Help with wing spars

What shape and design will make the strongest spars for their weight?

When using cap spars and shear webs should the spars be wider, taller or square?

Is a box beam with two cap spars and shear webs front and back stronger than a I beam style with the shear webs glued down the center?

Should the grain of the spars be set up to act like a leaf spring with the grain horizontal? Or should the grain run from the top of the wing to the bottom?

Thanks, Brian

Reply to
Brian Jolicoeur
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Lots of questions, and there will be much debate. The weight and strength question is not a clear cut problem, as using the spar in a wing will make things more complicated than a simple strength to weight question.

Wider will be stronger. The further apart the spars, the stronger.

An I beam would be stronger by weight, bit a box beam has benifits in torsional rigidity, and it is easier to glue ribs to it.

Grain should be run from side to side of the thickness of the spar, and lengthwise out the spar.

It is generally held that for medium and small models, the best compromise is to create a D cell, with top and bottom spars, one piece ribs, shear webs in between ribs, with vertical grain, and sheeting on the outside, running from top spar, around the leading edge, and back to the bottom spar.

In some full size wooden wing homebuilts, they use I-spars, with continuous plywood webs, with grain running on a 45 degree angle to the length.

Reply to
Morgans

The grain should run vertical. Horizontal shear webs are much weaker and the grain can actually split under stress.

Reply to
Paul McIntosh

Grain on shear webs should run vertical between the spars. Grain that runs the length of the spars is much weaker and can split under stress.

Reply to
Paul McIntosh

Paul's right. Look at any Q40 or FAI (180 MPH+) and they have the grain vertical, sandwiched between two layers of carbon fiber. I did some "dumpster diving" last year after a race and after dissecting the wrecks, I came to the conclusion it was common practice. It's light and extremely strong! Granted, I did build a twin for a friend that came with a spar having the grain run lengthwise and after many layers of carbon fiber tow and fiberglass, it would still flex. Run it vertical.

Greg

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Reply to
Greg

Thanks for all the replies. To recap, the shear webs should have their grain run vertical between the two spars. The spars should have their grain in the horizontal plane like a leaf spring. The strongest spar would be a one piece full depth spar. When using two spars with shear webs, wider spars would be stronger than taller narrow spars. I am looking to build a light and strong wing. I would like to stay away from two piece ribs so it looks like two spars one top one bottom with shear webs front and back to form a box.

Would 1/4" x 1/4" spruce spars be OK for a 64" "D" tube aerobatic wing? Thanks again. Brian

Reply to
Brian Jolicoeur

I'm not an engineer.

I play one in my dreams.

I actually think an I beam or it's equivalent (shear webs on one side) with the grain arranged as you described (spar grain parallel to surface of the wing, shear grain perpendicular to surfaces) will give a more rigid spar.

Just my opinion. I won't kill or die for it.

Sounds reasonable. Study some kits. You'll see that this is a very popular and successful design.

Probably. You could go to 3/8" wide by 1/4" deep without adding too much weight.

The UltraSport 1000 uses laminated spars that are triple deep near the root, double-deep in the middle, and just a single spar out at the tip. I don't have dimensions right at hand, but none of the three pieces were 1/4" thick.

You have to be careful when building a multi-layered spar like that to make them in the right order. I got one right and one wrong--had to fill it with balsa to get it to look less hideous. I don't remember now how I did it wrong, but I did. For every idiot-proof design, there is a design-proof idiot.

Marty

Reply to
Martin X. Moleski, SJ

In general, the principle of efficient beam design is to get as much stress-bearing material located as far apart (top and bottom "flanges") as possible. To an extent, having a wider, thinner flange works better, from this standpoint, than a square shaped one, but there are other practical considerations -- see below. The other necessity of beam design is that having the flanges in this configuration doesn't do any good if you don't have a reliable way of keeping them separated without the whole thing buckling. That's why the shear webs run with grain vertical, not because they actually withstand shear forces better in that orientation (they don't), but because they are less prone to buckle when the beam wants to collapse.

