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Thread: Composite Wing Spar for an Ultralight

  1. #16
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    Re: Composite Wing Spar for an Ultralight

    Lots of designs use extruded foam with cap strips for ribs. Yes, solvents will destroy them.

    Continuous white bead foam will be a LOT heavier than separate extruded foam ribs. The white foam is very weak, so if you used it you'd need more ribs or just to make it solid. And if it's solid, on a typical ultralight, it will be heavy. Let's say you have 130 square feet, 28 foot span, 15 percent airfoil, and the foam takes up 80 percent of it. You're talking about maybe 58 lbs of white foam! 3/4 inch wide ribs with 1/8 inch cap strips (guessing) might be more like 0.4 lbs apiece. If you use them every 18 inches, that's 8 lbs. Built up ones are probably even lighter.

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    Re: Composite Wing Spar for an Ultralight

    Quote Originally Posted by Bart View Post
    OTOH, some buy in S. Africa designed and sells kits for a motorglider (50' span?) using the cheapest white bead foam (that crappy stuff your computer was packed in) from home supply places like Lowe's. It's low density, and easily hotwired. Being low density, rather than heavier Klegicell ribs every 6" or so and hollow between, he just uses 100% foam core. Like surfboards. That way, skin is 100% supported, and no stress concentrations. Dissolved easily by gasoline, though, so you must make absolutely sure to avoid gasoline or other solvent contact ever.
    Since that foam isn't too resistant to fatigue (growing cracks) it's probably degraded to several small pieces of foam. That's also the moment your wing skin will warp and break off..

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    Re: Composite Wing Spar for an Ultralight

    My plan was to use 3/4 inch foam insulation with 1/8 inch cap strips, just like lr27 said, but I was thinking somewhere between 18 and 24 inch spacing. The wing dimensions I'm planning on right now are as follows:

    Span : 30 - 32 feet (not decided)
    Cord : 5 ft
    Airfoil : undecided

    These dimensions are partly dictated by the size of my truck vs. how far they will break down for transport.

    I think the idea of putting fuel in the wing spar is pretty cool, and not a bad one except that my ribs are styrofoam... that would make a leak potentially catastrophic...

    The only other thing I have to say is 'keep it coming'! I can use all the ideas I can get.

    Thanks!

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    Re: Composite Wing Spar for an Ultralight

    Quote Originally Posted by lr27 View Post
    Lots of designs use extruded foam with cap strips for ribs. Yes, solvents will destroy them.

    Continuous white bead foam will be a LOT heavier than separate extruded foam ribs. The white foam is very weak, so if you used it you'd need more ribs or just to make it solid. And if it's solid, on a typical ultralight, it will be heavy. Let's say you have 130 square feet, 28 foot span, 15 percent airfoil, and the foam takes up 80 percent of it. You're talking about maybe 58 lbs of white foam! 3/4 inch wide ribs with 1/8 inch cap strips (guessing) might be more like 0.4 lbs apiece. If you use them every 18 inches, that's 8 lbs. Built up ones are probably even lighter.
    I take your points, which all are valid concerns.

    Still, consider the following: Whisper Aircraft This 52.5' span aircraft and its design are no slouch. I'm not sure the wing weight difference would be as acute as you say. And, given the 18" gap between your ribs, that's 18" of unsupported skin which in turn has to be stiffer (i.e., heavier and/or thicker) than would be the case with 100% foam supported skin, as is the case with the South African Whisper motorglider. So, in large measure, what weight your 18" ribs taketh away, your skin puteth back.

    I had questions about the white styrene foam durability, so traded emails with the designer several years ago. He says it's fine. By now, there are a number of his planes flying an in construction.

    Maybe worth a look, to see if real world experience bears out your valid concerns about weight and durability of white foam vs. alternate choices.

    The jury may be still out, so forums like this are great to discuss pros and cons.

    Also, note how Whisper does its spar.

  5. #20
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    Re: Composite Wing Spar for an Ultralight

    I wouldn't use Expanded (bead board) white foam for anything structural no matter how light the load. It's not worth it. Just use some form of extruded foam. It has better water resistance too. Probably stands up better in sunlight. I think the beads in the other foam would tend to break apart under prolonged load, heat, sunlight, repeated deflections....any old thing really.

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    Re: Composite Wing Spar for an Ultralight

    I wonder, if you were using a fuel tank in the wings, how much extra trouble it would be to use a bandsaw and some other rib material. (Or built up ribs, for that matter.)

