Wing design for lightest possible wing

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by birdus, Jun 15, 2019.

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  1. Jun 15, 2019 #1

    birdus

    birdus

    birdus

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    If I wanted to build the lightest possible wing, how would it be constructed? For my sake, I'm thinking about a strut-braced wing, a la Luscombe 8 or Cessna 152.

    I'm thinking that the spars would be according to Jim Marske's designs: composite web or sandwich web, with the caps being made from carbon rods. I assume I'd want to use carbon fiber as the wrap or does it have qualities that would make fiberglass better? If carbon fiber's properties are okay, then obviously it would be lighter. I believe that if the spar won't be flexing much, then the carbon fiber would be okay.

    Then, the rest. I'm guessing that an old fashioned design (e.g., all-aluminum wing with few ribs, as on a later Luscombe) would not be the lightest. So, how about spruce ribs and covered in fabric? How about panels of carbon fiber sandwich? I think I would need ribs only at the ends of panels if I used carbon fiber sandwich panels.

    Other designs that would be lighter?

    From what I've read before (probably on here some), I believe that not overbuilding the wing might be one of the most important aspects to focus on.

    By the way, how about a fuel tank at the inboard section? I was thinking of just molding in a fiberglass or carbon fiber compartment.

    Thanks,
    Jay
     
    Last edited: Jun 16, 2019
  2. Jun 15, 2019 #2

    ypsilon

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    Lightest wing? Paraglider. From about 1kg for the whole wing, no spar needed.
    Obviously you are looking for something different, the problem is: We don't know what that is. It is thus quite hard to give any recommendations.

    In general: If you really want to go for the lightest solution, you won't be using wood anywhere in the wing,
     
  3. Jun 16, 2019 #3

    birdus

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    I clarified in the original post. The design would be for something like a Luscombe 8, or Cessna 152.
     
  4. Jun 16, 2019 #4

    BBerson

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    Lightest possible single strut would require fabric and a torsion structure. I think the lightest wing torsion structure is the diagonal rib structure such as used on the early Ercoupe and Taylor Coot. Double wire bracing also works but might take more time to build since ribs are needed anyway and serve double purpose.
     
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  5. Jun 16, 2019 #5

    birdus

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    What about with dual struts, like the Luscombes and J-3s use? I had wondered about that before. I assume they don't have a built-in structure to deal with torsion, and thus use dual struts. I'll look up the torsion structures you mentioned, as I don't think I'm familiar with them. (Looked them up. Seems like it would be tricky to get the airfoil right with the diagonal ribs.) The wings I've built for my Baby Great Lakes have compression tubes along with drag/anti-drag wires. Seems like you must be talking about something else, as I don't believe those would deal all that well with torsion, but merely with keeping the wing square in the face of the oncoming wind.

    It seems clear (if I'm understanding the mechanics involved) that the wings, themselves, would be lighter if I used dual struts, as that would eliminate the need for any internal structure to deal with torsion. Overall weight could be more, though.

    Any idea how close carbon fiber sandwich panels would be in lieu of a dozen ribs + fabric? Seems like it could be quite competitive, although I could be wrong. Spruce truss ribs are very light.
     
  6. Jun 16, 2019 #6

    BBerson

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    Ercoupe wing:
     

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  7. Jun 16, 2019 #7

    bifft

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    No trickier than with regular ribs. Just stretch the airfoil in the cord wise direction and not in the vertical direction.
     
  8. Jun 16, 2019 #8

    pwood66889

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    Birdus seems to have a high-wing design in mind. Leave us not confuse with low-winged ones.
    And is is not "early Ercoupe" it is all of them. Like the one I work on betimes.
     
  9. Jun 16, 2019 #9

    Himat

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    If you go for a carbon composite spar, my guess is that a cantilever wing is lighter. That have been discussed here before. With composite the strut atachment points get that heavy that no weight loss is achieved with that kind of design.
     
  10. Jun 17, 2019 #10

    proppastie

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    Thick...Carbon Dragon spar 12" thk. 64" chord at root.....18.75% thick. Spreading out the caps, they can be not as thick, and save weight. Empty weight of the original Carbon Dragon 44' wingspan UL glider was 147 lb.....It had a D nose, and fabric covered cantilevered. Plans on the Irish site in my signature block.
     
