Spar angle tolerance

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

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  1. Aug 20, 2019 #21

    12notes

    12notes

    12notes

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    I've got the rear spar straightened out now, I drilled out the rivets going from the rear ribs to front spar brackets (top skin was not yet installed) which allowed the bottom skin to move up/down, and adjusted the rear spar mount (flange on a bulkhead) with a rubber mallet. The ribs and brackets will need new rivet holes drilled spaced away from the old holes, which no longer line up. I wanted to open the top of the leading edge to replace the front spar to rear rib brackets, there's a couple of ribs/brackets that I drilled out the 6 rivets but will only be able to safely put 5 back in with proper clearances. The rivets holding those brackets go through the front ribs and spar, and were installed from the front.

    I don't think two ribs out of 24 having 5 rivets instead of 6 to the spar attachment bracket is a significant risk (please correct me if I'm wrong here!), so I'm going back to the plan of just re-drilling the existing brackets & ribs.

    There's 7 flutes on the top surface of a ~8" long front rib, the front flange is too narrow to get a rivet in, and I put the rivets in the middle of the non-fluted areas, so there's really no room for extra rivets
     
  2. Aug 20, 2019 #22

    BBerson

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    Drilling or driving out the steel pin is difficult. When rear access is possible I nip the rivet off on the back and that gets the pin out without a hammer. A good wire cutter ground flat on the back will nip them.
    If the top skin is off then it can still be twisted as needed. I think two vertical plum bobs are more accurate than a digital level.
     
    wsimpso1 and proppastie like this.
  3. Aug 20, 2019 #23

    wsimpso1

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    Before you go thinking that this is enough, sit back and check everything again.

    Going back to basics, you are reading 0.5 degree out on the length of the digital level, which is what, 12" long. Now if the aft spar were straight and 72" long, that would be one end about 5/8" higher than the other end. But is the spar straight? Let's remember that the "spar" is an aluminum angle. Is it still straight or did it get kinked? If kinked even a tiny bit, it could be fooling you. Our big reason to say you need extensive re-work is because we think the one end is 5/8 high, which puts the outer panel on that side at a serious difference AOI versus the other side...If the spar is really that far off. But if drilling and riveting has induced curves in the spar, you have other issues to resolve, like straightening the spar ...

    I like fixtures and water levels. With three water levels you can simultaneously check how high four corners of something are to each other within about 1/16" on each level. I imagine you are rethinking all of your processes now.

    If you really have allowed the spar to move back where it belongs in all axes by removing some rivets, then you can go about the task of jigging it to stay put while you reassemble the sections you have. Make sure that the spars and skins are in the right positions and then jig them there until reassembly is complete.

    The one jeopardy of having excess holes is that of fracture mechanics. If this puts rivet holes, both those that are riveted and those that have drill scratches, too close together, they will grow cracks. Too close together is generally precluded by having a minimum of 3x hole diameter between holes. You want to check that this is the case.

    Drilling out rivets is never nice, but help yourself by drilling just the heads and punching the body of the rivet loose. Then there are oversize shank rivets.

    Looks like the voice of much experience already present on here.

    Good luck,

    Billski
     
  4. Aug 20, 2019 #24

    12notes

    12notes

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    The rear spar is straight, the adjustment was made to the center spar mount after removing the rear spar to rib rivets and the rear spar to front spar bracket rivets. If the spar was bent, it's very unlikely that both wingtips would read the same angle as each other before and after the adjustment, unless it had a z-bend. Visual examination and running my hands along the surface (checking for burrs in the rivet and mount holes didn't reveal any bends, it laid flat on the freshly CNC cut 3/4" thick MDF slabs I am using for a jig for my leading edge tanks, and as a cross check, the 2nd rear spar I made last week (unnecessarily, at this point), slid in place and had the same problems. A final check with a laser level will be done tonight.

    I marked off 3/8 circles around all the rivet holes I could using a small strip of .040 with an 1/8" hole and a .052" hole (size of a Sharpie ultra fine point) 3/8" apart. This is how I determined that there was enough room for 5 rivets in some of the brackets that had 6. Fortunately, when I looked at the plans, and what I didn't notice before is that the brackets have 6 rivets going through the spar, but only 5 on the ribs, I had used 6 on both sides (see below, 5/8" side is spar side).

    My problem with drilling the rivets that hold the skins on is that the mandrel broke off in a manner that left it protruding from the center on about 40% of them, and it is sharp. It's like trying to drill on the tip of a nail, it just wont center and it pushes the drill off to one side while trying to drill the head off. Trying to punch the mandrel out ended up pushing the skin and rib down into a dent without moving the mandrel. I can only support the bottom of the rib/skin on 4 of the 12 ribs, so I don't see how I could get the skin off without destroying it and all the nose ribs.

    On the other hand, kudos to the Cherry BSC-32 rivets for relieving any potential worries I may ever have that they might fail. Those suckers are on there.
     

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  5. Aug 20, 2019 #25

    proppastie

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    Last edited: Aug 20, 2019
  6. Aug 20, 2019 #26

    12notes

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    I've got that same one. I also borrowed a much more expensive, self leveling laser one from work.
     
  7. Aug 21, 2019 #27

    12notes

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    So if I did drill the 3/32" rivets out and needed to enlarge a hole to replace one with an 1/8” rivet, is there an 1/8" blind countersunk rivet will a grip range that includes .040"? Smallest I can find is a BSC-42, grip range is .063"-.125".

    EDIT: Nevermind, I found them after an hour and a half searching. AK41-44BS201 is the part number.
     
    Last edited: Aug 21, 2019
  8. Aug 21, 2019 #28

    Turd Ferguson

    Turd Ferguson

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    Patch.
     

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