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Thread: Fuselage questions

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    Fuselage questions

    Hi there,

    Currently I am rebuilding a 1958 PA18. The fuselage has been media blasted and epoxy primed, no corrosion found!

    But we will be replacing a piece of longeron in the tail and one crosstube which are bent.

    For the lower longeron I have been searching what steel has been used, as I have heard that for $$$ savings Piper used 1020 mild steel from the rear seat back. Also I found out in this forum that all longerons have a wall-thickness of .035"... I have been measuring but I have .049". Anyone know this could have been standard? Is it correct that I can just use a 5/8 tube liner inside?

    For the 3/4 x .035" tube.. which size liner tube can I use?

    Of course all will be welded by a proffesional company.. but I have to order parts

    Then next question... I also contacted Univair but they seem very slow with replying these days or just reply not at all...

    I will be replacing the wooden stringers with metal ones... which p/n's do I order and how many?

    Thank you for the help guys..

  2. #2
    Steve Pierce's Avatar
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    The older frames had a lot of mild steel. We use 4130 to repair. Google search "longeron liner tubes site:supercub.org"
    https://www.supercub.org/forum/showt...ongeron-Tubing

    Be sure and look behind the U channel welded over the vertical tube in front of the right side D window. Pictures in this post. https://www.supercub.org/forum/showt...ge-Tubes/page2
    Steve Pierce

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  3. #3
    Crash, Jr.'s Avatar
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    5/8 od .035 wall tubing is what I show for the liner tubes for the longeron 3/4 .035 tubing.

    Here's the stringers from Univair: https://www.univair.com/categories/r...xtrusions.html Small ones go on the sides and the large ones go on the top of the tail. I forget which size is on the belly but I'm pretty sure its the large size. Measure your fuselage to get the lengths and don't forget to add a bit extra so you can taper the end of the stringer.

  4. #4

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    Be wary using the smaller aluminum stringers. I wrecked my very first PA12 covering job using them for the shoulders instead of wood (or all large aluminum ones) because they couldn’t resist the forces of the fabric shrinking. I’ve used wood stringers since.
    Last edited by Paul Heinrich; 10-27-2021 at 08:48 PM. Reason: Clarify the aircraft I screwed up covering was a 12

  5. #5
    Steve Pierce's Avatar
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    5 large stringers and 2 small. The top three and the bottom stringers are the large ones and the two stringers between the top and bottom longerons are the small ones. They are safety wired to the tubing.
    14978-00 is the bottom stringer.
    14983-03 top center.
    14984-02 top left and right.
    14979-00 left and right side 157".

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  6. #6

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    I am with Paul. I used Douglas Fir in 1969 to replace the factory Spruce scalloped stringers, and used safety wire and glue, instead of those horrible clamps. Half a century later, straight as an arrow.
    Aluminum is great, but if you do a cost/benefit analysis, wood wins.

  7. #7

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    Well, I am finished with the fuselage welding repairs, now I'll be ordering all the bigger parts, and so the stringers as well..

    They aren't available at Univair anymore!! Does anyone know a source for the aluminum stringers?
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  8. #8
    Steve Pierce's Avatar
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    Have you spoken to Univair?
    Steve Pierce

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    Quote Originally Posted by Steve Pierce View Post
    Have you spoken to Univair?
    Nope, but they took the stringers from their website

  10. #10
    Steve Pierce's Avatar
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    I think they are listed under a raw material part number. Have to look in my notes when I get back to the hangar.
    Steve Pierce

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    wireweinie's Avatar
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    I just looked them up and found them on the website. Try a search for 'Piper stringer'.

    Web
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    Quote Originally Posted by wireweinie View Post
    I just looked them up and found them on the website. Try a search for 'Piper stringer'.

    Web
    still can’t find the right stringers, were you sure?

    @Steve:
    thanks for your advice to look under the U-profiles. Both were corroded and one badly, will post a picture but can’t do this on my phone

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  14. #14
    RaisedByWolves's Avatar
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    Quote Originally Posted by Paul Heinrich View Post
    Be wary using the smaller aluminum stringers. I wrecked my very first PA12 covering job using them for the shoulders instead of wood (or all large aluminum ones) because they couldn’t resist the forces of the fabric shrinking. I’ve used wood stringers since.
    Oh man how did piper manage to do it to probably thousands of cubs from the factory?


    Sent from my iPhone using Tapatalk
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  15. #15
    Steve Pierce's Avatar
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    Univair still sells stringers. They took the raw materials off their website.
    1232-120 and 1232-162 larger stringer material that fit in the fuselage clips.
    1231-157 are the small side stringers that are safety wired to the tubes.
    Steve Pierce

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  16. #16

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    Tubing chart

    Clarity a bit off, sorry. May find this useful
    Forget about 1020/1025 tubing. 4130 for everything. Stronger, available, better, no weight difference.

    FWIW….Stoddards Aircraft in Anchorage stocks all sizes of the stringer material. They keep a pretty good supply. They may get it from Univair but it’s available.

    If it’s possible for you, experimental perhaps….Wayne Mackey uses or has used 6061T6 aluminum tubing for stringers, saw them on Jon Bush’s SQ12 at Valdez. The stringers on the sides I saw were 3/4” OD. Stout for sure.

    Oz
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    Last edited by OzAK; 12-14-2021 at 01:58 PM.

  17. #17
    nanook's Avatar
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    Hey Oz or Steve, why couldn’t you replace the Piper aluminum stringers with 6061T6 tubing legally, on certified pipers? The 6061 is as strong or stronger and less prone to bending under load. Probably cheaper than paying Univair’s extortion prices...

