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Thread: Composite Wing Spar for an Ultralight

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    Registered User Sir Joab's Avatar
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    Composite Wing Spar for an Ultralight

    Hey guys,
    I'm working on a design for an ultralight somewhat similar to Airdrome Airplanes' 'Dream Classic'.

    I'm a collage student studying Electrical Engineering (2 years so far)... I say this to communicate two things:

    1. I have a limited budget

    2. I can do the math, but my knowledge of materials is limited. So please be patient.

    I'm trying to design this plane with modern material technologies to make it lighter and, if possible, cheaper. I'm starting this process with the wing spar. My current idea is to use a Polystyrene foam core ( 6" x 2" cross section) and laminate it with 1 layer of 12k carbon fiber, and vacuum infuse epoxy resin.

    What do you guys think? I'm open to any suggestions or comments. Would there be better materials/methods for this project?

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    Registered User Norman's Avatar
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    Re: Composite Wing Spar for an Ultralight

    It'll be more expensive than aluminum and unless the wing is less than 6" thick you won't see any weight savings
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    Re: Composite Wing Spar for an Ultralight

    Some construction pics for a pretty impressive spar here:
    Parker P1 High Altitude Airplane
    Note the use of Graphlite. Pultruded fibers will probably be much straighter than any hand layup, so properties should be much better.

    However, with all those wires, doesn't seem like the spar is going to be all that heavy anyway. Maybe there's a safe way to use kevlar instead of wire, but I don't know what it is.

    If the specs are right, 242 lbs. empty is pretty low already!

    I haven't done the math, but perhaps if you did a thick cantilever wing with carbon spar and a simple carbon tailboom, you might make things lighter. It would definitely get rid of some drag, though that might get into some problems with the part 103 rules.

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    Re: Composite Wing Spar for an Ultralight

    Forget the composite--to expensive, not saving much if any weight, questionable quality unless done by experts.

    Instead, use extruded aluminum tube, preferably rectangular cross section, as round is sub-optimal stiffness to weight. Known engineering qualities, pretty tough, easy to get, cheap, off the shelf, hollow so you could put fuel in it for dual use as fuel tank, and/or make holes in the sides (webbing) to lighten it.

    If you come up with a better idea, let me know.

    PS: The ultralight forum on this site mentioned use of alu ladders for wing and fuselage structure on Flying Flea ultralights. Interesting idea, perhaps useful to your application. Make your own ladder spars?
    Last edited by Bart; March 28th, 2010 at 12:02 PM.

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    Registered User Sir Joab's Avatar
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    Re: Composite Wing Spar for an Ultralight

    Well, you popped my bubble about the wing spar design... and I was so proud of it too...

    Ir27, this is a great link, thanks for sharing! I'm getting ideas already.

    Bart, thanks for expanding on your answer, you have an excellent argument there... there's a reason planes have been built this way for so many years. And as for the ladder spar idea, that sounds pretty good. I'm going to see what I can do with that.

    To anyone else who may have something to say, say it! I'm looking for any ideas I can get. I want this plane to be as light as possible with a reasonable cost. That's it. (And safe...)

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    Re: Composite Wing Spar for an Ultralight

    We don't know your goals for this project, so we don't know why you're doing your own design. If the specs are honest, the Dream Classic is already quite light. It reminds me a bit of the Flitplane, though I don't know if you can get that one anymore. Lots of other open structure ultralights out there, such as Legal Eagle XL. (Steel tube fuselages can be fairly light, apparently.)

    Another light ultralight, although it looks different, is the Sky Pup. If you're a small to medium sized guy, it might be an option for you. Plans are still sold. Info at Machnone.org. Foam cored wood construction. Not sure which is cheaper these days. Just for an idea why unobtainium in the spar won't save you all that much weight, the spar for the Pup, which is for a cantilever wing with a bit more span (I think) should be around 26 lbs for the caps and web. Or so I calculate, approximately, from the plans. I think that's around 1/3 of the total wing weight or perhaps just a bit more. (I don't know what the center section weighs, but the outer panels are reported to be in the low 20's.) With bracing wires, that spar would be lighter. I'd guess a lot lighter.

