Wood to Metal Joints/Junctions for Replica Spitfire

Discussion in 'Wood Construction' started by Tom Kay, Nov 2, 2007.

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  1. Nov 2, 2007 #1

    Tom Kay

    Tom Kay

    Tom Kay

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    Hello All;

    I have been lurking at another aviation forum, but knew I'd be asking questions here sooner or later.

    My goal is to build a 75% scale Spitfire. Over the last few months, this goal has altered itself from an all metal Mk IX to a Mk XIV. I am really struggling with the decision to, perhaps, make this aircraft out of wood, not metal. I REALLY wanted it to be all metal, but it would obviously add some serious complications, new tools, a second person to buck all rivets, perfect shapes of various parts, that aren't adjustable after fabrication, cranked aluminum wing spars, etc. The leading edge skins alone would be a real challenge.

    So, at the moment, I will start to take wood more seriously. As well, the 75%wood fuselage mockup I have in my basement was child's play to build, although a lot more care would be taken for the real machine.

    One big question I have is about wood-to-metal joints or junctions. How, for example, do you transition from wood longerons to metal engine mounts? If I want try to have detachable wings, like the original Spitfire, how might I end a spar of wood and add a metal "coupler" ? I would probably make the stub spars on Frame 5 (familiar with Spitfire construction?) out of aluminum, like the real thing.

    So, any thoughts on wood longeron-to-metal engine mount transitioning?

    I'll have lots more as time goes by.

    Thanks, Tom Kay, Ottawa Canada.
     
  2. Nov 3, 2007 #2

    PTAirco

    PTAirco

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    In a word: lots and lots of bolts. Wood has a very low bearing strength and feeding concentrated loads into it is difficult. In the 30's in British aircraft it was common to have fuselages built in front and rear halves joined with fishplates with a series of bolts in the end of each longeron, for example. Lots of ply reinforcement helps to increase bearing strength, as does staggering the bolts. For wing root fittings large wedges of Walnut were sometinmes scarfed onto the wing spar roots, walnut having lots of bearing strength. You will end up with a large metal fittings and lots of bolts, so if you can ever get away with making something non-detachable, it will save headaches and weight. What about using a built-in bed-type mount for the engine? Like a WW1 ALbatross.
     
  3. Nov 4, 2007 #3

    Tom Kay

    Tom Kay

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    Thanks PTAirco;

    I kind of thought that lots of bolts would be the answer. But do you happen to have some pictures of the type of joint/junction I mentioned? Seeing it always helps.

    Thanks, Tom.
     
  4. Nov 4, 2007 #4

    PTAirco

    PTAirco

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    If I had a decent scanner , I could give you lots of examples, but here's something typical; lots of bolts, adequate spacing between them and fitting in double shear. Things don't always work out as neat as this though. This is the spar root of a KR2, but it's also what you might use at the longerons to hang the engine mount from.
     

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  5. Nov 4, 2007 #5

    Tom Kay

    Tom Kay

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    PTAirco;

    Thanks again. I am assuming that one of those orange looking metal attach tabs would be located at each corner of the spar, making a total of 4. I would have assumed that the metal tabs would be much longer than that to spread the load out more, and to accommodate more bolts. I also see that there's a rear spar, and I assume the same sort of metal tabs would be used there?

    If you want to pull a few g's, wouldn't you have to beef things up a bit overall?

    This info is helpful, and on target, so much appreciated.

    Tom.
     
  6. Nov 9, 2007 #6

    dgeronimos

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    That looks amazingly similar to the attach points on the spacewalker. Really. The spacewalker uses 4 attach points on the main spar. Each attach point is made up of three .1" 4130 straps, of decreasing length (13", 9", 5"). There are 12 AN3 bolts holding a pair of attach points to the spar.

    It really looks just like that. How reassuring.
     
  7. Nov 9, 2007 #7

    PTAirco

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    It's a very common method of attaching these types of wings. As far as strength goes - yes, they may look a little flimsy, but they are perfectly adequate. If we took at a guess at the spar being about 1.5" wide and those bolts 1/4" dia. - that pair of fittings would take about 13,000 lbs to tear out. If you can build a center section into your design and move these joints out as far as possible, you'll go a long way to minimising the load on the types of joint.
     