  11. Jun 17, 2019 #11

    wsimpso1

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    The lightest possible wing? A search of the design space is how this sort of thing is usually done, but we can shortcut the process somewhat. First thing to know is that this is hardly a simple task. It may be possible to build a wing that is light but is lacking in robustness to the slings and arrows of normal airplane operation. It may be that one scheme is lightest for one airplane but another is lightest for another. And a number of combinations are simply impractical, which would seem to drive you to follow conventional construction schemes, at least for the first trip through examining the options. I do wonder why you chose as your examples slow airplanes with metal skinned and structure wings, and single struts bracing each wing. Why not a cantilever wing? It may change which one is lightest under any given set of parameters.

    Let's get into the task at hand. You should probably define the wing you are interested in more fully. You can always shave weight by making the wing thicker. You can shave weight by going with lighter skinning, which will remove weight but can also increase drag, which is undesireable in some airplane, but may be an acceptable tradeoff in other airplanes. Define your design g's, FOS, wing loading, span loading, number and angle of struts, max drag at a target airspeed, max airspeed, and skin abuse allowed. With these defined, you can start designing wings to that task.

    A few things to know:

    Wing skins of most airplanes are designed to stand handling of the wing during construction and in use. The worst offenders could be anything from a starling strike to an airshow moron trying use your wing as a writing desk or a place to set a squalling, kicking toddler. In small airplanes, our wing skins are often sized not so much by flight loads as by foreseeable abuse on the ground. If the airplane is to be a human powered airplane with an effective life of hours, this may be an insignificant design issue, but if you are to take it to fly-ins and not post guards... Then if the airplane is faster, skin stiffness plays into keeping flutter at bay. Make the skin lighter and you may have to add more ribs, with it all being a tradeoff between skin thickness and number of ribs.

    Wing spars are not schemed out absent the wing skins. Spars and wing skins combine to supply stiffness and strength needed in bending and in torsion. So you will have trades to consider between skins and spars. Where rib structures are arranged in triangular patterns, structural skins can be replaced with fabric covering, as in the Ercoupe.

    There are many schemes. Cored skins faced with carbon-epoxy with carbon-epoxy spars and a few ribs designed to min strength and flutter resistance must be included, and may indeed be the low drag wing as well. But if you are scheming out a low speed airplane where drag is of less importance, you might do well to look at fabric covered wings made of wood, sheet metal, and yes, carbon-epoxy. You might even look into open structure wings with or without an enclosed D-tube. The rib structure aft of the main spar could be conventional or with ribs in a triangular scheme to replace more conventional internal wire bracing and compression ribs.

    On top of basic structures, there is much room for weight to be included or not within any one scheme. Fabric covering can be filled and painted to pretty terrific levels of finish and be heavy, or they can be kept light by applying only enough "silver" to protect the fabric and adhesives from UV and stopping there. Composite airplanes can be faired and finished to perfection for long laminar runs and very low drag, or they can basic finish plus light paint if low weight is more important than low drag.

    If you use thick cored skins, you can largely leave out ribs in composite wings. The trade is more than just skin weight vs rib weight. Internal ribs weigh something, but so do the attaching tapes, the transferred flanges and layer of adhesive added to bond the ribs between skins.

    One other possibility is going massive core. Yeah, that is a lot of foam, but you can skip the composite facings on the inner surfaces of the cored skin, the ribs, the tapes, transferred flanges, and adhesives connecting the ribs to the skins. below 5' chord, you can almost always build a lighter wing with a solid core than with a "hollow" wing...

    So I am telling you that to answer your question, you have to further define your needs, design wings in each material set and scheme within the materials, then compare them all. Do us a favor, report back on the parameters you choose and the results weight wise.

    Now as to your proposed scheme, the lightest carbon spar will be laid down of uni tape pre-preg in a dedicated composite mold, vacuum bagged in, and then autoclave cured. We homebuilders use Graphlite rods in simple fixtures because that is the best we can do without the dedicated molds and autoclaves. Not the lightest, but pretty good. A graphite cloth shear web on the bias is probably about the best you can do in a homebuilder's shop, but Uni pre-pregs for webs, autoclave cured, will be lighter at same strength.

    Putting an aluminum rib structure around a graphite spar is bad juju for a couple serious reasons. The first issue is that aluminum in contact with graphite fiber will have galvanic corrosion and come apart fairly quickly. Then there is the issue of greatly different load vs deflection capabilities when both are stressed to about the same fraction of their strengths. To make them compatible in load-deflection terms is to add weight.

