Orion's "Composite Think" in Plywood???

Discussion in 'Composites' started by Aerowerx, Sep 13, 2012.

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  1. Sep 14, 2012 #21

    BBerson

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    I was thinking thin plywood could be made in corrugated sheets to be used as sandwich core instead of the usual foam or honeycomb.
    It would be exactly the same as any ordinary cardboard box (which is made with cheap corrugated paper core).
     
  2. Sep 14, 2012 #22

    highspeed

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    I beg to differ when it comes to the metals. Every batch (heat) of material from the mill gets tested for mechanical properties and chemistry. The basic sample undergoes a standardized tensile test that tells you the yield strength, the ultimate tensile strength, the reduction of area, and the elongation at the fracture point. This is the fundamental stuff. Often a charpy impact test is performed to determine the toughness of the material. So you can "know" the properties of a material. Knowing the loads the material is subjected to might be another matter entirely, but the material properties can be pretty well known.
     
  3. Sep 14, 2012 #23

    SVSUSteve

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    Are you willing to bet your life and those of passengers on those numbers without a safety factor? I am not and neither are trained engineers. The other issue is that the strength of a material when it comes out of the mill in sheet form and its strength after being milled, shaped, drilled, riveted and then exposed to the fatigue of use are often two disparate numbers.
     
  4. Sep 14, 2012 #24

    Aerowerx

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    One of my previous incarnations was as a Measurment Scientist for the USAF, so I can speak with some authority on this subject. We calibrated test instruments that were used to calibrate the instruments that were used to test the aircraft.

    Every instrument had a range of uncertainty. We "knew" that the value was between 5.6 and 5.8 with 95% confidence, for example, but we still did not "know" the exact value. (Note that there was still a 5% chance that we had no idea at all!)

    The same thing would apply to testing a piece of aluminum. The instruments used have some uncertainty. Maybe the tech was having a bad day. Maybe it was raining outside. Same sample, different lab, different numbers. Same lab, different sample, different numbers. And exactly how many atoms of copper are in the piece of 2024? Can't say, can you?

    If we were able to "know" the properties of the material exactly then we would also be able to "know" the forces on a plane, and then use that safety factor of 1.00001! But, of course we don't, so we use the factor of 2.0, most of which is from not knowing the forces I would guess.
     
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  5. Sep 14, 2012 #25

    highspeed

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    Absolutely not. I have always assumed a factor of safety of at least 2 in my design exercises. And you're absolutely right about the properties of the part after milling, drilling, riveting, etc. I wasn't trying to say that you can look at a test report and design the component so the failure point is a hair past the yield point of the material. That would be foolish. I'm in Quality Assurance for a manufacturer in the oil and gas industry and most, if not all, parts are designed to require a minimum yield strength. The minimum includes the factor of safety, of course.

    Measurement uncertainty always rears its head. I agree, you can get close and know the strength of the material to within a certain range, but the sample is just that, a sample. You will always see variation in material. We'll see it from one end of a 10 foot bar of steel to the other quite readily with the appropriate test. Engineering is all about assumptions and generalizations, hopefully to the side of caution. One of the important things about all that material testing is that is shows if the mill is producing a consistent product with variation that is of an acceptable level. We often order supplemental testing of the material, and is always comes back with a slightly different number. If the difference is too great, we'll investigate further.
     
  6. Sep 14, 2012 #26

    Aerowerx

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  7. Sep 14, 2012 #27

    WonderousMountain

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    why?
     
    Last edited: Sep 14, 2012
  8. Sep 14, 2012 #28

    SVSUSteve

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    That's what I figured you recognized but I did not want to assume and risk something bad happening.

    So what safety factor is recommended for a wood structure? I have always used 3-4 depending upon the specific structure.
     
  9. Sep 14, 2012 #29

    Aerowerx

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    Ever seen two pieces of wood that even looked the same?

    The variation between one piece of wood and the next is a lot more than the variation between one piece of aluminum and the next.

    Whatever material is used, the structure should be designed with the minimum specification in mind, along with an appropriate safety factor.
     
  10. Sep 14, 2012 #30

    Aerowerx

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    I'm sure the 'experts' on here would have an opinion on that, but my opinion is that a FOS of 2 would be OK if you base it off the minimum specification of the particular type of wood. 3-4 would be even better.
     
  11. Sep 14, 2012 #31

    autoreply

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    Even 1.5 would do then.
    With the possible complication that your aircraft is so overweight that it won't fly anymore. Bumping up the F of S is what you often see with "amateurs". Doesnt' work, because overall the design gets worse, not better. Design for the appliceable loads, not more. Some "extra reinforcement" is pointless, unless that part is under strength of course.
     
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  12. Sep 14, 2012 #32

    WonderousMountain

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    Personally I like to work from the design envelope perspective, identifying what possible maneuvers stress the airframe to it's limit, then I apply material fatigue for 1000 hours minimum, and finally top it off with the 1.5 safety factor. This sometimes comes out to the 3-4 "safety factor" and sometimes is not much bulkier than the original 1.5, depending on the component. Heavy parts are downsized, or redesigned if possible. It takes longer for initial calculations, but is much better for understanding in the end. Vne is critical component of this, so choices have to be made early on.

