Could someone explain the rules for running balsa grain on a tapered wing sheeting. Are there advantages or disadvantages to running the grain parallel to the wing taper vs parallel to the spar? My wing has two different taper angles on the leading edge and a third on the trailing edge. The sheeting goes over the leading and trailing edge for a good glue joint. One advantage to running the grain parallel to the spar, on a one piece wing, is center section strength because of the continuous balsa run. Any thoughts?

I've always ran the sheeting parallel to the spar for strength. Running it perpindicular to the spar will allow the sheeting to flex more and allow the wing to twist more easily. You could however sheet the wing tips perpindicular to the spar.
Also when using shear webbing run the grain horizontally between the spars

With angles that slight, you're really splitting hairs. It wouldn't really matter either way,  but best bet is running parallel to the spar.

For angles that slight, use the one that gets the most coverage from your sheet of balsa, the deviation will be so small it won't matter. I'd lean toward parallel with spar for most strength.

As has been stated with spar. However, since it has to be glued over leading edge;which is tapered, that might be a bit difficult to flex it there since grain will not be parallel there. guess you will have to make decision.

Thanks for all the replies. In this particular case my wing leading edge starts at 45* and then drops to 30* towards the wing tip. So the angles are not slight. I would like to use parallel to the spar but at this point I have yet to find a production kit that does not have the grain run parallel to the wing leading or trailing edge, not the spar, at least for an inch or two. There must be a reason for this.

iflircaircraft,

"Also when using shear webbing run the grain horizontally between the spars "

Don't you mean the grain should run vertically between the spars as in from top to bottom.

It's basically impossible to run grain parallel to 3 different lines. So I assume you're thinking to sheet from the LE back with some, from the TE forward with some and possibly some down the middle. Trust me, it's been done.

That extra work might be worthwhile to some, but in fact balsa differs enough that it's really not worth the effort. What is worth the effort is to sort all the sheets by weight and strength. Use the strongest where it'll be down the spar and forward to the LE. Throw out the weakest and find better.

If you're sheeting foam, it's not so important. The foam provides the same support that webbing provides spars, only it does the support everywhere.

skylarkmk1
yes, that's what I meant. Grain Vertical
I had it right in my head but my fingers didn't listen!!

To throw another wrench in the mix... what is standard practice for shear webs... I've seen studies done regarding the vertical grain shear between the ribs.. and the same attached to the front of the two spars. I believe in front was strongest due to more surface for the glue joint and no gaps that are possible with grain between the spars...

Style A is more structurally sound.

The part about shear webs that does not make sense is, if you take two pieces of balsa, one with vertical grain and one with horizontal grain, the one with horizontal grain is much harder to break. I built two test spars, one with vertical grain shear webs and one with horizontal grain shear webs. While supported on both ends, I added weight until they broke. The horizontal grain shear web took 5 times as much weight to break. Where am I going wrong?

If you research shear webs, you will find that the strongest are those with the grain at 45 degrees to the vertical; bad thing, this changes by 90 degrees if you switch from positive G's to negative G's. Short story is that you are equally good having shear webs at either vertical grain or spanwise grain as far as shear strength goes. Now, if you are worried about crushing the wing by stepping on it or the dog walking on it, then having the grain vertical is the strongest for that external force. Shear forces are always at a 45 degree angle to the vertical so a pure shear web, for maximum strength with minimum material, the grain must be at 45 degrees to the vertical. Again, the bad thing is if you set it up for maximum resistance to shear for positive G's, you will have minimum strength for negative G's. Either vertical or span wise grain is the compromise for best strength for both positive and negative G's.

If 45 degrees is the best then get the best of both by installing shear webs on both the front and back sides of the spars. Upside is strength. Downside is weight.

What you need to understand is that we are talking about SHEAR.

"Shear" is when two objects (in this case, the spars) move in opposite directions to each other.

As air pressure lifts the wing, and since the weight of the fuselage is at the root, the wing tries to lift at the tips. This causes the spars to try to move in a shearing motion. (First image)

Although the pressure is at 45 degrees, the movement of the spars is relatively 180 degrees from each other. So a horizontal grain is more likely to split than a vertical grain.

You COULD add strips at 45 degree angles, but this will result in less gluing area. Of course, you could also cut full webs with a 45 degree grain, but for the most part, this is overkill.

The bottom line here is: I have built MANY planes with a vertical grain web and have absolutely tortured them without a single wing failure.

Your point is well illustrated. Shear web is for shear pressure.

I am of the opinion that running the grain parallel to the LEADING EDGE is the best plan. It allows for the easiest "roll" of the skin around the curvature of the airfoil while still providing good longitudinal stregth to the wing. Also, whatever method you choose, use the same angle on top and bottom, otherwise you may induce an unwanted twist. The type leading edge you describe (somehwat like a P51 Mustang) can be skinned in this fashion but add another piece to the skin that is skewed to the leading edge at the wing root. I also like to put spars in a foam wing. Use a hot wire to cut a groovetop and bottom, then place the spars in it with Gorilla Glue. Sand off the excess expanded glue before skinning (See other articles about using PU glue for skinning). Use same technique for placing gear blocks or servo boxes before skinning.

Guys, The main problem Mike is having is a bit different than you think.
This is literally a flat plate wing only 3/8 of an inch thick with a 42 inch overall span.
He is asking about grain orientation as it applies to what is essentially a stress skin wing.
If the wing were thick enough so as to be able to use a shear web the 45 deg setup is thought as what is being optional.
But were Mikes wing is so thin he is trying to come up with ways to use the sheathing both to stiffen the wing, and increase the torsional strength, because as it is now flutter is a major issue in his design.
So bending the wood to the airfoil shape is not an issue, coming up with a way to get the most strength both span wise and resisting twisting is.
His spars and alike are limited to 1/4 inch in height so that is all he has to work with, and he is trying to do it in wood as opposed to using exotic materials.

If that is the case, I would sheet the wing this way

Here is what the wing looks like it a bit unconventional, as it is a hybrid delta flying wing...

I prefer all grain run parallel to the spars unless there is a strength advantage to following the leading and trailing edge. One option is whats in black on the attached pic and someone else suggested what is in red. Your suggestion is doable but a lot of work on a multi taper wing.