Bear > To recap, the shear webs should have their grain run vertical between the > two spars.

Actually, in practice most builders glue the shear webs to the backside of the top and bottom spars; there's more gluing surface that way. It is nice if the shear webs fit snug between the rib faces, but not entirely necessary.

Definitely.

Yes, but also the heaviest. And you need to figure out how to interlace the ribs, which also are strongest if they are continuous structures through the spars.

Yes

Most people only have one web, on the backside of the spars, so it's a C shaped beam; not balanced, but that's a tradeoff. There are intelligent compromises everywhere in good design.

I'll stick my neck out and recommend hard balsa instead, to save weight. If your span was 72", or not sheeted from leading edge to front spars, then I'd consider spruce. If your airfoil is only 1-1/4" thick, then I'd consider spruce. But an aerobatic wing should have a fairly thick foil, and with greater separation, the balsa should be fine. You could use 3/16 x 3/8 too, but it's very common to use 1/4" square.

Reply to
Charles Wahl

Although the wording is not perfectly clear, I took it to ask about grain direction in a solid spar.

Reply to
Morgans

OOPS Got that one wrong. A taller spar is stronger, but then the wing would have to be thicker.

Reply to
Morgans

Charles,

I will bet my YS .91 that if you run the grain of the web parallel to the spars it will be weaker. I have had two wings built like this fail. Both were ARTFs that gave me no choice. Both used the same dimensional material as other designs but with the grain oriented wrong.

Reply to
Paul McIntosh

Got ya. In a sloid piece, the grain should run span-wise.

Reply to
Paul McIntosh

Should the shear webs be glued to the ribs and spars or just the spars to give the best strength? Brian

Reply to
Brian Jolicoeur

I've always cut my shear webs to exactly fit and glue to the spars and ribs. It creates a VERY strong unit. Dr.1 Driver "There's a Hun in the sun!"

Reply to
Dr1Driver

They should be a good fit between the ribs as well. That makes it one solid piece instead of a bunch of short ones. If a wing will fail, it will be at the weakest point.

Reply to
Paul McIntosh

Yea, in "solid" pieces, too!

Reply to
Morgans

Nope, only in sloid pieces. ;^)

Reply to
Paul McIntosh

Yep. You are correct.

What you are trying to build is a traditional I beam.

The I beam resists bending by means of resisting tension and compression in the upper and lower spars. That is a function pureley of teh cross sectional area of the spars....exceopet see later...and teh web between teh spars has to stop the spars buvckling and takes compressive load DOWN the web.

So grain should run spanwise on the spars, and vertically on the web, for best resistance to tensile and compressive forces (wood is weak across the grain)

As far as wide flat spars go, those stop the SHEAR WEBS buckling, and are useful if, and ONLY if, there is no leading edge sheeting etc to do the same job. If there is, square section is as good as anything else.

Cap strips on ribs are there to stop THOSE buckling. Arguably, with cap strips your rib grain should run vertically, but few go to that trouble, because teh ribs get weak at assembly time. Without cap strips, the grain should be horzontal, to resist buckling in the fore and aft plane.

So the traditioal wing, with square section spasr top and bottom, vertical grain shera webs, vertical grain ribs and cap strips over the lot, and a fully sheeted leading edge top and bottom, is in fact about as good as uit gets for a built up wing. The D box is very resistant to twisting, as the sheeting goes into tension/compression. Without the sheetng, the covering ends up doing teh same job.

Reply to
The Natural Philosopher

Mostly the spars. Ribs helps to make it a continuous structure, and aboid stress concentration, but its there to stop the spars buckling.

Reply to
The Natural Philosopher

Yep, I'd say they would even be overkill. I've used 1/4 sq. spars in a 78" aerobatic plane. I'd go for 1/8"x1/4", doubled up for the first few rib bays of the wing.....

That's assuming your building and material selection skills are up to the job...

-- Philip Rawson

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Reply to
philip-rawson

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