    There are really good reasons why structures tend to be in sets. I have seen:

    All aluminum;
    Aluminum spars, D-tube, ribs and fabric covering;
    All wood;
    Wooden spars, D-tube, ribs and fabric covering;
    All composite.

    I suppose that Composite spars, D-tube, ribs and fabric covering has been tried, but I do not know of any. While successful airplanes are made in each of the schemes, we rarely see bits shared between the schemes within the same wing because these schemes work together well and other mixes tend not to...

    Billski
     
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  12. Jun 17, 2019 #12

    proppastie

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    Phil Lardner in Ireland has done that with his "all Carbon Fiber Carbon Dragon"......He did a "black wood" design.....changed the wood to CF. Link to the Carbon Dragon site where his posts are in my signature block.
     
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  13. Jun 19, 2019 #13

    birdus

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    So, to clarify, I'm thinking of how one might redesign wings on a Luscombe for weight savings. Over the metal wings, I imagine it wouldn't be too tough to save 100 pounds (I heard someone say before that the fabric wings are 70 pounds lighter than the metal ones). On a plane that weighs around 870 pounds empty, that's significant. As I said, I'm thinking I would use Jim Marske's spar construction technique.

    Also, I was thinking making the leading edges from vacuum bagged carbon fiber would be neat. Lots of work to make a form, but bending the leading edges on my Baby Great Lakes wings was quite difficult. I don't think the experience would be any worse than that. I guess the D-section that would create adds lots of rigidity (not sure of other benefits). My thought was to epoxy that leading edge to the main spar.

    An earlier post said fabric is the lightest. Again, because of my experience building my Baby Great Lakes wings, spruce ribs are what occurred to me. To my inexperienced finger, they're quite light. Again, all I know is the aluminum compression tubes and the drag/anti-drag wires. Those components also seem quite light. That, and my familiarity, made me think of going that direction.

    Also, I assume I would just use the dual struts from a fabric Luscombe for torsional reasons.

    For airfoil, I was thinking of using one of Harry Riblett's GA airfoils, specifically, the GA 30-613.5.
     
  14. Jun 19, 2019 #14

    wsimpso1

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    Hmm, I just did a back of an envelope (literally) estimate of wing skin weight using BoKu's sailplane sandwich of 6 oz graphite fiber on both sides of 1/4" 8 pcf foam. Skins are about 60 pounds. You won't need many ribs, but they will be about 0.35 psf. Flaps and ailerons will be about 0.8 psf and you will have to do your spar per Marske. Then there will hard points for mounts, the strut, aileron and flap hinges and controls. I do suspect that if you want to maintain the real Vne of the Luscomb (they have been flown to some impressively high speeds), your lightest wing will be graphite- epoxy, but you get to decide...

    Structures are all very doable...

    Billski
     
    Last edited: Jun 19, 2019
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  15. Jun 19, 2019 #15

    blane.c

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    Here is a picture Fritz posted on one of the VP discussions of a lightened spar. VP LIGHTENED WING 1.jpg VP LIGHTENED WING 2.jpg VP LIGHTENED WING 9.jpg
    I mention it because the VP airfoil NACA 44xx is similar to the GA and similar work with the spars seems doable.
     
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  16. Jun 19, 2019 #16

    Victor Bravo

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    If you are willing to do fabic covering on a Luscombe wing, then making it out of simple solid core foam and skins may be less of a penalty than you think.

    The reason is that the only solid foam core is going to be the D-tube and the ribs, but half of the wing area is going to be eliminated between the ribs beind the spar. So although there is a measurable penalty for solid core Rutan style wings (compared to hollow molded modern stuff), that penalty is cut in half because half of the (Rutan style Long-EZ full core) wing just isn't there.

    So you would make a Marske style C channel spar using graphlite rods and bagged carbon cloth. As Billski said, it's probably about as good as you are gonna get in a garage.

    Then you'd hot wire the leading edge D-tube to fit the C channel spar. Glue the foam D-tube to the spar, and then vacuum bag the entire D-tube skins and spar together with 45 degree carbon skins for torsional stiffness.

    Then glue and glass the main rib sections to the back of the spar, using short cured carbon strips to tie the rib caps to the D-tube.

    Then glue on a Marske style C-channel rear spar which will carry the control hinges. Then add the small triangular trailing edge rib sections inboard of the ailerons.

