Spar/longeron material comparison

[Karl B²]

(If there are any good sites with data on this, please let me know.)

Just did some testing of a few pieces I planned to use for spars, LE/TE, etc. in a model that will use tubular CF spar, tubular CF or balsa LE, solid CF TE, balsa ribs, and nothing but shrink covering from LE to TE.

I've included linear density, flex (deflection/test wt), and flex/linear density. For the flex test I simply hung 24" over the edge of a table and measured the deflection with a test wt. attached to the end. The solid CF rods deflected under their own weight; deflection is measured from level, not starting (unloaded) weight.

The balsa and solid CF rods used an 8.5g test wt (binder clip); tube CF used a 209.2g wt (pair of pliers). All tubes are cylindrical, no taper.

3/16 x 1/4 medium balsa
6.33 g/m
(horizontal) flex: 4.303 mm/g, flex per g/m: .6795
(vertical) flex: 2.510 mm/g, flex per g/m: .3964

.068" CF solid rod
3.61 g/m, flex: 13.089 mm/g, flex per g/m: 3.6267

.094" CF solid rod
7.30 g/m, flex: 4.841 mm/g, flex per g/m: .6632

.205" narrow wall pultruded(?) CF tube
16.57 g/m, flex: .255 mm/g, flex per g/m: .0154

.226" narrow wall pultruded(?) CF tube
20.42 g/m, flex: .168 mm/g, flex per g/m: .0082

.252" pultruded(?) CF tube
30.27 g/m, flex: .102 mm/g, flex per g/m: .0034

"CX200" wrapped CF arrowshaft
18.52 g/m, flex: .138 mm/g, flex per g/m: .0075

Using the arrowshaft for the spar, the advantages of using a CF tube for the LE vs. shaping the balsa are ding resistance, strength, and ease of construction. Disadvantage is the extra weight (~15-20 g for a 60" span).

Plane will be a parkflyer class electric powered glider capable of operating off floats while lifting a pencam.

Thoughts/comments?
Karl

[Ollie]

The extra weight will hardly be noticable in the flight characteristics of a 60 inch span electric powered park flier except for a slightly higher stalling speed and a slightly shorter power duration.

It is not necessary to give up any performance yet have a stronger, stiffer spar at the cost of a little construction effort by using the carbon fiber more efficiently. For example you might use two 0.05 inch diameter carbon fiber spar caps with vertical grain 1/16 balsa shear webs between them.

If you don't want to give up the convenience of a tubular carbon fiber spar, you can still have one that is sufficiently strong and light by using a .125 inch OD, .07 inch ID pultruded carbon fiber tube for the spar. They are available from CST and Model Research Labs.

An even better structural shape for an all carbon fiber spar would be the 0.032 X 0.31 inch carbon fiber bar from CST.

[Karl B²]

Thanks for the input, Ollie. All CF I tested came from the local hobby shop, and I believe it's mfg'd by either CST or Aero Composites.

Yes, construction convenience is a factor :) I'm trying to balance performance, convenience, and cost. In constructing a CF/shear web/CF spar, could the CF be bent with a bit of heat, allowing a one-piece rod through the polyhedral breaks?

Karl

[Ollie]

If the polyhedral break is half or more of the distance from root to tip, the bending load at the break will be less than 1/4th the bending load at the root. Therefore the spar in the tip panel can be smaller than the spar in the root panel, saving weight where it will do the most good. Select a tip panel spar size that fits loosely into the inside diameter of the root spar. One way of making the joint is to plug the root panel tube with a balsa plug near the break and insert the tip spar in the cavity at the desired angle. The cavity at the break can be filled with microballoons and epoxy around the tip spar. This is best done before assembling the ribs to the spar. You could bevel the end of the tip spar where it goes into the cavity inorder to get the desired angle and have the tip spar emerge from the cavity at a concentric location.

Heat bending is not an option because the carbon fibers would have to slide relative to each other too much. Too much heat would destroy the strength of the cured epoxy. When the cured epoxy softens, the tube you are trying to bend will collapse.

[Karl B²]

Originally posted by Ollie
Heat bending is not an option because the carbon fibers would have to slide relative to each other too much. Too much heat would destroy the strength of the cured epoxy. When the cured epoxy softens, the tube you are trying to bend will collapse.
I was only considering bending the small solid rod if used top and bottom of balsa shear webs. Of course, in this type of spar a traditional ply dihedral brace through the rib would work as well, with a cut in the CF rod.

Guess it's time to start throwing splinters around and forget about the price tags of new pieces of CF...

Thanks again.