# STEEL SPAR CAP TO WOOD SPAR LOW COST

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by dougwanderson, Sep 12, 2018.

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1. Sep 18, 2018

### BBerson

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For a thin wing, I think a structural steel spar with thin non-structural wood caps might be an option. A steel spar is far stiffer.
The 1/8" wood caps could be bonded and screwed on the spar for gluing the plywood skin.

Sort of reverse of wood spar with steel caps.
The wood caps could easily be hand shaped to the airfoil curve.

2. Sep 18, 2018

### dougwanderson

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steel spar wood cap. what would you use for shear web or do a truss. maybe some spot welded stiffening also for rib connections.
or build the spar out of tubing truss with two part rib with glued ply to join the rib parts.

3. Sep 18, 2018

### BBerson

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A steel tube truss if the wing depth is large. Or a thin steel webbed I beam for a thin wing. The choice depends on your welding ability. I have a MIG welder that could make an I beam quickly, I think. But haven't tried yet. Might not be practical.
But I like to weld so might try something.

4. Sep 19, 2018

### lr27

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BoKu:
Are the higher modulus types of carbon too expensive to use for this? Maybe those aren't even necessary. Seems like if you make conservative assumptions about the strength of the layup, then the strain at yield ought to be LESS for the carbon. For instance, if you've got some super duper pultrusion, it might be good for 200,000 psi and have an elastic modulus of 20,000,000 psi. That would give 1 percent elongation at yield. ANC-18 lists Sitka spruce at 4,200 psi stress at the proportional limit and an elastic modulus of 1,270,000. That gives 3.3 percent elongation at yield. So the carbon ought to load up first. Hand layup tow should have a lower yield than a pultrusion, but presumably the stiffness would be almost comparable. Of course, if one uses certain uni that has obnoxious crossways stitching or adhesive that keeps the fibers from being really straight, the results would be different.

edit:
OOPS. As pointed out by ragflyer, I slipped a decimal point and the elongation of the wood is 0.33 percent! OTOH, if you're using hand laid up carbon tow, I suspect you couldn't count on more than 100,000 psi, so the difference would be less. Particularly if using high modulus tow. Also, if I"m interpreting things correctly, maybe the wood only has to go up to 1.5X the limit load, while it's customary to go to 2X for composites.

Last edited: Sep 19, 2018
5. Sep 19, 2018

### dougwanderson

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flat bar can be challenging to find but round bar is easy 1144 4140

6. Sep 19, 2018

### BBerson

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Aircraft Spruce has 4130 flat bar.

7. Sep 19, 2018

### ragflyer

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slight math issue... wood would be 0.3% (not 3%), so the wood will load and fail before the carbon in your example.

8. Sep 19, 2018

### lr27

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OOPS! Those pesky decimal points... have edited the original post

9. Sep 19, 2018

### lr27

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Just for fun, I compared some options for equal strength spar caps, based on prices from Aircraft Spruce. Baseline is a foot of 1 X 4 sitka spruce spar stock. That's good for something like 16,800 lbs going by figures in ANC-18. I used 90,000 psi for the steel, 40,000 psi for the aluminum (2024 t3). For the carbon, I required 22,400 lbs yield, assumed 100,000 psi for hand laid up tow. Epoxy about $20/lb, tow about$80/lb, 60 percent fiber content by weight. Found a figure of 26g/in3 for uni carbon. The figures indicate I should probably have used 13 percent more carbon, I suppose. The prices are probably all higher than they need to be. For instance, Soller Composites has stiffer tow for $30/lb and regular tow for$18 or $20 per lb. But that's probably true about the other materials as well. A further consideration is that some of these will be easier to taper than others, making them lighter in a real spar and maybe even making for less wastage. In any case, none of the options is all that expensive compared to, say, the cost of an engine. If you use the carbon, you might save a couple of percent on the weight. If that and a few other options are enough to use a smaller engine.... type cost weight size elongation at 16,800 lbs spruce$11.75 0.78 lbs 1 x 4 .33 %
4130 $2.77 0.67 .1 X 2 .3 % 2024$13.00 0.6 lbs .125 X 4 .34 % (roughly)
cf \$8.40 0.15 lbs! .224 X 1 .38 % (roughly)

If you were going to go this way, it would be wise to test the actual properties of the carbon, or to account for fasteners, fastener holes etc. for the metal. I suspect an oversized carbon spar cap is probably worth it. One dodge with the cap is to make sure that the non-structural wood cap over the carbon has the grain running in some other direction. With some species of wood, that might allow that wood not to fail until the carbon had. If not, it would still be flimsy enough in that direction that I doubt it would make the carbon fail prematurely. I think if it was my plane, I might just use some kind of foam there, at least in between ribs.

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10. Sep 19, 2018

### BoKu

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Jim Marske did a bunch of coupon tests of hand-laid carbon tow and tape. Getting good tensile properties is a slam-dunk. Getting good compression turned out to be problem. Nothing he did got to the 90ksi reported in some handbooks. Coupons he thought would go to 60ksi only went to 40ksi.

This right here is why I so love the Graphlite strips. Their modulus is only slightly better than hand-laid tape or tow. But you can reasonably expect 200 ksi in compression.

--Bob K.

Last edited: Sep 19, 2018
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11. Sep 19, 2018

### BoKu

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I have to defer to Billski for the real engineering. As for the economics, "too expensive" is relative to everything else in the context. Overall, risk and risk mitigation are usually far more expensive than carbon fiber.

* When you make a part you can't use, that's expensive.

* When your enterprise pays for each square foot every month and you take a lot of time to save a little money, that's expensive.

* When you make a part that requires compromises elsewhere in the assembly, that's expensive.

* When you make a part and you're unsure of its properties so you have to test every single example, or accommodate the uncertainty with lower capacity, that's expensive.

For my wing spars I went with Graphlite pultruded strips because I had confidence that they would give me their full strength and stiffness properties first time and every time even under relatively primitive fabrication conditions. We just stack them up in the mold trenches, close the shear wrap, and bag it and tag it. One shipset of finished 15m spars, ready to install into the upper skins, is basically a three-day job for a tech and two apprentices.

--Bob K.

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12. Sep 19, 2018

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