# 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 12, 2018

### dougwanderson

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I'm not building a plane with it I was playing with numbers for a spar and steel would make an excellent cap for a spar replacing a big chunk of wood saving weight and able to be deeper spar.

2. Sep 12, 2018

### wsimpso1

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You are asking the questions, you get to do the calculations. And let's remember, in airplanes, WEIGHT IS THE ENEMY. Keeping weight (that still makes strength) to a minimum is essential to flight...

Take the strength of any material you are considering and divide it by the density of that material to get specific strength. Accounting for differing FOS, take the max stress you will ever use in a material instead of the strength to get specific useful strength.

Your 100ksi steel at 0.283 lb/in^3 is about 353E3, with an FOS of 1.5 common in metals, that is 235E3. The more standard spar cap material is 7075 is about 742. E3, or 495E3 with the FOS. Graphite is about 6250E3, and with its FOS of 2.0, 3125E3. So that can give you some ideas as to why we might tend to use the materials we do and why steel is not used much in airplane structures. Look up the various woods you are thinking about.

Then, you can calculate total beam EI which is the sum of the EI for each of the pieces. Yeah, the caps contribute mightily, but so will the wooden shear webs ns the glue lines will have to be big enough. While you may think that the thermal and moisture induced dimensional changes look small, let's look at it another way - when components change dimension relative to each other, they load each other and the glue line. This internal stress is before you carry any live load. The live loads of flight then have to be carried with what is left of the strength...

Next is your calc of glue line strength. Gougeon Brothers Proset adhesive has a lap shear strength on steel of about 2000 psi, and shear modulus around 230E3 for failure in shear at a strain of 0.0087. Before you say that is plenty, let's remember that your glue line will be thin, so the actual dimensional difference it will tolerate is actually the lap shear strain multiplied by the thickness of the glue line. A 0.010" thick glue line will make the length difference 87e-6 inches per inch of length. So, compute your thermal and moisture induced length changes for the glued parts, compute the shear strain in the glue, and compare it to max strain available. You get to carry the live loads through the glue line after that deformation is already carried...

Steel is heavy. Differences in thermal deformations can significantly reduce load carrying of materials and of glue lines.

We like glass fiber and epoxy for good reasons. Graphite fiber and epoxy are even better. Aluminum and wood are both great. Even all steel beams can be very suitable. Mixing them together gets complicated and has to be done considering everything. Sometimes mixed material sets can be great, other times, it can really be a hinderance. Do all of the work, then make your choices.

Billski

3. Sep 12, 2018

### wsimpso1

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Actually, I would not use a wooden shear web with carbon fiber caps... Graphite caps and either glass or graphite cloth at +/- 45 degrees for the web. Make it a channel, with a foam core one one edge of the caps, half of the web plies around the outside (top of top cap, web, bottom of bottom cap) and half the web plies inside (bottom of top cap, web, top of bottom cap). The core is there to keeo the web from buckling, does not need to be thick - usually 1/4 or 3/8 divinycel is plenty. Wood is a heavy core at 30 to 40 pcf, while divinycel is 6 or 8 pcf...

The big trick in spar caps made of graphite fiber is getting them straight and low resin volume. Even tiny misalignments and increases in resin can make for substantial strength reductions. Graphite caps should probably be either vacuum bagged pre-pregs with resin content set for the vacuum level you will use (atmospheric bagging requires more resin than autoclave curing) or use Graphlite rods.

Jim Marske is the seller of rods and the authority on building with them... See his site and classes.

Billski

4. Sep 16, 2018

### lr27

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An alternative might be to look through ANC-18 and select an especially strong wood for the spar caps, so that they can be thin and as far as possible from the neutral axis. I wonder if you can get long, straight grained pieces of osage orange? (My guess is no, but it's strong.) An advantage of a wood or carbon spar is that it should be relatively easy to taper the thickness of the spar caps as required.

I've read Strojnik on epoxying things to metal. You need to be really religious about process control, which might not be easy on an amateur basis.