    Cover the whole mess with Stewart Systems fabric (water based, to not attack the foam), and you probably have about as light of a wing as you can get without an aerospace company or a university behind you.
     
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  17. Jun 19, 2019 #17

    wsimpso1

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    VB points out that a massive core might have value here. A couple points:

    If OP's Luscomb wing will contain fuel, the composite assembly containing fuel must be devoid of hotwired foam - gasoline WILL diffuse throughout the wing structure and the polystyrene foam WILL become a tiny little puddle of polystyrene at the low spot of each space that used to contain foam, leaving the skins unsupported. I have seen this in a post-crash teardown... Nope, a wet wing or even a wing wrapped around an independent tank can not be based on hotwired blue foam cores. Foam in these assemblies have to be immune to fuel. PVC is great, and PVC (Divinycel) is part of BoKu's very light and sturdy skins. Oh, and polystyrene is the only foam we know about that is both capable of hotwiring and won't poison you if you try. Polyurethane will poison you and your shop quickly if you hotwire it. Everything else we know about does not hotwire nicely.

    Alternatively, if the fuel tanks are in other assemblies, say a header tank like a Cub or Tailwind or in root strakes like a Long EZ, a massive core wing hotwired from blue styrene flotation billet can be substantially lighter than a hollow wing. We have discussed this in a bunch of threads. Short story is that a wing built on a hotwired massive core (Rutan style) is lighter than a wing with cored skins up to about a 6 ft chord. How can that be? Deleting all that 2 pcf foam requires stuff be added: Cored skins must have stouter foam, about as much facing on the outside as the massive cored wing, then add that much facing again on the inside of the core, a few ribs, bondlines on all of the faying areas between spars, ribs, etc to the skins, joints between skin pieces, doublers and covers over a number of access panels, etc. Below about 6 ft of chord, the foam weighs less than all that added stuff to make the wing hollow. So, if you are not putting fuel in the wing, you can make it lighter and make it more quickly with a solid core...

    Assemblies that do not contain fuel, like control surfaces, can easily be hotwired from blue styrene flotation billet, have skins bagged on them, faired and finished without worry over fuel. That is the light way to make control surfaces of any given skin facings. I have made the control surfaces and stabilizers for my bird this way. Quick build and light...

    So, do your homework on what your wings and other structures are doing, then size and iterate your structures up to strength, then do the weight estimates on the all-up weights of each of your options before making a decision. Massive cores may be a good choice on your big parts.

    Discussion on these topics are all available by using the advanced search tool here on HBA.com.

    Billski
     
    Last edited: Jun 20, 2019
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  18. Jun 19, 2019 #18

    wsimpso1

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    I have written about this before - the lightest solutions for any airplane varies with the mission:

    Slow airplanes are generally lightest in rag and tube;
    Fast airplanes are generally lightest in carbon fiber, followed by sheet metal, then fiberglass;
    Fast airplanes are generally faster in composites;

    This is all because of a few features:

    Slow airplanes can get away with fewer and lighter internal structures, and can be fine with non-structural skins (light fabric). Cubs and Taylorcraft and Steve Witman's Buttercup.

    Go faster and the skins need more support while the whole wing needs more stiffness. In fabric covering that means more ribs or more structural skins or heavier cloth or more diagonal bracing and compression struts or all of the above. Steve's Tailwinds. So you are then in range for metal or composite skins. In sheet metal airplanes, the faster it is the more ribs and thicker the skins...

    Try as you might, a composite airplane is generally going to have more laminar flow on the foils and be better faired everywhere than a sheet metal of fabric covered bird, thus lower drag and faster on given power.

    Composites need substantial skins to avoid damage during build and ground handling. These skins are OK for weight in the 150 knot and higher range, but are heavy for the 90 knot birds. And even faster airplanes begin to need more substantial skins, then carbon shines!

    Billski
     
    Last edited: Jun 20, 2019
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  19. Jun 19, 2019 #19

    proppastie

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    great general design rules......I guess the vapors from fuel in root strakes has not effected the Styrofoam?
     
  20. Jun 19, 2019 #20

    Victor Bravo

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    On a Luscombe type airplane there would be separate metal or plastic fuel tanks to prevent that damage. As long as they are not leaking it should not be a concern. No reason whatsoever to have permanent or integral tanks in this type of wing.
     

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