Spar angle tolerance

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by 12notes, Aug 15, 2019.

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  1. Aug 15, 2019 #1

    12notes

    12notes

    12notes

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    I've been using a digital level on my build, and have noticed a problem. I'm not sure if it's a precision issue with the level or a serious problem.

    The Hummelbird has a 72 inch wide center section of the wing that goes through the fuselage, front spar is built up, rear spar is a 1.5x1.5x.125" aluminum angle. Front and rear spars were installed and both leveled to 0.0 degrees, identical distances from each other at the ends.

    I installed the ribs, made the skins, measured at various random points along the way to confirm level. After I riveted the skins on, the front spar was still at 0.0 degrees, but the rear spar was at 0.5 degrees left to right. I have no idea how or when this got skewed.

    What would the tolerance on this be?

    I've tried replacing the spar with another piece of angle and shimming the center attachment, but that made little difference, 1/8 thick shim on one side (way more than I'd feel safe using) resulted in a 0.1 degree improvement. So it looks like I'll have to take off the bottom skin. I have a 0.00000001% chance of drilling that many rivets out without enlarging a hole in the ribs, replacing a rib means I have to take off the leading edge skins as the ribs are riveted together through the front spar. Basically, I'd be undoing the last 6 months of my build, the most unenjoyable slog I've had in the whole process, and re-doing it. I'm not sure I have it in me to do so, if this is really necessary, it may be the death of the project.

    Any suggestions on how to fix this?
    Is this something I could live with and fix with aileron trim, or a going-to-kill me type problem? What would be the effects of this? With an 18' total wingspan, could I fix it with the outer wings?

    I'm not going after perfection with this project, just good, so it won't bother me if it's imperfect as long as it's safe.

    Edit: Outer wing panels are inclined by 3 degrees, I believe, relative to the center section wing stubs.
     
    Last edited: Aug 15, 2019
  2. Aug 15, 2019 #2

    TFF

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    Where are you measuring? With twist you need to read this cord wise not just spar. What is the cord incidence at root and tip?
     
  3. Aug 15, 2019 #3

    12notes

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    I was measuring 0.5 degrees along the spar, from left to right. I did measure chord wise, but don't have the numbers in front of me. From memory, right wing tip had a larger angle than the left tip, by 0.8 degrees I think, oddly enough I recall root and tip measuring the same at one point, but I don't remember if that was when the spar was still attached to the skin or not (I'm guessing not). I'll put it back together and take better measurements again tonight.
     
  4. Aug 15, 2019 #4

    proppastie

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    I think .5 degree is a reasonable tolerance, as long as there is not too much twist,.....what is too much?.....more than you can trim out......The wing will level out in flight, and I am not sure you will be able to notice the .5 degree once you are in the cockpit. In any case you would split the difference when you set up your span-wise level reference. So now you are off .25 degree....not very much. You test a level by flipping it 180 degree and it should measure the same.....
     
  5. Aug 15, 2019 #5

    Dan Thomas

    Dan Thomas

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    Half a degree on a 72 inch center section spar puts one end of that spar 5/8" higher than the other. That throws the incidence between the wings a LONG way off and will result in a very wing-heavy airplane. Cessna, in their singles, uses eccentrics that move the aft spar up and and down maybe a quarter inch to adjust wing heaviness; imagine two or three times that much differential on a much smaller airplane! A big problem.
     
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  6. Aug 16, 2019 #6

    Turd Ferguson

    Turd Ferguson

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    Hold an EAA meeting at your build location and have everyone brainstorm the problem. Write down all the potential solutions, you'll need to throw them all out except for one.
     
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  7. Aug 16, 2019 #7

    BoKu

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    What sort of rivets are we talking about? Driven or pops? protruding or countersunk?

    I once drilled an entire face off of an HP-18 box spar, 576 AN570AD4-5s, without boogering any hole beyond tolerance. All it takes is practice and patience.

    My thinking is that if Dan Thomas did the trig right (and it sounds about right to me), backwards is probably the only way forward. The aft spar being 5/8" off is huge.

    When you reassemble, use a jig or fixture to maintain alignment. Clecos help, but they're not a superpower.
     
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  8. Aug 16, 2019 #8

    proppastie

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    I guess you are assuming the rear spar is correct and there is twist.....however if the rear spar is in line with the front spar of each wing, and no twist, I guess I do not see the problem.
     
  9. Aug 16, 2019 #9

    12notes

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    Unfortunately, my build location is cramped enough that no one thicker than me (5'10" 175lbs) would be able to get to the left side of the aircraft or in front of the wing at the moment.
     
  10. Aug 16, 2019 #10

    12notes

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    The skins are on with blind rivets. I'm sure you could drill them all out, but I'm not that good, or just have had bad luck.