    I'm sure you could make things a LITTLE lighter with carbon. But I bet it wouldn't be cheaper.

    If lightness is absolute priority, you might consider a biplane. With a beam that deep the spars don't have to be all that heavy. I recall reading that the 4 wing panels on the Jungmeister weighed 25 lbs each. That's an aerobatic biplane grossing 1300 lbs with lots of horsepower. Draggy, though. Not a very good glide if the engine quits.

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    Re: Composite Wing Spar for an Ultralight

    Oh, Marske (who sells Graphlite) says that Graphlite spar caps are cheaper than aircraft spruce spars. I don't know if that still holds true if you learn to grade your own wood.

    An appropriate aluminum I-beam will be considerably lighter than a rectangular tube with constant wall thickness, though you'd have to get torsional stiffness from sheeting. Alternatively, perhaps a box spar with shallow c-section top and bottom with riveted shear webs in between. You'd want to put some stiffeners on the shear webs, though. Or maybe there's a way to use plywood for this without having to bond it to the metal. Would make a lighter shear web. But it gets complicated.

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    Re: Composite Wing Spar for an Ultralight

    I must admit, my main reason for making my own design rather than using a preexisting one is the challenge.

    The inspiration for this design is a competition for R/C airplanes that's basically 'weight lifting'. The plane has a maximum engine size and airframe weight, and they see how much they can carry.

    I'm trying to make an ultralight that can carry a payload (pilot included) of 300+ lbs (though I only weigh 170).

    Flying is a second love... Engineering is my first.

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    Re: Composite Wing Spar for an Ultralight

    Well, you could do worse than a smaller version of Dan Parson's plane. Just small enough to get safely under the 254 lb. limit. Expensive, though.

    Seems like another way to go, if you could get some really light covering, is a very low aspect ratio plane that weighed very little, with enough depth that the structure could be very light. Facetmobile weighed 370 empty with 720 or something gross. With smaller engine, slightly higher aspect ratio, and somewhat lighter cargo, perhaps it would work.

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    Re: Composite Wing Spar for an Ultralight

    Google for info on these planes:

    Davis DA-11, <~200 lbs., 18 hp B&S , B&S lawn mower engine

    Colomban CriCri and Colomban's latest, whose name I forget, <200 lbs., B&S lawn mower engine

    Winton Facet Opel, ~242 lbs. empty, time to climb records to 10, 20, 30K', STOL, ~2,500 mile range, 40 hp.

    Also, DeBreyer Pelican, tailless, 12 hp, light & cheap

    May as well benchmark off these guys, who already designed and flew aircraft that prove their theories. What they did, you could too.

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    Re: Composite Wing Spar for an Ultralight

    Those planes may be light, but they're not what people usually mean by ultralight because they land quite a bit faster, though I'm not ABSOLUTELY sure about the Facet Opal and the DeBreyer Pelican.

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    Re: Composite Wing Spar for an Ultralight

    Quote Originally Posted by lr27 View Post
    Those planes may be light, but they're not what people usually mean by ultralight because they land quite a bit faster, though I'm not ABSOLUTELY sure about the Facet Opal and the DeBreyer Pelican.
    For an ultralight provide a larger wing area and corresponding tail surfaces and moment arms. If that stays within ultralight weight limits it will then have the required low stall speed.

    A Moni motor glider is an ultralight as far as weight is concerned but the wing area is too small for the required minimum flying speed. If a larger, lighter wing could be used then it would technically be a true ultralight per part 103.

    A Heath midget racer, for that matter, is an ultralight as far as weight is concerned but the wing is far too small.

