Help in seleting spar wood type and size for wing
 

Hi
Im trying to scratch build a 1/4 scale piper J3 CUB and need help in type of wood for spar, L.E and T.E.

could anyone which type of wood in which size should I use?
here is a snapshot
and attached the DWG file

Hi Hashem, It's good to see that you have started on the design of your model. I will attach a drawing of a wing I designed for a 50cc engine that weighs 20 pounds. You can get some ideas from it. I'll also email the dwg version. Dan.

I would use clear straight grain spruce from someplace like Aircraft Spruce & Specialties as my spar caps and medium density balsa for the leading and trailing edges of the wing.

Bob

Bob,

Kinda hard for someone where Hashem is to buy from Aircraft Spruce.

Bedford

He asked what type of wood, and thickness to use, how he gets it is of no concern to me. If somebody wants something bad enough they find a way to get it...

Bob

Hashem, looking at your drawing I'd suggest a couple of things.

First off I assume that your carbon tube will be used as a wing joiner to let you transport your model more easily. That's fine. But the tube is best put in between your spars and boxed in with filler so it connects directly to the spars instead of using the ribs as support.

While old growth Sitka spruce is the standard for aircraft spars over here in North America you could consider some of the European pines or other softwoods as an alternate that would be easier to obtain. What you want is some wood that is flexible and spring like but does not break too easily. Some soft woods are flexible but not springy. If a piece is bent it tends to stay bent. You want one of the woods that springs back to straight to use for spars. Generally this means it will be a slow growth wood with probably 6 or more growth rings per centimeter when you look at the end of the wood. And obviously you want a wood where the grain is fairly straight. To be a good spar stock the grain ring runout where the grain twists and runs side to side instead of along the wood must be less than a full runout in 30 cm. Anything much less and if you test break a sample you will find that it snaps with short pointed ends. That's no good for a spar.

Something you might want to consider since you are building a quarter scale version. If you make the wing struts functional and load bearing you can then switch to a two spar setup just like the original Cub used and use the struts to carry the flight loads. This means making the strut connections VERY strong. But the advantage is that you don't even need a joiner tube at the center. Instead the wing just needs a carry through box through the fuselage. The two spars would end up being located on the ribs at the same spots where the struts connect on the real Cub.

Hashem, there is most likely a supplier in your part of the world that can provide similar products. Good luck!

The size should be enough to avoid buckling of the spar under compression loads, whatever type of wood you can get.
This technique helps reducing the size of the needed spars:
http://www.airfieldmodels.com/inform...raft_wings.htm

Now, the location of the spars along the airfoil is favored to be at the highest point for the simple reasons of keeping a good shape of the airfoil (formed by the cover) between the ribs where is more critical and more efficient structurally (more distance between top and bottom).

oh thanks guys for well informative posts. I learned new things and will absoloutley consider them.
BMatthews would you add some images about wood grain runout becaue of bad english understaning! (sorry).
I added one: which one do you mean?
http://www.rcuniverse.com/forum/atta...mentid=1927925

Everyone. Hashem and I have been sending emails to each other discussing his progress and drawings for the J3 cub, He is a very skilled CAD user as you will see from his drawings. I have "marked them up" with red lines and sent them back to him. I will post the drawings, my comments and some images I took during my Aeronca Champion build. Since this new RCU is pretty bad, I'll do it in several posts. First is an image of his parts drawings. They are impecable and a example of a pure scratch build.

Wing and Struts
Use shear webs of vertical grain balsa between the ribs and glued to the bottom and top spars. Sheet the center section of the wing with 1/16 balsa. The two plywood dihedral braces should be 1/8 plywood. Use very heavy balsa for the inner ribs that are glued to the braces. There are no hinge supports shown for the ailerons. How is the wing going to be attached to the fuselage? Use basswood for the struts. I used a foam wing so I had no images.

Empennage
Use 3/8 balsa for all the tail pieces. ј is too thin. You will not be able to sand the laminated 1/16 balsa because of the glue. Make the center section of the horizontal stabilizer solid balsa for a more secure attachment to the fuselage. Use a seperate servo for each elevator. I sheeted my verical and horizontal surfaces with 1/16 balsa.

Fuselage
Make the firewall from 1/4 aircraft plywood. Make the landing gear blocks a single piece. Use 3/16 wire for the landing gear. Reinforce the fuselage doublers at the top and bottom of the window cutouts and the vertical pieces with a hard wood post. You will need a sheeted portion of the fuselage where the pushrods exit for the elevators and rudder. Add a plywood plate for tail wheel mount. Add two more gussets up front under the wing. How are you going to attach the windshield up around the wing?

Hashem, what you posted in your sketch is the end grain. ANY of them are fine. The runout I wrote about is seen from the side of the wood. Not from the end.

Each of those lines you show has a line that runs along the length of the wood. If those grain lines run from side to side in less than the thickness of the wood times 10 to 12 then the wood can split too easily to be used for a wing spar.

Here is a good example of what I'm writing about;

http://www.rcuniverse.com/forum/atta...mentid=1930722

See how the split in the wood is angled to the side? That's runout. You can typically see this by examining the grain on all the sides to see if it is angled or not.

It is rare to have perfect grain alignment. There is almost always SOME runout. The idea is that it should be less than 1 in 12 to 1 in 15 for use as a wing spar. So if you can see some runout on a 6x12mm spar then on the 12 mm face the grain should not cross from one side to the other in less than 150 mm's. If it is less than it could fail too easily to be used for a wing spar.

Let me know if you are still confused. Online translators do not always convert things correctly. Or if you are using your own English I would be happy to expand where you are unsure.