Enlarging wing rib lightening holes

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by 12notes, Jan 27, 2020.

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  1. Jan 27, 2020 #1

    12notes

    12notes

    12notes

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    I'm plugging along on my Hummelbird build, and I've hit a snag. I've been working on leading edge fuel tanks in the outer wing sections, which have been done several times before, but there are no plans. I'm a little stuck on building the end caps, the last couple of solutions I've tried to get a minimal gap in the corners have been failures. I've already made the ribs for the middle, but am contemplating a different solution, and wanted to run it by some folks.

    The original design has a 2" diameter and a 1.5" diameter lightening hole in the nose rib. The outer wing has 7 ribs and is 6 feet long. My thought was it would be much easier to make new ribs with a 4" lightening hole in the rib and insert a cylindrical fuel tank made of aluminum tube. My initial thought would be that this would make the rib too weak, but when you fill that hole, the hole can't deform much. Plus, the skin and spar form a D shape which is fairly stiff on it's own. I don't know how much the lightening holes were calculated or "looks about right" on a design such as the Hummelbird. My reading has not yet covered this detail, and I couldn't find much in the search about it.

    I could insert more ribs, or make them out of thicker material, if needed.

    Any thoughts?
     
  2. Jan 27, 2020 #2

    lr27

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    To be really sure, you'll need to test a couple of old ribs to destruction and compare with new ribs. Or do some engineering analysis. It's probably best if the holes are flanged.

    If we don't have Hummel builders here, I'd be surprised if there wasn't an on-line group of Hummel builders, some of whom have solved this problem
    Does Hummel still support the plans? Maybe they have a supplemental sheet about this.
     
  3. Jan 27, 2020 #3

    cheapracer

    cheapracer

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    Draw a 4" hole around the 2" hole.

    Drill say 12 x 4mm holes around that 4" circle (think of a clock).

    Make 12 evenly spaced cuts from the 2" hole to the 12 4mm holes (think of the hands of a clock).

    Bend all the fingers out and use a hose clamp (whatever clamp) to attach to the tube.


    rib to tube.jpg
     
  4. Jan 27, 2020 #4

    Hot Wings

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    I'm presuming that the problem with the gap is related to sealing the tank? If so how big are these gaps? For a data point consider that the wing tanks in the BD-5 have gaps on the leading edge of the ribs on the order of a half a inch. These gaps are simply filled in with rubber blocks glued in place with Pro-Seal. I always though the method was a bit of an after thought, but it does seem to have been proven to work.

    Maybe you are just over thinking this and a bit of sealant is all you need?
     
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  5. Jan 27, 2020 #5

    12notes

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    I may be overthinking it. I made a test rib using a method similar to cheapracer's 2 piece rib idea, sealed it with RTV and closed end rivets, and it leaked at the corners when I filled it with water. I don't see a way to get a flange on the nose of the rib other than using a second piece. I was thinking of brazing the flange on a flat rib, but ok not so sure on that. Maybe I should try some fuel safe rubber material.

    These are not part of the plans, and the Hummel Facebook group is not one I trust for these types of question.
     
  6. Jan 27, 2020 #6

    Hot Wings

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    Fuel containment is not an area I feel comfortable experimenting with. The BD-5 method is not one I would have ever decided to try - but it was in the plans. If I were to try something like this on my own I'd be very motivated to do the whole part 23 fuel tank pressurize and shake testing.

    Something closer to a conventional formed, riveted, and Pro-Sealed rib would cause me less anxiety.
     
  7. Jan 27, 2020 #7

    TFF

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    If you do build tube fuel tanks, they un-port if in a spin. The fuel also rushes outboard making it harder to stop the spin. Regular flying is no issue. AA1 has an AD added because it was a problem; no intentional spins. The AA5 went to conventional tanks. Two friends just bought the rights to an extended range tank for the AA1.

    Making the holes bigger is probably not as big an issue as having the weight of the tanks sitting in the ribs. I would probably have doublers or thicker ribs where the tanks sit.

    If not a fiberglass tank or one with a bladder, they have been assembled with sealant. Some better than others.
     
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  8. Jan 27, 2020 #8

    12notes

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    Wing fuel tanks just refill the header tank, so un-porting in a spin isn't much of a concern, and Hummelbirds aren't spin tested, so it wouldn't matter that the engine stopped if the spin is unrecoverable.

    I just looked up the weight of the thinnest aluminum tube I could find, and 4" diameter .049" 6061 tubes with caps will add about 10 lbs in weight (+3% empty weight!) and $500 in cost to the plane, so it's not a good option anyway. Back to sealing what I've got then.

    Conventional forming the rib leaves no flange at the very nose, and an extremely small flange for the few inches around the nose. Also leaves a gap at the corners of the back flange from spar to skin. This is why I tried this style of rib flange: https://www.homebuiltairplanes.com/...ordinary-plane-build.24786/page-5#post-374248 . It did a great job of sealing the nose, and only leaked at the back corners. Maybe I'm underestimating what kind of gaps the sealer will seal, I did a rather quick and dirty version of the rib for testing, and didn't have as many rivets as it probably needed.

    I'm still curious about enlarging lightening holes, not for my project, but as a general principle.
     
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  9. Jan 27, 2020 #9

    pictsidhe

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    If you are stil curious...
    Either flange the holes or use cheapy's method. Then proseal the ribs to the tube. That will be stiffer and stronger than the original.
     
  10. Jan 28, 2020 #10

    Riggerrob

    Riggerrob

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    Two suggestions: flanges and extra ribs.

    Tools for flanging lightening holes are readily available. How much does Aircraft Spruce charge?
    Can you rent those flanging tools?
    There are a few threads about hammering your flanges around MDF blocks.
    Even fancier is hydraulically forming flanges in one shot with those MDF form blocks clamped into a hydraulic press. Pumping on a bottle jack applies even pressure.

    The other question is whether ribs with larger lightening holes will be strong enough. Try the same solution as the Sonerai aerobatic wing by adding a few extra ribs and spacing them closer.
     
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