Composite acft--why so heavy?

Discussion in 'Composites' started by Vigilant1, Oct 28, 2011.

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  1. Oct 28, 2011 #1

    Vigilant1

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    I'm wondering if it's practical to design and build a small composite aircraft that's as light as a metal one. It would seem possible given the very good strength to weight properties of foam sandwich construction, but I'm not seeing it in the weights of aircraft now being designed and built. Example: A Sonex has an empty weight of about 650 lbs with the VW engine. A composite Personal Cruiser weighs 650-750 lbs empty. The heavier cruisers do tend to have a heavier engine (Corvair) that accounts for about 50 lbs of the difference, but still . . . . The Sonex is longer, seats 2 instead of just one, and has a wing with 98 sq feet of area, vs just 76 sq feet for the Cruiser. The Sonex is bigger in every respect, yet weighs less. To build a Personal Cruiser with the Sonex's wider fuselage, extra seat and controls, and 20 more sq feet of wing and required additional tail volume--the empty weight would be at least 75-100 lbs more than the Sonex. That's a lot.

    The folks who market the Vision have suggested that aircraft could be built to LSA specs (meeting stall speed by adding LE slots, building with with graphite spar caps for reduced weight, etc), but the slots are a big aerodynamic price to pay when the Sonex achieves the same result simply by using more wing area (and possibly a more appropriate airfoil).

    Are the aircraft designs I've cited outliers, or are they good representative cases of glass and metal construction? Would it be practical to design and build a "plastic Sonex? of comparable size, weight, design load factor, and performance?
     
    Last edited: Oct 28, 2011
  2. Oct 28, 2011 #2

    Jan Carlsson

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    The PC have a longer wing, probably thinner wing in percent, sure it is thinner in inch, that make it heavier, the glass fuselage is heavier then metal. but smoother.
    it also have a higher MTOW
    I haven't seen the acual weight of the prototype...


    Jan
     
  3. Oct 28, 2011 #3

    Himat

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    The lightes, stiffest and strongest materials do not allways make the lightest and strongest structure.
    With tiny dimensions it can be awkward to get the load path's reasonable and you end up with adding more volume and weight.
    In some cases finess ratio is as important as strenght of the material, think buckling of a tube. A tube with thicker walls in a "weaker" material might have a higher buckling load. If the "weaker" material is less dense, the tube might end up lighter.
     
  4. Oct 28, 2011 #4

    autoreply

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    Absolutely. Specific weight (lbsf per lbs mass) they are superior and if properly designed, it should be possibly to design considerably lighter. But some prejudices just stick and for example "rag and tube" or fabric composite aircraft are very rare, even though they have been built satisfactory. For the design you mention, the Long-EZ for example is slightly lighter (though usually has a much heavier engine), while considerably faster.


    Finding true, apples-to-apples comparisons is often very hard though, because different materials appeal to different types of pilots and flying. The strong feelings about a material from many people don't help either.

    If you for example compare a metal aircraft with a composite one, we find it fully acceptable that we can dent/deform the metal leading edges. In composites we find that completely unacceptable, even if there wouldn't be any structural problem. Those different expectations make a 1:1 comparison virtually impossible, but the fact that even a homebuilt (inferior materials strength and manufacturing) composite can compete (weightwise) with metal tells me a lot.
     
  5. Oct 28, 2011 #5

    Hot Wings

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    ,I think a lot of the weight problem some of the composite planes have comes from what Orion has pointed out several times - Metal thinking. That combined with what has also been recently pointed out in another thread, that there is such a large range of strengths for composts depending on environmental factors, leads to having to build with larger margins than metal. That removes some of the perceived advantages.

    Add a further margin for less than ideal home builder techniques and methods and it's a wonder they are as light as they are.

    But I agree with Autoreply that a lighter plane can be designed and built with composites. If the industry could find a way for the average home builder to build to the same standards that can be used in a factory setting with all of the variables controlled to the same degree of perfection, then I think you would see a steady and profound shift toward composite planes.

