Torsional stiffness

[Curare]

Not sure if this should go into the stunt design forum, or the building forum but I put it here, feel free to move it as you guys see fit.

I've just completed a tailplane for my chipmunk, with an internal warren truss structure which has then been sheeted with 1/16" balsa. It's light but it's not as stiff as say a 1/4" plank of the same size. It appears to be ok in bending but torsionally it's a bit iffy. I can twist the tailplane about 25˚ before I'd worry I'd break it.

Now I know torsional rigidity is a big think on wings, but on a tail, how big a deal is it?

The next question is, if I'm having this problem on a tailplane, am I going to create twisty flaps if I do the same thing there (sheeted warren truss)?

Cheers

Greg

[Curare]

For reference, I grabbed the stock (feels like mahogany) sheet stab and gave it a twist, For the same amount of force I'd guess I'm getting about 2/3rds of the deflection of the sheeted stab, but half the weight, easily.

[Tim Wescott]

It sounds like the stiffness difference has more to do with the difference in wood density than the construction.

Sig Chipmunk?

I fly a Twister with an un-sheeted Warren truss that's about 3/8" thick.  It's probably about the same size as your Chipmunk, and I certainly haven't had any problems that I know of.

[Steve Helmick]

There is a lot left out in the structural description...like thickness of the stabalizer. But I think that once covered with CF, tissue, or silkspan and dope, you'll be ok.

It's pretty easy to understand that a Warren Truss "l\l\l\l\l" (which I think is technically correct) or "lVVVVVl" (which I think is not correctly called WT) is less stiff than "lXXXXXl". The closer the triangles are to equilateral, the better. The smaller the triangles, the better. Maybe Randy or Howard would comment on this. For sure, the thicker the stabalizer, the stiffer in both torsion and bending.  Steve

[Douglas Ames]

Is it a profile?

[Curare]

Steve, I've kept the thickness the same as before, so overall, it's still 1/4"

In hindsight, I probably should have gone for a 3/8" build up stab, trading the extra thickness for torsional ridigity. Extreme fibres and all that jazz.

It's a IVVVVI style truss, with a big centre brace for mounting.

Douglas, no it's a full fuse Sig Super Chipmunk.

Well when I say it's a sig, it's getting less and less like the box every minute!

[Brett Buck]

There is a lot left out in the structural description...like thickness of the stabalizer. But I think that once covered with CF, tissue, or silkspan and dope, you'll be ok.

It's pretty easy to understand that a Warren Truss "l\l\l\l\l" (which I think is technically correct) or "lVVVVVl" (which I think is not correctly called WT) is less stiff than "lXXXXXl". The closer the triangles are to equilateral, the better. The smaller the triangles, the better. Maybe Randy or Howard would comment on this. For sure, the thicker the stabalizer, the stiffer in both torsion and bending.  Steve

   The "Warren Truss" is a good structural design but it doesn't work nearly as well as you might expect, the way it is used in modeling. Its intended to take loads in what we would call the fore/aft direction, like the sides of a bridge. Not necessarily to keep the side of the bridge from twisting out of plane.

   The stab rigidity is critical, at least as important as the wing.

     Brett

[Serge_Krauss]

The 3/16" stab below seemed pretty resistant to twisting when sheeted with 1/16" balsa and either doped silkspan (previous one) or .56-oz FG and epoxy (this one). They were marginally lighter than solid balsa of no greater total thickness and seemed slightly stiffer linearly and torsionally. I did no quantitative experiment on them. Planes with trussed flaps without the balsa sheeting have been pictured here, and builders seemed satisfied. I've always wondered about how really rigid they were though, especially with plastic coverings.

[Dennis Moritz]

Doped covering should do it. Our balsa constructed models are composites when handled classically. The doped silkspan plus balsa structurally sound and lite. Balsa without the covering, not so much. Also balsa varries in weight, grain, inherent rigidity. Light weight contest wood in particular needs silkspan/dope covering to be rigid and effective in load-bearing.  

[SDF]

I suffered tailplane flutter on a stunter -  not pleasant. Light weight glass cloth and epoxy top and bottom completely cured it.

[Jim Thomerson]

I have made a number of 1/4 thick stabs, Warren truss, and 1/32 covering, then tissue or silk.  I have been impressed that they were much stiffer than the same stab made out of contest balsa. 

[Serge_Krauss]

One thing I failed to mention above is that that core was cut with the grain running spanwise. If I'd made separate half-span leading edges with the grain running parallel to the leading edge and then sheeted with the spanwise-grain 1/16" balsa, it would have been torsionally stiffer yet. The real key though, besides weight saving, is that the diagonals have their grain running obliquely to the span and chord. You don't get that with single-piece stabilizers.