Switching to wood?

Discussion in 'Wood Construction' started by mcrae0104, Aug 6, 2017.

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  1. Aug 6, 2017 #1

    mcrae0104

    mcrae0104

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    I am considering switching my design from aluminum to wood. (Low-wing, single seat, approx. 1,250 lb gross.) I am familiar with ANC-18 and have been studying the Falco plans and construction manual to learn what I can (thank you, Sequoia--what a gold mine!). I have a few questions for those who have experience, or at least reliable knowledge of this construction method.

    Weight estimation. Raymer uses the same coefficient for wood and metal in his initial W[SUB]e[/SUB]/W[SUB]0[/SUB] equation (a=1.19, see Simplified p. 20), suggesting that they are similar. Later, when dealing with weight estimation by component, he provides statistical equations for components but says they're for conventional aluminum construction. Then he suggests that a wood fuselage could weigh 60% more (!) due to minimum gauge effects, and you need to do more design/analysis to find the weight. Perhaps I'll just have to do this to complete the trade study, but do you know of any references that address weight estimation of a structure similar to a Falco or GP-4?

    Wing attachment. In aluminum, my design includes an overlapping sailplane-style spar (with an adaptation of Dan Weseman's Panther wing folding mechanism). I can do this with wood box spars (though probably with a weight penalty) but my main concern is that with wood movement, the spars might swell enough inside of the carry through box to bind and prevent removal. Has anyone seen this done in wood?

    Glues & Epoxies. I find the idea of using an epoxy adhesive appealing since it would eliminate masking off glue joints when sealing interior surfaces before assembly, but this gets us back to the whole paint color and temperature discussion just like composites. If you have experience or have done testing of wood/epoxy joints at elevated temperatures, I'm all ears.

    Finishing. Some plywood-skinned planes are finished with a layer of glass cloth (but no mention of this in the Falco manual). Why is this done, just to prevent spliced seams from possibly opening up over time? Perhaps to protect thin skins from hangar rash? I do not see a useful structural advantage to a glass skin on one side only, and would hesitate to include its value when sizing skins. By the time a wet layup us smoothed over the entire plane, I would imagine this is a considerable weight penalty (even 20 lb would be significant).

    (Edit: forgot to add this one)
    Laminar flow. Is it realistic to think that plywood-skinned wings can be constructed true enough to take advantage of laminar flow?
     
    Last edited: Aug 6, 2017
  2. Aug 6, 2017 #2

    BBerson

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    A thin layer of fabric on plywood will prevent paint cracking. Paint or varnish alone tends to shrink and crack.
    Old gliders used cotton fabric. Modern wood gliders used Dacron fabric on plywood.
    I think Tony Bingelis used the lightest glass cloth, 3/4oz model fabric.
     
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  3. Aug 6, 2017 #3

    TFF

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    You have to think glass and wood planes passed in the night when it comes to certified, so there really was never a chance to substitute fiberglass cloth on something like a Bellanca wing. Unless your name is Burt, we all copy certified plane design. The simplicity of adding the hard shell finish of fiberglass for the same weight of dacron or cotton is hard to beat. All wood planes are covered. With using FG, its like painting a Corvette. Spray primer, sand smooth, spray paint, which can be car paint, buff( gently). If you do dacron, its just like a covering a Pitts. Glue fabric, fill weave with dope or polybrush, fill more with silver dope or polyspray, which is going to be your filler, and then top coat. Best not to use auto paint, but many do. Certification cost is too much for the market of certified today. The one minus of fiberglass is its permanent. You can remove the solvent based processes for repair. Remember you are only putting one layer of 1 1/2 oz cloth on the plane, you are not building a cowl that has to stand alone.
     
  4. Aug 6, 2017 #4

    BBerson

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    The solid precured fiberglass skins used by a couple of experimenters in the 60’s seemed to work.
    Search archives for "Jupiter 1" and "Ol' Ironsides". And Strojnic. Precured fiberglass doesn't shrink like plywood so much and get that "starved horse" ripple look. No pin holes either, cured on flat sheet of glass. Easy to see through for bonding.
     