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    Re: Composite Wing Spar for an Ultralight

    Quote Originally Posted by lr27 View Post
    Oh, Marske (who sells Graphlite) says that Graphlite spar caps are cheaper than aircraft spruce spars. I don't know if that still holds true if you learn to grade your own wood.

    An appropriate aluminum I-beam will be considerably lighter than a rectangular tube with constant wall thickness, though you'd have to get torsional stiffness from sheeting. Alternatively, perhaps, a box spar with shallow c-section top and bottom with riveted shear webs in between. You'd want to put some stiffeners on the shear webs, though. Or maybe there's a way to use plywood for this without having to bond it to the metal. Would make a lighter shear web. But it gets complicated.
    Strojnik discussed all of this in his 3 books, which in my opinion is where everybody should start if they want to save time and wasted effort in the design process.

    Anyhow, a 4" square alu. tube of 1/8" wall thickness weighs ~2.2 lbs/linear foot. This would be a much more robust spar than, say, a BD-5 round tube spar of ~3" diameter. Not only is it thicker than the Bede, it's also got the median point of the flanges much further apart, making it vastly stiffer. Stiffness is what we're typically looking for. Anyway, Stojnik calculated various spar types, including I-beam with plywood web, as I recall about 1.7 lbs/linear foot, but a lot of work to make. A square spar, in addition to being a box beam with much better torsional strength, can be lightened significantly by making round holes in the sides (webbing), bringing it to about the same weight as the too-much-work-but-still-too-twisty I beam. Or, lighten the outer sections of the square tube spar near the wingtips, and use the middle section for a fuel tank, saving that weight. After all, it's at the thickest point of the airfoil, best possible place for variable weight as fuel is burned off.

    Bottom line: Square or rectangular alu,. tube spar is cheaper, stronger, stiffer, more predicable structurally, and probably lighter since it includes the fuel tank. And, it's ready to go pretty much when it comes off the delivery truck.

    Thoughts?

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    Re: Composite Wing Spar for an Ultralight

    You've got a good point there... I'm definitely leaning toward an aluminum spar now, though I'm still torn between a square channel and the ladder spar idea... The square channel appeals because of it's ease...

    Another part of the picture, which I failed to tell you guys (sorry!) is that I'd like to use 3/4" thick Polystyrene foam sheet for my wing ribs. It's very light weight, easy to cut to shape with a hotwire and very cheap. So I need to consider how to attach it to the spar (Epoxy to aluminum?), and how to limit the load that each rib will receive. (maybe just more of them...)

    Is this a bad idea too?

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    Re: Composite Wing Spar for an Ultralight

    Quote Originally Posted by Sir Joab View Post
    You've got a good point there... I'm definitely leaning toward an aluminum spar now, though I'm still torn between a square channel and the ladder spar idea... The square channel appeals because of it's ease...

    Another part of the picture, which I failed to tell you guys (sorry!) is that I'd like to use 3/4" thick Polystyrene foam sheet for my wing ribs. It's very light weight, easy to cut to shape with a hotwire and very cheap. So I need to consider how to attach it to the spar (Epoxy to aluminum?), and how to limit the load that each rib will receive. (maybe just more of them...)

    Is this a bad idea too?
    That foam may lack adequate strength, but Klegicell or whatever Columban used on the CriCri (prolly OK after 30 years empirical use) would be better, but it's more dense at 4-5 lbs/cu. ft. than insulation foam at ~2 lbs./cu. ft.

    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&#37; 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.

    How about ladder spar using two smaller alu. tubes, so you have a place to hinge ailerons? Could still put fuel in hollow tubes.

    Thoughts?

    And, with alu. tubes, data is already published on strength, manufactured per specs, so you don't have to worry about all the time and expense of the home made spar done in your garage, which may or may not be up to snuff. With extruded alu. tube, you could hang the sandbags on it as soon as the UPS guy delivers it, to test strength. No testy no usey.

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