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    Re: Composite Wing Spar for an Ultralight

    Quote Originally Posted by lr27 View Post
    I wonder, if you were using a fuel tank in the wings, how much extra trouble it would be to use a bandsaw and some other rib material. (Or built up ribs, for that matter.)
    Not sure I understand your question. But, it seems that a hotwired foam that is impervious to fuel or similar solvents would be best and easiest. 100% support of the wing skin means use of thinner skin is acceptable, saving weight and cost there. Columban's CriCri used thin gauge aluminum skin over Klegicell ribs, adhesively bonded. With a straight, un-tapered wing, he just used a household vacuum cleaner to suck and hold the alu. skin down against the ribs for adhesive to set up. Surely this would work with a properly contoured, 100% foam core as well. Some research shows that up to thickness of ~4", a solid foam core has lightest weight. With no stress concentrations, a web-less spar may be possible.

    Len Neimi's Sisu sailplane of the 1950's was a world beater. He used several sheets of thin alu. in a built-up laminate skin/spar. I'm not sure what he had for web between upper and lower surfaces. Skin texture and prevention of oil canning was excellent, which accounts for the excellent performance of his plane.

    3M makes some very good adhesive, 2 sided tape, which holds very well if surfaces are properly cleaned. No rivets or stress concentrations, so thinner skin can be used, since rivet grip and joint strength is apparently what drives skin thickness and therefore weight.

    Anyway, for those who dismiss use of white styrene foam out of hand, why not withhold judgment until we hear more from the South African Whisper motorglider guy? He's now had a few years to work with this stuff, so may be it's OK.

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    Re: Composite Wing Spar for an Ultralight

    Never heard of a foam that was safe to cut with a hot wire that wasn't attacked by fuel.

    4" is very thin for an unbraced wing. Would require VERY heavy spar if span was not really short.

    The skin in most small aluminum aircraft is so thin already that you can dent it with your finger.

    We used to use transfer tape on foam wings, now use epoxy. If you place anything wrong with the transfer tape, it will rip up the white foam.

    As far as the white foam goes, we already know what it weighs. An ultralight has a wide chord which means lots of volume and therefore lots of weight if it's solid foam.

    Quote Originally Posted by Bart View Post
    hotwired foam that is impervious to fuel or similar solvents would be best and easiest.

    100% support of the wing skin means use of thinner skin is acceptable, saving weight and cost there. snip
    3M makes some very good adhesive, 2 sided tape, which holds very well if surfaces are properly cleaned. No rivets or stress concentrations, so thinner skin can be used, since rivet grip and joint strength is apparently what drives skin thickness and therefore weight.

    Anyway, for those who dismiss use of white styrene foam out of hand, why not withhold judgment until we hear more from the South African Whisper motorglider guy? snip

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    Re: Composite Wing Spar for an Ultralight

    Quote Originally Posted by lr27 View Post
    Never heard of a foam that was safe to cut with a hot wire that wasn't attacked by fuel.

    4" is very thin for an unbraced wing. Would require VERY heavy spar if span was not really short.

    The skin in most small aluminum aircraft is so thin already that you can dent it with your finger.

    We used to use transfer tape on foam wings, now use epoxy. If you place anything wrong with the transfer tape, it will rip up the white foam.

    As far as the white foam goes, we already know what it weighs. An ultralight has a wide chord which means lots of volume and therefore lots of weight if it's solid foam.
    Perhaps you should familiarize yourself with various designs such as Rutan's Longeze, with its solid foam core wings and fuse panels, yet manages to fly, for decades, using fuel which would dissolve the foam if in contact, which it isn't. The Rutan method is essentially like making surfboards (fiberglass wrapped carved foam), from which the idea was copied by others, then by him, 35 years ago. Hot wiring is surely the most efficient way to get an airfoil in a garage workshop. The Rutan method does, however, sop up too much heavy, expensive, and perhaps brittle epoxy, and when you're done, you don't quite know what you have, structurally. Alu. is much more predictable, qualitatively, and best suited to flat wrap applications, but not compound curves, so makes good wing skin.

    As for spar thickness, consider that when you double the thickness between flanges, you get 8-fold increase in stiffness, and stiffness is mostly what you're after rather than strength, per se. Take the BD-5, which had problems, but the wing spar was not noted among them. It had a round tubular spar of ~3" diameter, carrying an aircraft perhaps twice as heavy as I envision. If a ~3" round spar (inefficient because being round has not enough metal far enough apart for the flanges, but too much metal in the webs) was structurally sufficient, then a 4" square tube would be much stiffer, with its flanges further apart and more metal in them. A 4" tubular spar of 1/8" wall thickness would be much more robust than the BD-5 spar, and would weigh ~2.2 lbs. per linear foot, or 30-40 lbs. for an aircraft of comparable size to the BD, a CriCri, DA-11, etc., but would also serve as the fuel tank, saving weight, expense, and time normally spent on that. If fuel were carried mid-section, the outer sections of web could have lightening holes cut, to save a bit of weight further out on the wing.

