STEEL SPAR CAP TO WOOD SPAR LOW COST

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dougwanderson

Well-Known Member
The answer to sticking with a material is mainly to get away from the "ain't gon'na happen " to the I got the design ready to build. I know I'm not smart enough to pull odd mixes off. Asking if it will work instead of building test coupons means it ain't gon'na happen.
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.

wsimpso1

Super Moderator
Staff member
Log Member
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

wsimpso1

Super Moderator
Staff member
Log Member
Good or bad idea? instead of carbon fiber spar caps on a wood spar what about a normalised carbon steel flat bar 100,000 psi yield. clean sand and epoxy onto wood spar. maybe a layer of glass over top of steel to help with seperation?
more heavy than carbon fiber but lighter than wood alone spar. Also with steel easy to laminate 2 or more bars at hi load areas.
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

lr27

Well-Known Member
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.

lr27

Well-Known Member
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.

TFF

Well-Known Member
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.

Jimstix

Well-Known Member
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

lr27

Well-Known Member
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.
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.

wsimpso1

Super Moderator
Staff member
Log Member
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

dougwanderson

Well-Known Member
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?

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BoKu

Pundit
HBA Supporter
...would a foam carbon fiber spar be compatible with wood ribs and ply skins?
If you design to deflection and not to strength, sure. You just have to make sure the spar is stiff enough so that the skins don't break when the spar assumes its limit load shape. For a low aspect ratio wing that should be pretty easy.

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