    Last edited: Aug 6, 2017
  5. Aug 6, 2017 #5

    Aerowerx

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    I think he is talking about a single thing layer of glass over external plywood surfaces. It is in the plans for my (currently on hold) build.

    This is for protection, mainly.
     
  6. Aug 6, 2017 #6

    Pops

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    I have been doing work on designing an aluminum wing for the JMR Special in addition to the wood wing of Geodetic construction. I was talking to Bob Barrows (designer of the Bearhawks) about the difference between the wood wings and the alum wings and he said that it will be hard to get a set of alum wings rated for the same G's at a lighter weight.
     
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  7. Aug 6, 2017 #7

    Aerowerx

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    Just to clarify, Pops....

    Do you mean it is harder to get an aluminum wing that is lighter than a wood one (with same load rating)?

    I was crunching some numbers a couple of weeks ago. Don't remember the details, but I thought I saw that, with 6061T6 aluminum sheet and plywood sheet of the same strength, the plywood would be thicker but lighter. Can't remember the details right off, so take that as conjecture until proven otherwise. Particularly since I always seem to have trouble finding specific strength data on aircraft grade plywood.
     
  8. Aug 6, 2017 #8

    BJC

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    That is an interesting thought experiment that will better prepare you to appreciate the opportunity to develop an elegant design in composites.


    BJC
     
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  9. Aug 6, 2017 #9

    Norman

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    Yes but it ain't easy. Poisson's ratio is going to drive you crazy. Every bend over an open area will try to warp at a 90 degree angle to the intended bend. Panel joints are stiffer than the base wood so they become troughs (flat spots count as troughs between waves). When you soak the wood water does not get to the internal layers because of the waterproof glue so the internal grain should be aligned in the direction that bends easy dry ie the face grain should be parallel to the nose ribs or perpendicular to the leading edge if it's swept. After you get the plywood ironed down and glued on you'll have to check it for waves. The proper tool for this is a dial indicator but the waviness tolerance realy isn't all that bad at the Reynolds numbers at which mere mortals fly, basically you can feel the waves and troughs with your hand and if the waves are big enough to feel they will look awful under shiny paint so you'll want to fill in the troughs anyway. There will be a lot of filling and sanding and that's where the glass layer comes in. The veneers in that plywood are only 1/3mm thick so it won't take long to cut through the top layer. Then after you've got the naked wood faired you get to ruin it by putting cloth and paint on it so you can start the whole smoothing process over. The edges of the leading edge tape form little aft facing steps which are much worse than waves. You can fair these steps with some extra paint thus turning the step into a wave. The local Re at the leading edge is really small so hopefully the boundary layer can override this little wave without going turbulent. It isn't hard just tedious. As I recall the only tools we needed were an 8" wide putty knife to spread the micro and a long soft rubber sanding block. Back in the '50s and '60s when sailplanes were made of wood with laminar profiles this is how competitive pilots spent the winter.
     
    Last edited: Aug 6, 2017
  10. Aug 6, 2017 #10

    Pops

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    Yes, That was what Bob Barrows was saying.
     
  11. Aug 6, 2017 #11

    Dan Thomas

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    Try building a small wooden boat, then cover it with only paint. Being in the water isn't the problem; it's the sun that does the damage. The wood will dry out over time, some UV will get through, and the finish soon starts cracking along the woodgrain. Once that happens, then water does indeed become a problem and the wood's life expectancy is shortened a whole lot.

    Been there, done that. Also have seen it on homebuilt wooden sailplanes that were simply painted.

    You really, really need fabric over the wood to protect it. The lightest polyester, held down with Poly-Brush, will do it. Cover that with Poly-spray and a topcoat. The fabric is there to protect the finish from the woodgrain's cracking influences, and the finish protects everything including the fabric. MEK will dissolve the finish and allow removal of the fabric for repairs or fabric replacement. If you have paint on wood, it cracks and flakes and removal risks the wood being damaged by sanding or scraping or the use of paint strippers.
     
  12. Aug 7, 2017 #12

    Aerowerx

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    A clarification....

    A thin layer of glass cloth over just the external plywood areas (in my case the entire fuselage, and D-tube), with epoxy. Then cover the whole thing with your favorite covering cloth.
     
