Hurricane Mk103

Discussion in 'The light stuff area' started by pictsidhe, Aug 6, 2018.

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  1. Sep 18, 2018 #121

    BBerson

    BBerson

    BBerson

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    The core would prevent skin buckling and that is nice. But does the core cancel the low e to weight ratio? Balsa core plywood might have superior modulus of e to weight ratio.

    The Driggs DJ-1 wing was skinned with 3/64" plywood.
    I would guess an ultralight might use thinner skin, perhaps 2/64" (1/32").
    Aircraft Spruce has 1/32" birch ply for about $2/sq.ft.
    I don't know what it weighs.
    Might be 3.5 pounds per 4'x8' sheet or .1 pounds per square foot.
     
    Last edited: Sep 18, 2018
  2. Sep 18, 2018 #122

    BBerson

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  3. Sep 20, 2018 #123

    pictsidhe

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    Been putting a spreadsheet together for loadings. Yawn. I can't do much structural design till I know all the loads.

    I also had a coro bending experiment running. My first go at tight radius bends. I filled the flutes with salt. I bent it with an outer 'mould' of Al flashing till the fluting failed. I managed about 3/4" radius at that point. It's extra rough on the inside as I initially set the oven too hot and it partially melted. That also meant it was stuck to the flashing, which I didn't discover till I was bending it. Might try a flashing sandwich next time as well as not letting it stick...

    20180920_035844.jpg

    20180920_035911.jpg
     
  4. Sep 20, 2018 #124

    lr27

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    You're comparing apples and Ferraris. There's no need to go to carbon to outdo PP. And balsa is generally stiffer for the areal weight than carbon or almost anything else without a core. OTOH, I'm sure foam core board is quite a bit stiffer for the areal weight than Coroplast. Skinning 1/8" or 1/4" of blue foam with 1/64" ply would far exceed the properties of Coroplast. Even skinning with light fiberglass would. Plus glue actually sticks to these materials.

    Running a few quick numbers, I expect birch plywood to be comparable to the same weight of coroplast in bending stiffness, but completely superior in shear. That's a very rough estimate, but I'd be flabbergasted if birch ply was much worse. Any lighter plywood should be much better in bending, too. Coroplast is actually inferior in bending stiffness to the same THICKNESS of balsa in one direction, anyway. Two if you make balsa plywood. I guess I should just shut up, though, and let you do the test. I don't have any fresh Coroplast, just some that's been weathering outside.

    It's too bad balsa isn't more consistent and cheaper. We could probably just make our aircraft out of balsa, except for the hard points.

    Some further noodling around reveals that blue foam would be much stiffer in bending than the same weight of Coroplast, though I think it might not be better in shear. Maybe just cover your plane with thick blue foam sheet and lightly glass the outside. Highload 40 or 60 might be best.
     
  5. Sep 20, 2018 #125

    lr27

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    BBerson:
    I've seen at least one ultralight design with a 1/32" ply sheeted d-tubes. The VJ-23 rigid wing hang glider used 1/32" ply for the shear web! Also 1/32 "popular ply" (poplar ply?) for the wing d-tube sheeting. The Woodhopper plans seem to show 1" styrofoam for sheeting on the first few inches of the wing.
     
  6. Sep 20, 2018 #126

    pictsidhe

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    I'm not building a Ferrari. I don't need ultimate strength to weight. I don't need a carbon, glass or ply skinned structure. Or a composite sandwich. I want something light with sufficient stiffness and abusability that isn't too hard to build or cost a small fortune in materials and/or tooling. Or that dissolves if you spill fuel on it. Haven't a few Rutans crashed due to unseen fuel damage?
    Skinned hiload was one option I wasn't overly keen on before I hit upon coro. To make it suitably light, it needs very thin skins, which will be very prone to hangar rash.

    coroplast is lighter than the same thickness of balsa. Balsa is expensive and a lot more fragile. How heavy is balsa once you have sealed both sides? I looked at balasa cores too. Expensive, heavy due its sponge like nature.

    Oh solid carbon doesn't out-stiff the same areal weight of Coro. I guess the apples win there.

    I'm going back to my design calculations now. Feel free to carry on telling me why you think that my numbers are all nonsense.
     
  7. Sep 20, 2018 #127

    BBerson

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    I have been looking at clear Mylar for D-tube wing skin. Mostly because it is clear and see through for drilling rivet holes.
    It comes in .002", .005", .007",.010" and .014", I think.
    Seems Mylar (polyester) might be the stiffest plastic?
     
  8. Sep 20, 2018 #128

    radfordc

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    I wonder if Steven K. Wood knew that? https://en.wikipedia.org/wiki/Wood_Sky_Pup
     
  9. Sep 21, 2018 #129

    lr27

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    My reference to the Ferrari was that you were talking about carbon when there are many materials which are much better than Coroplast and cheaper than carbon.

    You gave a modulus of 180,000 for the polypropylene. Even if you used a solid sheet of the stuff instead of the Coroplast, that's floppier, at least in one direction, than 6 lb balsa. But it weighs more than 11 lb balsa does. (The figure I saw was 0.15 lb/square foot for 4 mm Coroplast.) And the balsa has that modulus all the way through. There are ways to work with balsa without turning it into ballast, otherwise we'd never make model airplanes out of it. I suppose if you have a termite problem in your hangar, and you don't fly often... In any case, I know balsa is expensive and I wasn't necessarily suggesting it.