    Rutan and his disciples back, when he was actively engaged in the homebuilder market, did a very good job of getting the the needed information and training disseminated to the field so that light planes could be built quickly and efficiently. In the years since much of this "old school" practical approach to building has been lost and replaced by the common misconception that composite planes are time consuming and to finish and come out heavier than specified.
     
    Last edited: Oct 28, 2011
  6. Oct 28, 2011 #6

    orion

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    There is a substantial difference between academic design and real world application. If we take a simpler, academic look at airplane structures then we can make a case for being able to build stronger and lighter airplanes however, as many have discovered, the real world practice of actually doing so is much more fraught with weight penalties than what the simplified examination might reveal. If we look at modern construction materials, it is really hard to beat, weight wise, aluminum construction. Yes, tube and fabric is probably lighter still but for this discussion I am not considering this as equivalent just simply due to surface quality and geometric stability.

    Metal thinking is one such cause of heavier composite aircraft however it's not the only one. First off, virtually all components of an airplane have to be designed with stiffness in mind, which at 1/5 the stiffness of aluminum (for woven fabric) and only about 20% less weight, pretty much takes fiberglass out of the running. Additionally, given that fiberglass is also subject to issues of fatigue, material choices, safety factors and structural arrangements will lead to assembly weights that will easily surpass any equivalent structure made from aluminum.

    This then leaves only graphite and yes, if we look at light airplane construction from a generalized viewpoint, there could be theoretical weight savings as a result of part molding and building a more optimal structure, one that is specifically designed for the material and minimizes part count (joints and attachments tend to be significant sources of weight buildup). But even the most optimal composite structure can still be subject to weight penalties - we only need to take a look at the ICON program where a continued weight savings effort has kept the aircraft from achieving production status.

    Although we've never taken a look at the trades in great detail, several cursory examinations have consistently showed that for small aircraft, the aluminum reinforced monocoque is still the most efficient structural arrangement. But with the benefits of molded components, I do feel that composites, even heavier ones, are a more optimal material choice for our industry.

    I think that in several programs way too much emphasis is put on weight, sometimes to the detriment of safety. Within limits, for a given material choice, the aircraft will weight what it will weigh. Departing from established and proven design practices just to save a pound or two here or there is not wise and may bite the designer down the line.
     
  7. Oct 28, 2011 #7

    BBerson

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    There was study comparing composite and aluminum in Sport Aviation, as orion said, the aluminum was lightest.
    I think the sandwich requires a heavier minimum weight that is based mostly on ground handling and is heavier than one sheet of metal.
    If a single sheet of composite could be made that was stiff enough and dent tolerant like aluminum, that would be ideal, I think.
    BB
     
  8. Oct 28, 2011 #8

    Vigilant1

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    I'd been hoping to use solid-core construction for the wing panels (to save construction time), but that might also cause a bigger metal/composite weight gap. Billski pointed out in another thread that solid core wings can be lighter than built-up composite wings, but you have to do the math. If the wing is thick, the solid wing may be heavier. In these small LSA designs that require a low "clean" stall speed, the wing is often thick and, due to the included wing volume, a built-up wing (of composite or metal) may be lighter.

    I like the idea of building in composites, but the real world is dragging me to aluminum.
     
  9. Oct 28, 2011 #9

    Topaz

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    Well, note the fine print on what Billski is saying. Solid-core is generally going to be lighter for the average homebuilder than some of the usual "gimmicks" we often see here to "lighten" a solid-core structure - cutting out holes in the foam, non-optimized cores, etc. A fully-optimized composite sandwich structure is almost certainly going to be lighter than the equivalent solid-core structure, even in the real world, but such optimization is more the purview of the big aerospace majors. Not something we generally have the experience or toolset to replicate in our world.

    As BBerson mentions, Sport Aviation published a report some years ago comparing sample wing sections designed to the same strength criteria, in various construction materials and methods. Not surprisingly, optimized metal skin-stringer (traditional high-end aerospace stuff) was the lightest. Optimized composite sandwich was next, followed by conventional (homebuilder, RV-4 style) metal construction. Solid-foam composite was the heaviest. To some extent that article is quite dated, in that we have better analysis and optimization tools now, even at the homebuilder level. But IMHO the general trends still hold, since those tools are equally applicable to metal as composites.