  8. Nov 11, 2007 #8

    Tom Kay

    Tom Kay

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    Thanks for the last two replies. This is what I'd like to see, namely, examples of how wings attach to the fuselage at the root. It also confirms for me that my design intuition isn't all it's cracked up to be, because I wouldn't find it easy to trust small attach straps like that, despite the calculation of 13,000 pounds. Stress analysis and strength calcs are my weakest link.

    So, any other examples available?

    Cheers, Tom.
     
  9. Nov 21, 2007 #9

    Tom Kay

    Tom Kay

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    An upgraded wing joint. Critique please !

    Hello;

    I have been asking for ideas on how to join the wood Spitfire replica wings to the fuselage in much the same manner as the full sized original. This includes aircraft enthusiasts/designers outside of this forum.

    Lynn Williams is the designer of the Flitzer biplane. He has extensive knowledge of wooden aircraft structures, and he has offered many ideas on the issue of wing attachment to what's called frame 5 on the Spitfire. I will share his original concept drawing, and my additions as they pertain to the 75% replica Spit. Most of the load was carried by the front spar and leading edge skins, which is why I haven't yet worried to much about the rear spar.

    You can't see it, but the wood stub spar, which is an integral part of frame 5, is extended outward to the limit of the purple metal plates. Same on the wing spar. When you slide them together, wood almost butts right up to wood. The wing spar is slightly narrower than the frame 5 stub spar, so its metal plates slide neatly inside the plates attached to the stub spar.

    PTAirco, you'll probably agree that there are obvious similarities between the KR2 and this concept from Lynn. It's a question of how much beef is used to hold the wing. The metal plates on the wings also hold the landing gear legs and related equipment.

    I should mention that Lynn has not applied any stress analysis to his concept. It's just an instinctive first iteration, but I'd like to see how well this could work, first by building and torturing a mockup. Then by having someone a lot more math-worthy than myself do a set of calculations on this approach.

    As always, please add any comments, because the whole point of doing this is to learn the best way to attach the wings at the root in a safe manner.

    Cheers, Tom.
     

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    Last edited: Nov 21, 2007
  10. Nov 21, 2007 #10

    PTAirco

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    The Flitzer is a wonderful little design. I had the opportunity to look the original one over over a period of three days while mannning a PFA stand in the Royal Air Force Museum, many years ago. I also saw the original drawings and stress analysis at the PFA HQ. The analysis was very thorough for the wings, so thorough in fact that the fuselage stress analysis was limited it to the sentence: "The fuselage has been passed by inspection...." I assume it meant that if you looked at the wing analysis, you got the impression the guy knew what he was talking about so, just take his word for the fuselage!

    As far as the above sketch goes, it looks perfectly reasonable. One thing about stacking many layers of metal though - you can't assume that the load will be shared perfectly evenly between them and you should allow a factor for this. Apart from manufacturing tolerances , also consider thermal expansion of a long metal strap vs a short one. Maybe it's of no consequence here, but worth keeping in mind.
     
  11. Nov 22, 2007 #11

    Tom Kay

    Tom Kay

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    Hi Again;

    Some good points about thermal expansion, etc. Lynn Williams had suggested a conical bolt, not that I actually knew what that was before he showed me, but basically it helps keep the three plates centred or registered with each other.

    I might take the approach that all three plates are minimally but firmly attached to the spar (6 plates if you count both sides) and then gang-reamed all through the stack. This would mean drilling/reaming right through the spar wood as well, but that's acceptable for the reamer. Lynn also said that the stacks of three plates should be edge welded around the attach end, but not further out at the "leaf" end, where they are different lengths, because they are in shear there. I hope I got that right !

    If I recall correctly from school, identical metals will expand at the same rate. But this isn't a perfect world, so adding some safety factor where the straps are concerned would help. I wish I were able to do stress calcs for this, but not a bloody chance. Wouldn't know where to start.

    I haven't seen the Flitzer, but had my first look at the set of drawings today. What a work in itself. Great drawings. It would be a hoot to fly on a nice calm summer evening.

    Well, thanks for the reply. Stay tuned !

    Tom.
     

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