    Blessings,

    Wonderous Mountain
     
  13. Sep 14, 2012 #33

    SVSUSteve

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    If done carefully and only for extremely critical components (in the case of the Vireo, this would be the spars and the roll cage which are directly connected), it should not be an issue. Everything else that is critical to safety, it is a factor of 2. All other structures were designed with a factor of 1.5 I went with that approach and still came out with an aircraft that meets the LSA standard.
     
  14. Sep 15, 2012 #34

    Barracuda

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    Those tophat stringers are perfectly suited to being moulded in composites, to make them in wood would be a pain. There's no reason you can't make/buy the tophat sections and bond them to plywood, it would work well. It would be versy easy to make up a mould on a table and lay up stringers like that out of biaxial etc, then just cut to length.
    Cored plywood structures have been used in boats for ages, often cores of Nomex or even honeycomb are faced with a ply to give strength and stiffness to a bulkhead with a huge reduction in weight. As long as the ply is prevented from deflecting, it can do it's job as well as a much thicker, solid ply section.
    You'll find ply moulded furniture is made in large, bespoke hot presses where the adhesives can be applied uncured, the ply loaded in wet and the whole lot heated and pressed until adhesive cure. To replicate it in ply, you would be vacuum bagging and trying to ensure correct adhesion between all the various materials.
     
  15. Sep 15, 2012 #35

    Dan Thomas

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    1.5 safety is what Cessna uses when they build their airplanes. For instance, the 172 is rated at 3.8 positive Gs, IIRC, so it should take at least 5.7 Gs to break one. Now, they might be looking at yield rather than ultimate, so maybe it will bend some first at that load before it breaks.

    I wouldn't want to go find out.

    As far as compound curvature in plywood: this can be very hard to achieve. My son and I built a small boat that had some minor compound curves and it was a bear to fight that 1/4" pine ply onto the frames. Steaming may have helped, but those fibers aren't inclined to stretch much. Bending a really thin piece along the grain will place enormous stresses on the grain and it will permit that, but adding another dimension of curvature is really difficult.

    Some boatbuilders do it by laying up the hull with several layers of thin veneer strips. I think the deHavilland Mosquito was built this way. The builder is basically making his own plywood in the shape he wants.

    See this: Gentry boat plans a

    Dan
     

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  16. Sep 15, 2012 #36

    Aerowerx

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    Seems that this thread has gone far beyond my initial simple question!

    Refering back to Orion' truss-type sparless composite structure, all I wanted to know is, if it was done in plywood how it would compare with the typical built-up wood ribs and spar.

    I dont' see why the plywood would have to be molded. Why not just cut some long plywood strips and place them inside the wing spanwise, arranged in the truss type configuration? Glued directly to the skin at top and bottom. Yes, some type of reinforcement might be needed at each joint.

    As for the skin, maybe 1/8th or 1/16th inch ply that was steamed or soaked could be bent enough to form the surface. Although some type of form or mold would be needed to keep it in place while drying.

    Or maybe a geodetic type arrangement of 1/16th inch ply strips, and then cover that with a solid layer of 1/16th inch ply?
     
  17. Sep 16, 2012 #37

    wsimpso1

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    Good schemes exist for each material. The one Aerowerx is asking about is a good one for composites, but I suspect a very difficult and somewhat heavy one for wood. The ultimate way to know is to design an optimized one in both carbon and in veneer, and compare the weights. The way that some folks would propose is to compare the strength to weight ratios of the materials, but that does not take into account things like minimum gauges for each, adhesive weight, and the many regions of each that are not optimized...

    I would build plywood in the conventional way, and if I wanted to build a hollow tail, I would build it via the method in the original post.

    Billski
     
  18. Sep 16, 2012 #38

    orion

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    Keep in mind though that the original idea of this concept/discussion was the minimizing of part count. Doing this in wood would be very time consuming and would most likely end up with as many parts as in a conventionally built up wing. Then given the issues of wood working, the resulting structure will probably be measurably heavier and one would still need to address how to form the skin shape. Bottom line, this is unlikely to work in wood.
     
  19. Sep 16, 2012 #39

    TFF

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    Building truss wood ribs is really easy and for wood light. only a little slow. If the trusses get replaced with longitudinal ply that go straight to the skin, its going to need some blocking to keep in place all the way down the skin. To replace a solid wood spar requires a lot of ply which is weight. You would end up with a bunch of wasted strength in not useful directions with the ply, along with the weight. The Fokker Triplane used a box spar which is I bet where the jodels get their ideas. 2 sets of 2 spruce spars boxed with ply and then boxed together. Fokker DR-1 Photo Gallery I bet it could be simplified.
     
  20. Sep 17, 2012 #40

    bmcj

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    Followed your link to Ron Sands and found this picture. It makes sense though... if I had three hands, I might build a plane with three wings too. Think of how much faster you could build!

    ron_dogs.jpg
     

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