Composite spar in a wood wing

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by buzzypeterson, Apr 4, 2011.

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  1. Apr 4, 2011 #1

    buzzypeterson

    buzzypeterson

    buzzypeterson

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    Hey guys, I am building a double eagle and am thinking about the spar. It uses a ladder type construction similar to what the minimax uses. Do you think I could substitute a composite spar? can you glue a wood rib to a composite spar? Is there anywhere you can buy a long plank of composite that I could just drop in? There must be companies that make beams and such?
     
  2. Apr 6, 2011 #2

    wsimpso1

    wsimpso1

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    You may be able to buy something off the shelf, but I would seriously doubt it. Virtually any beam you find out there (designed for some other wing) will either be bigger and heavier than it needs to be or will not be strong enough someplace. In either case, you would have been better off (either from safety or weight or both standpoints) with the plans spar. Now maybe if the Double Eagle wing is actually the wing used in another airplane, and somebody is making composite spars for it, you might be successful.

    In general, I encourage you to work from the plans. To replace a wooden spar with a composite one is not just a drop in process. You will have to determine all of the loads that would go into the wooden spar, then use that knowledge to tailor the structure. And composite structure analysis is much more complicated than with metal and wood.

    One question? Why do you want to deviate from the established design on such a major structure?

    Billski
     
  3. Apr 12, 2011 #3

    buzzypeterson

    buzzypeterson

    buzzypeterson

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    Thanks for your imput.. with your imput and a few friends i'm going to stick with stock on the eagle the only reason I wanted to deviate is that the spar seems so dinky, it's maybe a 2x1 inch piece of wood on the top and on the bottom, that go the length of the wing.. then everywhere there is a rib there is a vertical between the two. I curently fly a hi-max which has the identical wing and it's great but the eagle has double the weight so it gets a little bit iffy in my mind. A second quick question though, you seem to be very knowlegable on composites, and I know next to nothing so far. I aquired some prepreg composite that expired for comercial use from a QA guy at an airline. It's in my freezer, The label says:

    Heatcon composite systems
    HCS2403-015-05
    BMS8-219 ST285
    Out tune 19 hours

    My friend said they use a vacum bag then put it in a kiln. I am making a cowling for the himax and my question is, can I just make a form out of foam and then unroll this bad boy and let it sit for a few days to harden? Do I need a kiln? Do you know anything about it? Iv'e tried to find info on the web about it but can't find much.
    And lastly I would like to know where I can get information on using spray foam to make a mold. I was trying to use great stuff from a can.. All I seem to be doing is making a gloppy mess that flops onto the floor. Am I just putting on to much at a time or should I be using something different?
     
  4. Apr 12, 2011 #4

    psween

    psween

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    I can't help much with the pre-preg question, other than to say that expired material could be fine or it might not work at all. There's no good way to find out without doing some pretty extensive testing. Also, most pregs need heat to flow the resin, and without both heat and pressure you are very unlikely to get a usable part.

    As for composite spar in a wood wing, I'm building one now, for a KR-2S variant. A lot of analysis has to go into the rib to spar joints and all other interactions between materials, and I'd say the chance of finding a pre-built beam are exactly zero. I built up my spar with pultruded caps, CBID webs, and Rohacell core. It took me almost 30 cure cycles and was a royal pain, but I'm happy with the result and it should save me 30 lbs. off the stock spruce spar. Not an approach I'd recommend though!
     
  5. Apr 12, 2011 #5

    Topaz

    Topaz

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    Seems and is are entirely two different things. IIRC the Legal Eagle and Double Eagle have a fairly good reputation. Apparently that "dinky" spar is all that is required for the loads imposed by the aircraft. "Making it stronger" without actually doing the loads and structural analysis can actually make a structure weaker - your "non-dinky" spar could cause a failure in an attached structure that is having load transferred into it by your over-strong new spar. Loads it wasn't designed to take. People have been killed this way, by "making the airplane stronger."

    Unless you're qualified to make a structural change, it's best to stick to the plans. If you've vetted the plans properly - the aircraft has no known issues in that area - you should have reasonable assurance that the airplane is strong enough, despite how it looks to the untrained eye.
     
  6. Apr 16, 2011 #6

    JMillar

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    psween, what is the approx weight of the stock spar? Could you share more details on how you did yours?
     
  7. Apr 17, 2011 #7

    psween

    psween

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    It's hard to find a weight for the stock spars, but based on the dimensions and average densities I think the spruce/ply main spar is around 60# with steel attach fittings. Mine weighs 28# and allowed me to make some other changes as well. I designed the depth around the new KR airfoils and put the dihedral break at the fuse side instead of outboard, which will allow me to use half span plain flaps.

    As for construction, it's basically a C-channel. I built a female tool out of MDF and bondo and covered it with Teflon tape for release. I used 1.25" X 0.125" pultrusions for the caps. The cap thickness tapers as you go outboard, so only 1 strap at the tips, stepping up to 3 at the root. The web is 7 oz. plain weave carbon BID, with .25" Rohacell core. The reason for the many cure cycles is that I interleaved the web plies in between the cap plies. The web plies also drop off as you move outboard, going from 7 core 7 at the root to 3 core 3 at the tip. It's got hardpoints and build-ups so I can use the KR style main gear mounting brackets. All plies were vacuum bagged into the tool, and I used a resin system that requires a 250*F postcure. I used leftover material from work, so it was free materials, but it would have cost me around $2500 if I had to buy it. It was great for my situation, but probably not worth it for most.

    Patrick
     
    Last edited: Apr 17, 2011
  8. Nov 23, 2013 #8

    dana62448

    dana62448

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    Hey psween, Looking at this post from three years ago, wondering about more details on the shear webs. How many layers of 7 oz. and on which side of the Rohacell? What was the spar depth and length and do you remember the weight for the finished piece?
     

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