    I remembered that I used attach brackets instead of a rib flange for most of the rear ribs to front spar, if I were able to drill out some of these, then level the back and attach to new brackets, I think I might be able to leave the bottom skin attached to the ribs, but I'm not sure if that might add a wrinkle to the bottom skin, or get the rear spar drill holes out of alignment.

    Sorry, I haven't been able to get out to the garage to check the measurements tonight, or check which ribs have the brackets, I think it's the outer 4 of 6 on each side. Or I hope it's at least those. I remembered I used the brackets on more ribs than the plans called for, but not sure if that was nose ribs or rear ribs.
     
  11. Aug 18, 2019 #11

    12notes

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    Drilling out the brackets from the ribs, along with a few taps from a rubber mallet on the bulkhead flange the spar attached to, allowed me to adjust the spar to 0.0 degrees. It's currently held in place with clamps. Removing the brackets to replace them would require removing the leading edge skin, as the rivets were installed from the front, I'd like to avoid that if I could. Obviously, the holes in the ribs and brackets no longer line up, but there is more than enough room to drill new holes in the brackets and ribs and reattach them in the proper places. Is there any downside to this, as long as proper clearances for the new rivet holes is maintained? I'm not OCD enough that the additional "lightening holes" would bother me, as long as it's safe to leave them.
     
  12. Aug 18, 2019 #12

    proppastie

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    depends on how close the new holes are to the old holes...If proper minimum rivet spacing, (3 dia,minimum, normal spacing is 8-10 dia.) an empty hole in the bracket should not be a problem. A whole set of new holes again at normal to minimum spacing to the old holes should not be a problem.....However I will add it is hard to properly advise you without drawings, so you are sort of on your own. My assumption here is you will be re-position so there is no twist ...if only the dihedriel is off I sill do not understand how 1/2 a degree in dihedriel only is a problem
     
  13. Aug 18, 2019 #13

    12notes

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    It's a half degree of twist I am attempting to correct, front spar is at 0 degrees left to right, rear spar was at 0.5 degrees left to right.
     
    Last edited: Aug 20, 2019
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  14. Aug 20, 2019 #14

    12notes

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    When I said I don't have good luck drilling out rivets...I wasn't kidding.

    This is even when using a rivet removal drilling tool. The Cherry 3/32" countersunk rivets leave some stem protruding on about 40% of the rivets, hand drilling is impossible when they're like that. I thought I'd test the tool, I only had the drill turning for less than a second, it didn't like them either. I can't take this skin off without completely destroying the skin and all the ribs.

    No idea how to fix this hole.
     

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  15. Aug 20, 2019 #15

    TFF

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    You have to take a punch and knock the center out first.
     
  16. Aug 20, 2019 #16

    proppastie

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    Leave it, maybe fill with epoxy for cosmetic reasons.....add 2 rivets one each side 1/2 way to the next rivet .
     
  17. Aug 20, 2019 #17

    proppastie

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    If thin skin need to back up far side to not dent skin,....not easy to do needs 3 hands
     
  18. Aug 20, 2019 #18

    12notes

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    Thin skin, no way to support the backside except on 2 ribs, it's the leading edge. Punch just starts a dent without moving the stem. Can't add extra rivets due to fluting of ribs.
     
  19. Aug 20, 2019 #19

    Rockiedog2

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    lotta good experience in the answers
    I been here before as have others. Lots of it depends on your personal tolerances. It probably can be fixed but not IAW 43.13. And you may never feel relaxed flying it, personal tolerances again.
    I know of an RV6?7?8? (sorry don't remember which but no matter here...190-200 mph airplane) that has 3/8* twist chordwise in the wing at the tip per digital level. The builder flew it as is and last I heard it had near 1000 hrs on it. He said it never did have the RV "hookedup" feel but was ok. Rigged it pinching the trailing edge of the aileron per Van instructions.
    As some said yeh you gotta jig the spars somehow or they will find a way to walk on you. I don't drill the rivet out. Like TFF said knock the mandrel out then mostly drill the head off and knock the rest out with a proper homemade punch. Pic attached. Yeh it may bend the rib a little...personal tolerances again, the replacement pulled rivet will straighten it to some degree...if you gotta have it perfect 43.13 then throw it away and start over. It all depends on where the new hole falls in relation to the wrong hole. It will probably be ok with a some less than 43.13 requires but that's a personal judgement call. What you got has been dealt with countless times. I drilled/punched the whole top skin off a 701 wing panel. 025 ribs and 016 skin.
    You just gotta decide you gonna fix it if it's possible and go at it. It will become clear at some point if it's the right thing to do to try to salvage it.
    Welcome to homebuilding

    PS sorry couldn't post the pic of the punch
     
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  20. Aug 20, 2019 #20

    proppastie

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    Must have lots of flutes if you can not find space for a rivet...does not have to be anywhere but more than 3 dia. away from another. You probably will only need one extra rivet 3 dia. away from the bad one.
     

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