    Read Strojnik, all three volumes. He goes into considerable detail about various spar and other aircraft components, pros and cons of each, comparative weight, etc.. His best weight spar idea was ~1.7 lbs/linear foot using alu. angle pieces as flanges, epoxied to plywood web. Assuming his epoxy bonds were perfect, he'd still have to add more weight to resist torque, and yet more weight for the fuel tank, bringing it to significantly more than just using a square or rectangular extruded alu. spar in the first place. He did, however, use such tube for his tail boom on the S-2 motorglider, which is a feast of clever ideas which leave the rest of us wondering why we never thought of them. His Laminar Magic plane was a scaled-down S-2 and set records for speed vs. power in its weight class. Google for it.

    Anybody interested in light, small, efficient planes of excellent performance for limited power should consider how Strojnik, Columban, Davis, DeBreyer, and Winton et al did what they did.

    For efficient aerodynamics, read Bruce Carmichael and Sig. Hoerner.

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    Re: Composite Wing Spar for an Ultralight

    Quote Originally Posted by lr27 View Post
    4" is very thin for an unbraced wing. Would require VERY heavy spar if span was not really short.
    The 1100 lbs MTOW Lak-17a has a 18m (60 ft) span wing with an aspect ratio of 30 and thus a root spar depth of about 4". To what I recall the total spar weight is around 10 kg (22 lbs), excluding the metal parts to bond both parts of the spar together. A Lancair IV or a Cirrus 22 would need about the same spar weight, assuming all are rated to 5 G's.

    The only reason they don't use foam is that you need place for the water ballast and thus you have a complete wet wing (200 kg or so)

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    Re: Composite Wing Spar for an Ultralight

    Ok, if you want to use graphlite spar caps, I concede the spar would be pretty light. But it's not necessary on an ultralight with bracing wires.

    On the Skypup, which is a cantilever ultralight, the WOOD spar caps weigh something like 10 lbs! For a wire braced ultralight, wood caps could weigh much less.
    Quote Originally Posted by autoreply View Post
    The 1100 lbs MTOW Lak-17a has a 18m (60 ft) span wing with an aspect ratio of 30 and thus a root spar depth of about 4". To what I recall the total spar weight is around 10 kg (22 lbs), excluding the metal parts to bond both parts of the spar together. A Lancair IV or a Cirrus 22 would need about the same spar weight, assuming all are rated to 5 G's.

    The only reason they don't use foam is that you need place for the water ballast and thus you have a complete wet wing (200 kg or so)

  12. #27
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    Re: Composite Wing Spar for an Ultralight

    Well, it's comforting if the foam isn't vulnerable to the fuel. One little mistake....

    I'm fairly familiar with the Rutan method. The fiberglass, although laborious, allows you to sand in a nice laminar airfoil. If your core is perfect and your bonding process is totally under control, perhaps aluminum makes better skin. It's not clear to me that it's going to be much lighter, since you have less control over the thickness. If you want to have a lighter skin, I bet vacuum bagged or infused glass on either side of, say, dense 1/16" or 1/32" foam, would make a lighter skin for the stiffness, though you'd then need a separate spar. None of this is relevant for an ultralight which has a larger wing and must be much lighter.

    I have hot wired cores and helped with large (20' X 6') vacuum bagged layups.

    The Longeze has the advantage of relatively small wing, so the foam doesn't weigh all that much relative to the ultralight that started this thread.

    GIven it's mission of lofting a heavy pilot, the ultralight that started this discussion probably has a span on the order of 30 to 35 feet. A spar that was 2.2 lbs per foot would be 66 to 77 lbs. The entire cantilever spar for the 31 foot Sky Pup wing weighs something less than 25 lbs, not counting the steel joiners. A wire braced version would weigh far less. Let's be pessimistic and say 20 lbs. With the use of a non-foam shear web, it would probably be even lighter. An extra 46 to 57 lbs counts for a lot on a 254 lb plane. (Powered ultralights, at least in the US, are required to weigh 254 lbs or less.)

    I own and have read all 3 of the Strojnik books. I'm afraid at the moment I can't find the figures on the spar you mention. However, the limit (not ultimate) bending moment is given as 163,930 in-lbs (page 182, Laminar Aircraft Structures). Your square tube has a moment of inertia of 4.9, a section modulus of 2.4, and stress at the limit bending moment of about 67kpsi, far beyond the yield strength of the 6061-t6 that Strojnik uses in his spar. Using 7075, which would appear almost adequate for the limit load, won't be anywhere near enough for the ultimate load. I'm fairly certain Strojnik's wing skins, which have to be there anyway, especially for laminar flow, take care of the torsional loads without additional weight.