    If you compare solid polypropylene to solid carbon, it's about 1 percent as stiff as carbon in the good direction. So I wouldn't say the "apples" win. If you compare a similar configuration of carbon to Coroplast, it will probably also be, say, 50 or 100 times as stiff for the weight. Solid anything, almost, is inferior to cored construction except maybe in shear if the surface isn't so thin it buckles.

    I just checked Boulter Plywood's site. Okuome plywood is maybe 150 percent more expensive than Coroplast, but it's lighter and the physical properties would be far superior.

    If you're worried about fuel damage, you could always use urethane foam or something instead of polystyrene foam.

    Coroplast is lighter than the same thickness of moderately heavy balsa wood, but I've seen balsa nearly as heavy as spruce, and also usable balsa that weighs as little as 3.5 lbs/ft3.

    If you could actually make a part 103 compliant ultralight using a lot of Coroplast, you could make a much lighter one with better materials. With the better materials, you'd get the same performance with less power, requiring a lighter, and perhaps less expensive engine.

    I suppose I should probably drop this. You'll probably figure it out as soon as you start testing materials or parts, but then you'll have to redo the design, unless you test now. So I suggest, if you don't believe my numbers, do some fair and careful tests.
     
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  10. Sep 21, 2018 #130

    lr27

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    There's a fair amount of wood in the Sky Pup too. The foam is mostly used as a core material. I think that's a better approach, but there are a lot of ways to do better than using polypropylene as s structural material.
     
  11. Sep 26, 2018 #131

    pictsidhe

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    A monocoque 2mm coro H.S. with no internal structure is strong enough in torsion and spanwise bending stress for double figure g, but the bending shear combined with torsional shear will fail the LE at 3.8g with the cp at the LE. That's only 2.5g + safety. Torsional shear is also fine at lots of g so the thin skin can stay. I'll add some truss shear webs once I work out where they will also be most useful for chordwise loading. By truss webs, a cross section of the profile will be a truss. The shear webs will reduce the amount of ribs I need so the weight penalty isn't as large as it first seems. XPS is a possible web material if ti looks better than coro.

    I'm a bit rusty on the finer details of structural analysis so I'm spending a fair bit of time working out how to do it again. Hence the slow progress. I was dubious about being able to do the H.S. in coro, but it is looking workable. I don't know how unfavourably the weight will compare to stick and fabric yet. The stress numbers look fine for the elevator. That's good as rigid skinned control surfaces work a whole lot better than fabric ones, thouhg mostly only an issue for ailerons. I'm getting an urge to work out the stresses of the Minimax H.S. and scale that design to a Hurricane H.S. so I can see how Coro compares on paper.
    If nothing else, I'll have learned how to design a monocoque lifting surface.

    I intend to test some coro parts such as a H.S. to failure to learn how it fails and if practice approximates my theory. Should the numbers tell me it will work.
    This is experimental construction!

    Now, would Airfix let me rip off their box design for selling 2/3 scale plastic aeroplane kits?
     
  12. Sep 27, 2018 #132

    lr27

    lr27

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    Have you also looked at stiffness and buckling?

    That's going to be a very big box if you package the parts the same way Airfix does. ;-)
     
  13. Sep 27, 2018 #133

    pictsidhe

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    I haven't looked at buckling yet as I was expecting to need shear webs. Stiffness is OK.

    I had a quick look at the Minimax plans earlier and it's H.S. is unsuitable for a Hurricane shape. But the area is very similar so I can still do a weight and strength comparison. The Legal eagle empennage construction looks a better starting point for a conventional construction.

    Yes, a very big box. Since I'll likely be restricted to 8x4 coro sheets, an 9x5x3-ish box may do it. Something that will slot into the back of a fullsize PU even if it did hang out the back a bit seems fine. A giant Airfix box would look very, very cool. especially If I had a stack of them at flyins. They wouldn't be too heavy. 160lbs maybe with everything but the engine and paint. One person per corner would be easy. One person per end would work, too. Failing that, a crudely printed 'Acme Plastic Fighter Aeroplane' stamp on the boxes....
     
  14. Sep 27, 2018 #134

    lr27

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    You could make it easier for people by selling the correct color paint in giant, Testor's style jars. Or, I guess, since it's a British airplane, Humbrol. If you used two boxes, 9X5X1.5, they'd look more like model boxes. ;-)
     
  15. Sep 27, 2018 #135

    pictsidhe

    pictsidhe

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    I'll be testing exterior latex as it is very available, cost effective and I think will work. It'd be daft to mail it. Roundel paint doesn't need much, so a set might be good in pints. Humbrol didn't have accurate matches when I was using it. It was sorta-close-ish.
     
  16. Oct 2, 2018 #136

    blane.c

    blane.c

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    I wonder if you could "bubble" the canopy and canopy frame, I don't know about you but my fat head is going to get wedged into a scaled down version of that.
     
  17. Oct 2, 2018 #137

    blane.c

    blane.c

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    Browning 12 guage auto's solenoid fired.
     
  18. Oct 2, 2018 #138

    blane.c

    blane.c

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    Well you know this but, at slow speeds aileron effectiveness falls way off and control to turn and or pick up a wing falls upon the rudder.
     
  19. Oct 2, 2018 #139

    blane.c

    blane.c

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    A six cylinder two stroke at six thousand RPM could have the same staccato as a 12 cylinder four stroke at three thousand RPM? With the right tubes.
     
  20. Oct 2, 2018 #140

    Tiger Tim

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    How goes the battle with Coroplast? Learn anything new about working with the stuff lately?
     

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