    But the key here is that the article also points out the relative amounts of work to build each type of structure. Fully-optimized structures, in either metal or composite, are very time-consuming (and expensive) to design and build. Solid-core composite may be the heaviest, but it's also by far the quickest and easiest to build in, if you pay any attention at all to the right techniques. Conventional metal structures aren't much harder, but they do require more expensive specialized tooling (for bucking rivets, cutting and bending sheet, etc.).

    As always, the most important thing is this: Which material are you most comfortable working in? You're going to spend hundreds, if not thousands, of hours working with this stuff. You'd better like what you pick. Shaving a few pounds of structural weight is worthless if you never finish the airplane. Designs in our world are rarely so edge-of-the-envelope that you don't have wide latitude in choice of building materials.
     
  10. Oct 28, 2011 #10

    Othman

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    This is turning into an interesting discussion.

    Like many of the forum members I've gone through several "paper" airplane designs, some metal, some wood, some composite etc.. Each construction material/method has its pros and cons, which are discussed extensively over numerous other posts. What I found is that I was never quite satisfied with the end results mainly because of the compromises that were necessary for the selected material/construction method. My latest project is a hybrid of materials and constructions techniques, and so far I am happy with where it is going. Rather than selecting one construction type and applying it to the entire airframe, I am trying to utilize the "appropriate" type to achieve the structure I want and the overall look I want. One can argue that the "con" of the hybrid approach is that you need to have a building with a wider skills set and possibly a wider range of tools.

    This isn't a novel idea, as it can be seen on many airplanes. The T-6 (or Harvard) is a good example. The forward fuselage is welded tube with removable aluminum skin panels, and the aft fuselage and wings are semi-monocoque. The University of Toronto Ornithoper I worked on a bit back in university was a real hybrid. The center part of the fuselage (thorax) which housed the engine and transmission is welded steel tube, the forward and aft fuselage are riveted aluminum tube, the wings have a kevlar D tube spar and foam ribs with spruce caps. Some sections of the wing are sheeted in plywood. The tail surfaces are traditional wood construction... Project Ornithopter

    Putting engineering considerations aside (as important as they are), there always the "artistic" design aspect. What look are you going for? Do you want something like a cub replica or do you want something sleek and fast looking. I like to think an airplane design is expression of ones personality (but what you end up with is probably an expression of how deep your pockets are). That being said, if you were able to design the ultimate STOL super-rugged super-utility back country bush plane that looked something like a Lancair IV, would the true (or wanna be) rugged north country bush pilot want it? They probably have something more traditional in mind.

    At the end of the day, what do you really want? Design it right, design it safe, and like Orion said, the aircraft will weigh what it weighs.

    Ideally we all need 5 different aircraft to suit our seasonal moods.
     
  11. Oct 28, 2011 #11

    Vigilant1

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    Sorry for the diversion to a detail (and at the risk of entering the "gimmick zone" identified by Topaz):

    In a solid core wing for a typical LSA acft (e.g. approx 170 KIAS Vne, 14 lb/sqf wing loading) is there a practical, lighter, alternative to a monolithic piece of 2 pcf foam? For a 95 sq ft wing, 2pcf foam might weigh 65+ lbs. If there were a practical way to meet the buckling loads with lighter foam, or a "tailored billet" of foam (maybe 2 pcf from the spar forward, 1 pcf aft of the spar) there's a potential to shed 30+ pounds without adding much to the build time at all.

    Again, apologies for the detour.
     
  12. Oct 28, 2011 #12

    Topaz

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    Or, and no disrespect intended, if you're like most people you could shed 30+ pounds from the take-off weight by eating right and going to the gym a couple of times a week. 60+ pounds if you can get your significant other to join in the fun. :gig:

    It all comes down to a balance of design and construction "cost" (time + money) versus the level of optimzation in the aircraft. IMHO, worrying about 30 pounds at anything before detailed structural design is just misplaced effort. Worry about designing a mission-appropriate aircraft first. Baseline a construction method that makes sense for you, and worry about detailed weight savings "gimmicks" later. There's too much to do in the early stages to worry about such things.
     