    The 4" X 4" spar is irrelevant to the ultralight anyway. If we assume that the ultralight uses even one set of bracing wires, to halfway out on the wing, then, for a 32.5 foot wing, the maximum bending moment at 6g's will be less than 24,000 in-lbs, or about a seventh of that on the root chord on the S-2. If you use more wires, the bending load goes down more. (calculated per page 180).
    Quote Originally Posted by Bart View Post
    Perhaps you should familiarize yourself with various designs such as Rutan's Longeze, with its solid foam core wings and fuse panels, yet manages to fly, for decades, using fuel which would dissolve the foam if in contact, which it isn't. The Rutan method is essentially like making surfboards (fiberglass wrapped carved foam), from which the idea was copied by others, then by him, 35 years ago. Hot wiring is surely the most efficient way to get an airfoil in a garage workshop. The Rutan method does, however, sop up too much heavy, expensive, and perhaps brittle epoxy, and when you're done, you don't quite know what you have, structurally. Alu. is much more predictable, qualitatively, and best suited to flat wrap applications, but not compound curves, so makes good wing skin.

    As for spar thickness, consider that when you double the thickness between flanges, you get 8-fold increase in stiffness, and stiffness is mostly what you're after rather than strength, per se. Take the BD-5, which had problems, but the wing spar was not noted among them. It had a round tubular spar of ~3" diameter, carrying an aircraft perhaps twice as heavy as I envision. If a ~3" round spar (inefficient because being round has not enough metal far enough apart for the flanges, but too much metal in the webs) was structurally sufficient, then a 4" square tube would be much stiffer, with its flanges further apart and more metal in them. A 4" tubular spar of 1/8" wall thickness would be much more robust than the BD-5 spar, and would weigh ~2.2 lbs. per linear foot, or 30-40 lbs. for an aircraft of comparable size to the BD, a CriCri, DA-11, etc., but would also serve as the fuel tank, saving weight, expense, and time normally spent on that. If fuel were carried mid-section, the outer sections of web could have lightening holes cut, to save a bit of weight further out on the wing.

    Read Strojnik, all three volumes. He goes into considerable detail about various spar and other aircraft components, pros and cons of each, comparative weight, etc.. His best weight spar idea was ~1.7 lbs/linear foot using alu. angle pieces as flanges, epoxied to plywood web. Assuming his epoxy bonds were perfect, he'd still have to add more weight to resist torque, and yet more weight for the fuel tank, bringing it to significantly more than just using a square or rectangular extruded alu. spar in the first place. He did, however, use such tube for his tail boom on the S-2 motorglider, which is a feast of clever ideas which leave the rest of us wondering why we never thought of them. His Laminar Magic plane was a scaled-down S-2 and set records for speed vs. power in its weight class. Google for it.

    Anybody interested in light, small, efficient planes of excellent performance for limited power should consider how Strojnik, Columban, Davis, DeBreyer, and Winton et al did what they did.

    For efficient aerodynamics, read Bruce Carmichael and Sig. Hoerner.

  13. #28
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    Re: Composite Wing Spar for an Ultralight

    I've since read that the spar for that 40+ foot winged ultralight sailplane is actually only 3.5" deep at the root.

    Using glass on foam is expensive and heavy. Using carbon fiber in any "volume" is very expensive and I'd think just not worth it for an ultralight or light sport plane that I personally assume should be somewhat economical and practical.

    The wing above though and the use of carbon fiber in the spar caps however limits the use of the expensive material and places it on the spar(s) exactly where it will do the most good for increasing bending moment resistance and lowering deflection. I think that using carbon fiber for this can then make a lot of sense.

    It could be used to stiffen a wood or aluminum wing spars caps or in a completely composite spar. The rest of the sturcture could be conventional, more economical and easier to work with.

    This could be used to make some very efficient, and long cantilever winged powered gliders.

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    Re: Composite Wing Spar for an Ultralight

    Yes, you could make a long cantilever wing. Bu the guy who started the thread wanted to build something like the ultralight in the reference he gave. Wires everywhere. That's probably even lighter than a carbon fiber spar. Nothing like a spar that's several feet deep, which is what using wires or struts gives you.

    I think a carbon d-tube on a typical ultralight might cost $500 to $1000 in carbon cloth. In Finnish 1/32 ply from Aircraft Spruce, maybe more like $300-$350.

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    Re: Composite Wing Spar for an Ultralight

    For a really clever simple spar check out the MAG 01 ( French use a translator )
    Lots of other construction ideas and photos
    Uses pultruded carbon rods for caps that are bonded onto ply stiffeners
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