Is carbon fiber really all that expensive? You can get carbon tow for as little as $18/lb at Soller Composites. You'd probably need two or three times as much of it as Graphlite, of course. How much would you have to pay for the steel? I could be wrong, but I suspect it's possible to work out a way to lay up tow under a certain amount of tension, so that things stay straight. Some careful testing would be in order, increasing the amount of material you['d have to use. If you want to combine wood and carbon, I think you can find downloadable plans for the Carbon Dragon and the Woodstock on line. I haven't heard anything terrifying about them, though you'd want to look into that. Plus better techniques may have been developed since then. Someone mentioned welding, but that's going to reduce the strength except maybe if you heat treat the whole thing afterwards. Unless the failure mode is buckling. In that case, the steel's strength loss might not make that much difference. It would help to know the allowable dimensions and anticipated loads on this beam. If there's enough depth available, wood isn't all that bad, and the technology is well developed. 5. Sep 16, 2018 ### lr27 ### lr27 #### Well-Known Member Joined: Nov 3, 2007 Messages: 3,216 Likes Received: 463 P.S. Come to think of it, if depth is not constrained, the wood spar could be lighter, stiffer and stronger than the steel-capped one. mcrae0104 likes this. 6. Sep 16, 2018 ### pictsidhe ### pictsidhe #### Well-Known Member Joined: Jul 15, 2014 Messages: 6,224 Likes Received: 1,599 Location: North Carolina Speaking of ANC-18, I have a pdf, it's poor quality and scrolling through it is wading through treacle. Does anyone have a link to a good one? 7. Sep 16, 2018 ### rotax618 ### rotax618 #### Well-Known Member Joined: Nov 1, 2005 Messages: 641 Likes Received: 215 Location: Evans Head Australia The original Benson type rotorblades consisted of a tapered steel 1/8” spar screwed to laminated plywood blade with countersunk screws. The blades were subjected to extreme flexing and tension without failure or de-lamination. There was some problem with curving due to moisture mainy because there was only one spar and it was attached to the bottom of the blade. dougwanderson likes this. 8. Sep 16, 2018 ### lr27 ### lr27 #### Well-Known Member Joined: Nov 3, 2007 Messages: 3,216 Likes Received: 463 9. Sep 16, 2018 ### dougwanderson ### dougwanderson #### Well-Known Member Joined: Jul 21, 2014 Messages: 137 Likes Received: 28 Location: Lakeville, MN USA wood wing with plywood skin is what i was playing with numbers for. so a foam and composite spar would be bad choices with ply skins?and all foam and fiberglass wing$$i did look at 3k tow i dont have experience with composites. maybe play with it. this thread was looking for experience and knowledge with a steel spar cap and thanks all for the conversation as for depth 5.5 inches momet at center 91000 inch pounds 10. Sep 16, 2018 ### wsimpso1 ### wsimpso1 #### Well-Known Member Joined: Oct 19, 2003 Messages: 5,674 Likes Received: 2,924 Location: Saline Michigan The problem is we do not have experience with a steel capped wooden spars... If you want to do the math, and do not know where to start, I always recommend the beam theory chapters in Timoshenko and Gere "Strength of Materials", but any Mechanics of Materials text will have it... My/I = stress; max stress = 10,000 psi/1.5 = 6666 psi; y = 2.75"; M = 91000 in-lb; Rearranging, I>= My/max stress = 37.54 in^4. I rectangular = b*h^3/12; Rearranging b = 12*I/h^3; I = 37.54, h = 5.5; b>= 12*37.54/5.5^5 = 2.708" So, a solid spruce spar 5.5" deep and 2.75" wide would do it. You could work with thinner, somewhat wider caps and plywood webs and get to similar I. I found that 7"x1" caps gets you there with out including webs, and plywood webs would add more. Thing is that plywood on the bias has its own E, so after you size the webs to carry your shear (you did not say what your shear was), you have to include them using composite beam theory. Also in the same text referenced above... You could also do similar stuff with steel. Steel is not only stronger, but it has E of 30 Mpsi while sitka spruce is about 1.5Mpsi. Have fun, Billski dougwanderson likes this. 11. Sep 17, 2018 ### BBerson ### BBerson #### Well-Known MemberHBA Supporter Joined: Dec 16, 2007 Messages: 11,553 Likes Received: 2,159 Location: Port Townsend WA Most cantilever wood spars have beefy steel bars for the end attach fittings, about 12" or something. These are bolted, never bonded. Since you need end fittings anyway, you might extend these bolted steel straps out to 4 feet or six feet or whatever and get what you want that way. The steel could get progressively thinner and no steel on the last four feet to tip. The additional 4130 steel might be a few pounds. And a few dollars. The steel caps would take all the cap loads and the web could be plywood. 12. Sep 17, 2018 ### lr27 ### lr27 #### Well-Known Member Joined: Nov 3, 2007 Messages: 3,216 Likes Received: 463 Is this for an existing design? 5.5 inches seems a bit thin for a wing, at least at the root, unless maybe it's a sailplane? If you can add more depth, you won't need the steel and its complication. 91,000 inch lbs seems to suggest the wing is short and the aircraft light. Or I could have missed a decimal point someplace. The Sky Pup, which is a well regarded ultralight, uses wood spar caps and foam shear webs, though that's supplemented by plywood at the joints. The spar is something like 11 inches deep at the very center, tapering down to 8 5/8" within several feet. Root chord is 50 inches. The famous Carbon Dragon combines traditional wood construction with wood and carbon spar caps. Possibly more complicated than it has to be? Anyway, I suspect that a foam and composite spar with ply skins can be done reasonably well. Someone mentioned extending the steel joiner reinforcement, but unless that curls up over the top, it gives up some spar depth. Strojnik has shown that, with sufficient care, you can use aluminum spar caps and plywood webs, but he was working to a pretty high standard. I'm guessing that, with similar effort, you could make steel work. But why? 13. Sep 17, 2018 ### dougwanderson ### dougwanderson #### Well-Known Member Joined: Jul 21, 2014 Messages: 137 Likes Received: 28 Location: Lakeville, MN USA Remember this was a mathematical learning playing around what if. Put highest modules of rupture on the spar caps economically. Yes it is a short wingspan 4x8 wing panels 220 inch total one piece spar full cantilever 1.5 okoume ply skin. for a single place tailwind like aircraft all up weight gross 550 to 600 lbs. using 6061 t6 welded (t3 at weld) fuselage .058 wall 3/4 to 1.5 tubing (very economic sizes around$400 for fuselage) rated 4g crash 9g flying (aluminum fatigue).