  13. Aug 7, 2017 #13

    TFF

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    No need to do that if using dacron covering or if covering all with FG stop there dont add the dacron. All wood gets an epoxy penetrating sealer no matter what. Adding fiberglass is just weight, and if you have to have FG, you might as well make the part completely FG . It is not going to make it tougher if weight is a concern mixing materials. The Dacron is waterproofed with its chemicals.
     
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  14. Aug 7, 2017 #14

    deskpilot

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    I'm experimenting with a different type of skin for my Thruster. Half sized 'socks' sewn from Lycra, stretched over the frame and then coated with ordinary f/g resin. Depending on how many coats of resin, toughness and flexibility can be adjusted to your requirements. In your case, a single, thick coat rolled on would be sufficient, imo. Lycra can be made to fit any shape, even concave in your case and although fairly expensive ($35Aus per meter) here in Oz, you only need half a much material due to it's elastic quality. Another advantage is that you don't have to paint it. Lycra comes in many colours and retains it's colour even when coated. I believe it will also be un-effected by UV's.

    DSCF4929 (Copy).JPG DSCF4931 (Copy).JPG
     
  15. Aug 7, 2017 #15

    Victor Bravo

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    We still had to do that in the 1980's with the fiberglass wings too. 6 foot long rectangular aluminum tubes with 400 and 600 grit sandpaper glued to the faces of the tube. "The Torture Board" it was called. I never had to do it myself thank goodness, but I was there helping others do it a couple of times.

    Then there were the guys who wanted the first 25% chord polished perfectly smooth, and then the top surface from 25% back to the rear spar sanded to 400 grit with fore-aft sanding strokes only... trying to emulate the NASA shark skin research findings.
     
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  16. Aug 7, 2017 #16

    Norman

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    Those guys had it easy with their hard surfaces and lack of compound curves. :cheeky: We had to wet sand the paint with 600 grit wrapped around a soggy sponge. It's impossible to get into the corners of the aft bib beys without sanding through to the primer on the edges of the wood.
     
  17. Aug 7, 2017 #17

    mcrae0104

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    Thanks all for the good discussion on fabric or glass covering on wood skins. Makes perfect sense.

    This makes me feel better about going forward with some initial structural sizing and weight estimation. About the only other data points I have are the empty weight fractions and fuel fractions of the Falco and GP-4, which are very close to my aluminum design.

    Yes, perhaps it's a baby step toward that. Right now the two main points attracting me to wood are the smooth surface and the fact that virtually any part shape can be made of wood (tailoring the spar, for instance). There are some downsides, of course.

    Thanks, Norman. I know a lot of sheet metal wings use a laminar foil (even if they don't achieve much laminar flow), but your comments in the other thread lead me to believe its possible to realize some performance benefit in wood.

    Now, any thoughts on the other questions, such as the possibility of the overlapping spars swelling inside the carrythrough?
     
  18. Aug 7, 2017 #18

    mcrae0104

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    Do you have a copy of ANC-18 or ANC-19? I think you will find what you want there.
     
  19. Nov 22, 2017 #19

    wanttobuild

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    Maybe you could use a bushing and pin attach like the RV12 uses, so really the spars wouldn't really come into contact.
    I don't guess I could talk you into a wood honeycomb fuselage? It would be so strong, it would be one big ass beam.
    Honeycomb came out of left field for a building material i have considered, but am totally sold on it now.
    Ben
     
  20. Nov 22, 2017 #20

    Topaz

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    My understanding from Strojnik is that it can be done, but expect a fair amount of sanding/filling in a more-difficult and more-time-consuming material than filler-over-fiberglass, and be very careful not to go through too much of the wood, making it too thin. If you go the glass-cloth-over-plywood route, to protect the wood and seal the structure, it makes it easier (you're back to filler over glass cloth), but be sure to allow for the thickness of the fabric when setting up your ribs and so on. Otherwise you'll end up sanding all the glass cloth right off, just to get the wing down to the proper airfoil.

    EDIT: Ninja'd by Norman, who gave you a better answer anyway. Glad you're getting such good answers!
     
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