm looking for a quick/cheap/lowweight way to eliminate a torsion problem on my airframe. It's a flat 3D view style model but with low wing. I have a carbon rod on the lower portion of the fuselage (actually points out nose, sorta looks like a gun). But one rod does not provide significant torsion resistance.

I don't want to put a second complete rod along the top, as I believe that would be excessive weight and I think there's a better solution.

I'm considering a cut carbon rod (4-5 inches) glued on the side at an angle near my tail section. [it's going to be a pusher].

Any other suggestions how to provide torsional regidity to a 3D view style flat foamie?

2 small rods, or even kevlar thread or dental floss, glued on each side is far superior to a single rod or tube. The farther they are spead, the better, for strength. Also, done on diagonals, they can provide anti-twist as well as anti-bend.

i have just reinforced the weaker areas. This typically is at the ends of where the wing joins the fuse. I put a 3" segment of bamboo to "bridge" the change from the wing to no wing. Without the added support I get fatigue cracking at that junction.

edit: my bad, thats not that much for the torque of the motor, but for the bending of the airframe. This is more so from general abuse(crashes) and large forces (rudder).

This will solve your problem. I had the same dilemma with the Shockys and this fixed it. Sandwich a piece of depron about 2 1/2 in wide, in this case to suit the firewall size, between two yardsticks or suitable molding, and clamp in the middle and both ends. Using a heat gun, bend it at the edge of your bench about a ft. at a time to about a 45 degree angle as it's hard to do all at once without breaking. Then bend it the rest of the way to 90 degrees. Make it a gradual curve instead of a sharp bend to prevent a stress point. After it was bent I tapered it from 1 in. to a side at the front, to 1/2 in. at the back of the fuse.I put hockey tape on the high points of the bends to give it some tensile strength. Plane is 30 in. span and weighs 8 oz. ready to fly with a Little Screamer. Is very rigid vertically and horizontally. Ply firewall sets into the square shape at the front with doublers on the inside that the firewall sits up against. Doublers also from the firewall to the front so it isn't going anywhere. Angle pieces are foamsafe C.A.d to the sides of the vertical fuse. Any questions? Hope this helps.

Torsional stiffness is all about cross sectional area, which is why the above works so well. That little bit of depron encloses and area about a square inch, or about the same as a 8" high flat sheet of 3mm depron.

You can prove it like this: Take a sheet of paper - almost no torsional strength.
Then roll it into a tube and tape/staple the ends together - huge torsional strength!

With a shockflier type fuselage, you can just add a lengthwise strip to enclose a triangle of space.

I added triangle balsa as pictured below to a 3d plane with lots of potential for torsion. The smaller 3D plane I am building right now is going to simply have a piece of Depron, beveled to match up with the fuse / wing and glued into place where the triangle balsa is now. The wood, will certainly be less likely to break than the Depron, but it's also easy to glue. However the more weight youhave on something like that, more power it takes to fly the more damage there is in a crash. So a strip of Depron or FFF should really add a great deal of strength without all the weight. The Nasty has only 4 torsion triangles around the wing / fuse area with a single CF spar running the length and it's very stable.

Originally Posted by RobinBennett

Torsional stiffness is all about cross sectional area, which is why the above works so well. That little bit of depron encloses and area about a square inch, or about the same as a 8" high flat sheet of 3mm depron.

Yes! I had the same problem with my 3DFoamy Tiburon, and this improved a lot by adding some 6mm depron triangle pieces on the fuse. See the red parts on the attached picture. I actually have four of those, running all the way back to the elevator. Very rigid, no flex any more.

That works better, but would be much stiffer if you were to fill in all four corners. More like a torque tube. A good example would be to take a cardboard tube like a Monocote tube abd slit almost from end to end. Now take it by both ends and it is easy to twist. Now glue the slit with hot glue or CA and it will be impossible to twist.

>That works better, but would be much stiffer if you were to fill in all four corners

Only twice as stiff (twice the area)
Even one fillet often adds sufficient stiffness compared to the original design.

The tube experiment is very good.

Triangulate that is the answer , this plane has a triangle foam fuse , very stiff, in all directions.