  13. Oct 29, 2011 #13

    wsimpso1

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    No detour. Look up the threads on that topic.
     
  14. Oct 29, 2011 #14

    rtfm

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    +1 to Topaz
    Hi,
    There's a time to every purpose under heaven (although, having said that, what else are you going to think about as you drift off to sleep at night...) :) I can't even figure out where to position my wing, and I'm already building... But I'll spend the time figuring that out when I actually need the information.

    Right now my drifting-of-to-sleep thoughts are all about how to build my wing. I have run the entire gamut of methods, settling on each in turn as the optimal method for me. And that includes solid foam core with glass skin, foam core with plywood skin, foam core with aluminum skin, ribs with glass/plywood/aluminum skins. The fact is, I have no real idea what I'm going to end up with.

    When I figure out WHERE to put the bloody wing, THEN I'll decide on how to build it.

    Happy falling-asleep-ponderings.

    Regards,
    Duncan
     
    Vigilant1 likes this.
  15. Oct 29, 2011 #15

    Toobuilder

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    Thread drift: So you do that too? I thought I was the only one who pondered endless scenarios on some immediate technical detail while trying to get to sleep... Glad to see I'm not insane.
     
  16. Oct 29, 2011 #16

    Topaz

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    Well, it doesn't mean that exactly. Just that you're not alone. ;)

    Sorry. Couldn't resist. :)
     
  17. Oct 29, 2011 #17

    Vigilant1

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    Searching for threads/posts on solid core alternatives . . . I know they are here somewhere.

    Aside: Google and other search engines are really good, but I'm waiting for AI to really make this better someday. Word searches are good, but surely we'll be able to do better soon. If the info being sought doesn't happen to have some handy unique terms, today you are SOL.

    I mentioned the "card catalog" and the "Readers' Guide to Periodical Literature" to the youngster behind our local community college reference desk recently. She looked at me as though I were a triceratops.
     
  18. Oct 29, 2011 #18

    martinlowrren

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    aircraft should be heavy because there is many things in heavy material i mean it's doesn't made by tissues.<br>(not funny)<br>the lightest,stiffest and metallic strongest materials do not always the lightest or strongest structure.
     
  19. Oct 31, 2011 #19

    wsimpso1

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    Well, lessee, we were talking about "foam" and "fiberglass" and I posted my comments, and it is all about "composites", so use those words in the keywords, and my moniker in the posts by window, and see where that takes you. If that does not narrow it down enough, we were also talking about weights, which for panels means "lb/ft^2". Another set of threads can be chased using the phrase "minimum gage". Once you are around a topic a bit, you will learn the lingo, and then the searches run better.

    As to "Why..." well, the world has gotten used to things like aluminum and fabric coverings, but when people see a composite airplane, they tend to think of it being a good writing desk or a place to sit a squalling kicking toddler. Airshow morons will make you crazy. Then there is surviving bird strikes and general handling. This all pretty well sets the minimum skin on something that is sturdy enough as 3 UNI - 21 oz/yard^2 of cloth. Do a wet layup on a full core of blue foam, and the skin will weigh a bit over 42 oz/yard^2. Vacuum bagging will pull a few ounces per square yard off of that. After that there is micro fairing and then sealing. It all adds to the weight.

    Then when you design to stiffnesses (failure criteria are all in strain space) it can grow a little too. For instance a wing is a beam with a lot of bending moment relative to the shear load, a torsion bar, a pressure vessel, and set of mounting brackets, sometimes a fuel tank, and an aerodynamic device comprised of a beam or two, a skin that may also include part of the beams, and other associated parts. They all have to move as one, and yet the materials all behave differently in different directions. Not so much an issue in metal - it all behaves the same in all directions...

    If you want light and you have low wing loadings, go fabric covering. Higher wing loadings, go aluminum. Want seamless, low drag, high wing loadings, go composite. Want to finish your airplane? Pick the material sets that suit what you like to build in...

    Billski
     
  20. Oct 31, 2011 #20

    Hot Wings

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    If you want light and you have low wing loadings, go fabric covering. Higher wing loadings, go aluminum. Want seamless, low drag, high wing loadings, go composite.

    Simple, elegant, and accurate summation!
     

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