14. Sep 17, 2018

### lr27

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With a wood spar cap, I think you're looking at maybe 4 or 5 square inches spar cap cross section, tapering off quickly. Less cross section with a thicker airfoil. Yours is pretty thin unless it's designed to be fast. Maybe even then. I'll admit I'm a bit confused about the g's.

15. Sep 18, 2018

### TFF

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Cantilevered changes a lot. A W10 Tailwind spar is 4 3/4" X 1 1/4". 4'x8'and strutted. Any spar out of any material is going to be massive in its context being cantilevered. If you want a cantilevered wing, I would build a big Cassutt wing and attach it like a Cassutt. The fittings of a half on a split panel wing and the fuselage are going to be large. One piece laminated wood spar and be done. Otherwise you are going to have to build a high wing mini Bellanca Viking. The Spacewalker has a huge spar being cantilevered.

16. Sep 18, 2018

### Jimstix

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See the standard textbooks on beams of different materials. As mentioned above, often the biggest challenge is transmitting the shear forces from the steel cap to the wooden spar. Most common solutions for steel caps on wooden spars include lots of flat-head screws passing through the caps vertically into the wooden spar below. Thin caps are prone to buckling, so rolling the fore and aft edges may be required to meet needed bending resistance. Steel is an excellent aircraft material that may not belong on your wooden spars. Another approach would be to embed Marske's pultruded carbon rods in the wooden spar. Just watch the nails. Jim

17. Sep 18, 2018

### lr27

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The Tailwind is twice as heavy at gross. If wood sailplane wings are possible, and they are, then such a short wing will not require an infinite amount of wood. The Sky Pup has a cantilevered wing, grosses 400 lbs with a 31 foot span, and has a top cap only 1/4 inch X 4 inches. Of course, the wing is twice as thick in the center and it's probably not stressed for as many g's. OTOH, it doesn't require sitka spruce or doug fir for the spar caps.

18. Sep 18, 2018

### wsimpso1

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Steel fittings bolted to wooden spars was mentioned... The reasons those airplanes use steel fittings is that they use bolts to connect one piece to another. All of the failure modes have to be prevented, bearing, tear out, bolt shear, etc. Steel on steel is sometimes the light and compact way to do that. The classic way is to bolt the fittings to the wood, even when they assemble the fittings and bolts with epoxy on everything...

Billski

19. Sep 18, 2018

### dougwanderson

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did i mention it was a tailwind like not a tailwind. and yes i did look at a cassutt wing 5.5 ish tall by 3 wide at center.

This whole post was on the feasibility of wood spar with steel cap strips. and the answer is probably NOT. a 5.5 tall by 4in box spar tapered to 2 inch at wing tip maybe best with contoured to air foil shape?

would a foam carbon fiber spar be compatible with wood ribs and ply skins?

Last edited: Sep 18, 2018
20. Sep 18, 2018

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