Building a "spare" wing or tail to test it?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by SVSUSteve, Aug 8, 2019.

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  1. Aug 8, 2019 #1

    SVSUSteve

    SVSUSteve

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    Okay...so some of you may be aware that I have a math learning disability. It's a weird situation where I can apply theory and figure out interrelations but I repeatedly failed basic algebra etc.

    Despite having plans to have more experienced folks look over my design before I build my design, I am really leaning towards building a "spare" wing and tail to really test my calculations (plus, it's a "dry run" practice for my building skills). Since these are going to be wooden structures-- only the cockpit (the "shark cage" as my wife calls it)-- is primarily metal, the issue of cost and hassle are somewhat lessened versus metal or composite. Also, working with wood is fun.

    Plus, I get to break stuff in various ways. That's always a treat. LOL

    I know this is done with commercially built aircraft but I am curious if any of you all have done this?
     
  2. Aug 9, 2019 #2

    mcrae0104

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    You are aware that wooden aircraft are unsafe, I presume? :D

    Kidding aside, this can be a legitimate design approach. I have not gotten to the stage of building test articles, so I'm sorry I can't help there. But for general structures (and/or algebra) PM me any time. If it's above my pay grade, I'll holler for Billski!
     
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  3. Aug 9, 2019 #3

    BBerson

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    Usually start by testing only the spar, if possible. Then the ribs separately. Then a final torsion/load test of the whole wing.
     
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  4. Aug 9, 2019 #4

    Voidhawk9

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    Why not build it to fly it and still ground test it. If it passes, great. If not, then build another (improved) one.
     
  5. Aug 9, 2019 #5

    ScaleBirdsPaul

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    We are doing this for the Litefighter. Since the wing is built with a inboard and outboard sections we built a test article outboard sections and loaded it up past ultimate. That verified that we could predict strengths and deflections. Our inboard section will be only be tested to limit load and inspected prior to the first flight.
     
  6. Aug 9, 2019 #6

    pictsidhe

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    I am quite likely to test my 1st stabiliser to failure. Maybe another for creep.. I am using a very experimental construction. As in, nobody has done this before. The materials are not expensive...
     
  7. Aug 9, 2019 #7

    proppastie

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    My tail that failed the load test is now a piece of aviation art on my wall......I test to limit only....... and if there is no distortion I will fly it.....my limit is fairly high though.

    If you plan to sell plans or are paranoid about liability if you sell .....it might make sense to test to destruction.
     
  8. Aug 9, 2019 #8

    litespeed

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    A excellent idea in view.

    If you have the time, its a new design and you are developing skills in building.....a no brainer.

    You will then be confident of the design and your building skills.

    And as you say, breaking things is great fun.
     
  9. Aug 9, 2019 #9

    wsimpso1

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    First article is used for a test in stages up to limit loads, then to destruction. That way, you can get a pretty good idea of how much shortfall you have.

    I would do a decent job of design by the book, and build the first article complete for test. Testing just spars, well you can get into instability and get early failures. Being as the tail is a small wing, doing a downtown job on it including test will give you a lot of feedback on your methods for both.

    Let's remember something really significant - when you design something with beams and ribs and skins and hardpoints, well, the main spar is easy to design properly. It is all of the bonded joints and connections between parts and the rest of the airplane that are likely to have gotchas in them. If all of the joins are good and the basics are good, it will all move as one piece under load and be as strong as your downtown analysis indicates, if not quite a bit better. But if you have some surprise weak joins or the parts do not really move together or something buckles or cripples early, then you get your surprises. This is where testing gives benefits - you find out about the things that were either not built well or had design issues on things you either did not cover in design or had issues you did not even anticipate.

    I have presided over breaking a lot of stuff. Anytime you can adjust load with small steps it is better. That is one of the neat things about using a whiffletree instead of sandbags. You know exactly how much load you had when you heard something creak or pop or collapse. The other side of things is that you usually can not dial exactly a certain amount more strength - it will usually be in steps and will usually be either a little under or a little over, and you are going to go a little over... Also, sandbags distribute loads better than straps like with a whiffletree.

    So, what will the structure of the wing look like? Main spar, drag spar, ribs, structural plywood skins? That tends to be a really solidily assembled structure with few gotchas.

    Billski
     
  10. Aug 9, 2019 #10

    ScaleBirdsScott

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    I learned a lot of lessons building the test articles that I am glad were not in the final product, where the final product will actually end up going a lot easier/faster because the 'gotchas' were all in the sample.

    Of course I'm not talking about major design changes here, more like "oh if I drilled these holes first I wouldn't be spending 2 hours on this step" and "if I trimmed this flange 1/4" further away from the end than I already did, I wouldn't be sitting here with a Dremel tool trying not to nick the spar caps" and "Next time don't drill ALL the holes to 5/32, just the ones you want to be 5/32"
     
  11. Aug 9, 2019 #11

    SVSUSteve

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    Thanks!
     
  12. Aug 9, 2019 #12

    SVSUSteve

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    Will do. Thanks for the advice.
     
  13. Aug 9, 2019 #13

    SVSUSteve

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    Because I would rather know the actual margin of safety and the only way to do that is to destroy the wing. Plus, you all want to see the video of the wing coming apart under load or during a flutter test.
     
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  14. Aug 9, 2019 #14

    SVSUSteve

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    Thanks Billski. I was really hoping you would respond. Orion (may his memory always be a blessing) aside, you're one of the most experienced engineering folks I have had the pleasure to know who I am not afraid to approach with questions or fear of seeming like an incompetent amateur.

    And, yeah, it's pretty much the design you described. I decided to go with it since it's something solid and reliable.
     
  15. Aug 9, 2019 #15

    Heliano

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    After reading the interesting postings here, here's my comment: if you want to test your one-of-a-kind aircraft intended for personal use, you can test it to the limit load only. HOWEVER: 1 - carefully check the geometry before applying loads, 2 - apply the loads for three seconds (that is the certification requirement) and, once the structure is relieved of the load, recheck the geometry. If any important difference is found, make sure you have a BIG trash bin.
     
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  16. Aug 9, 2019 #16

    ScaleBirdsScott

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    A "failed" structure isn't trash, it generally makes for very nice modern art. Or even a practical design, such as a decorative display shelf made from some aircraft wing, or maybe throw some legs on and make a fun coffee table.
     
  17. Aug 9, 2019 #17

    SVSUSteve

    SVSUSteve

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    Not my kind of art

    Or a dining room table for 20 people. LOL
     

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