  18. #18
    Steve Pierce's Avatar
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    You don't think the tubing would be more prone to getting bent than the I beam to extrusion?
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    nanook's Avatar
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    The big I beam is not bad, but still a little floppy. The small I beam is a joke. The thick walled 6061T6 is the strongest and straightest.

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    From both points of view, excellent questions. That’s one that’s going to have to be answered I guess. Part of that might be like the apples to oranges thing. Also there’s a point where the strength to weight over the given length where it has to be strong enough to support the design load ie the pressure of shrunken fabric over the distance of maximum span between fuselage members.
    The stringer extrusions are made of what alloy? That will answer part of it.
    When I hit the house tonite after work I’ll dig out a hunk of each size of stringer plus refer to some notes I think I know exactly where they are where I weighed a foot of each plus some tubing weights of a couple different wall thicknesses.
    Heres a couple photos of the interior left side of Jon’s SQ12 where you can see the tubing Wayne used.
    Right by the flap handle you can see the wall thickness is .035. Compare the tube to the fuel lines right there that we all know to be 3/8” OD and that puts that stringer tube at at least 3/4”, maybe even 7/8”.
    I’ll see what I can come up with for a “test” tonite and report back.
    Yes, I’m kinda nuts about this material strength and substitutions stuff. We run into this a fair amount here at work. I typically end up being “The Guy” that figures it out one way or another and giving the green light to keep things rolling.
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  21. #21
    nanook's Avatar
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    The biggest problem with the Piper intrusion is keeping it straight. The round tubing will resist rolling over. The tall intrusion not so much. The small intrusion structurally is weak and tends to wander. Seems you could substitute the 6061T6 for the intrusion on the basis of being as strong or stronger and more aesthetically pleasing...
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    Quote Originally Posted by nanook View Post
    The big I beam is not bad, but still a little floppy. The small I beam is a joke. The thick walled 6061T6 is the strongest and straightest.
    Nook, I completely agree. I’m building a scratch built -12 clone and have already determined that I WONT be wasting any money on the I beam stringers, especially not the small size. I’ll bring a definitive answer to this table tonite or tomorrow for you experimental guys at least. I think the Ibeam material is either 3003 or 5052 alloy, both quite inferior strength wise to 6061.
    Everyone is after a material that won’t deform under the shrinkage load of their fabric install and I think most guys would agree that something stronger and at least as light if not lighter that will take a hit now and then from a leg or some sort of other bump would be a bonus. Everything has a limit but as you say, better would be great. Also, gotta agree wholeheartedly about being extorted. There are plenty of participants here I’m sure that have much deeper pockets than mine but no one wants anything but a fair shake when it comes to their parts.

  23. #23
    Steve Pierce's Avatar
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    I was thinking small size tubing. That makes a lot of sense with the large diameter stuff.
    Steve Pierce

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    Quote Originally Posted by Steve Pierce View Post
    I was thinking small size tubing. That makes a lot of sense with the large diameter stuff.
    I think you’re spot on on that one Steve. The definition of “small size” may differ between us but with that I think, say, 3/8” or less as small. I’m going to get a couple of pics tonite of what I think is my solution for attaching this round tubing to the fuselage using exactly the same fastener for holding it on, safety wire, and standoffs that can be made any length if need be.

    Oz

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    Quote Originally Posted by Steve Pierce View Post
    Univair still sells stringers. They took the raw materials off their website.
    1232-120 and 1232-162 larger stringer material that fit in the fuselage clips.
    1231-157 are the small side stringers that are safety wired to the tubes.
    Thanks Steve, but why can't I still find it when I enter these part numbers then?

    I really like the aluminum stringers and don't want to put wood back in

  26. #26
    Steve Pierce's Avatar
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    Quote Originally Posted by bertievdbunte View Post
    Thanks Steve, but why can't I still find it when I enter these part numbers then?

    I really like the aluminum stringers and don't want to put wood back in
    Because they took the raw materials off the website. People were trying to buy 80 sheets of aluminum and huge amounts of tubing since they couldn't find it elsewhere. Call them or shoot them an email.
    Steve Pierce

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    Steve Pierce's Avatar
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    Quote Originally Posted by OzAK View Post
    I think you’re spot on on that one Steve. The definition of “small size” may differ between us but with that I think, say, 3/8” or less as small. I’m going to get a couple of pics tonite of what I think is my solution for attaching this round tubing to the fuselage using exactly the same fastener for holding it on, safety wire, and standoffs that can be made any length if need be.

    Oz
    Yep, to me small is 3/8" and large would be 3/4" or bigger.
    Steve Pierce

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    Quote Originally Posted by Steve Pierce View Post
    Because they took the raw materials off the website. People were trying to buy 80 sheets of aluminum and huge amounts of tubing since they couldn't find it elsewhere. Call them or shoot them an email.
    Ahh, that makes sense yes! Already e-mailed them few days ago, I think I will call them in a few hours.. thanks

  29. #29

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    Quote Originally Posted by Steve Pierce View Post
    Yep, to me small is 3/8" and large would be 3/4" or bigger.
    Ok, here’s what I have, fwiw. Nothing real scientific here but measuring my fuselage the longest span between tubing members where stringers are held on is 22” so I used that. Laying various sizes and wall thicknesses of 6061 T6 tubing across a couple blocks of wood on the bench top and applying weight to each, after using a Piper aluminum stringer (large size, the small stuff just doesn’t make it) for comparison. The Piper stringer is sufficiently strong but tries to lay over under a load of roughly 30 pounds. Obviously adequate because lots of it in use on lots of airplanes. We’re looking at alternatives here and maybe this should make its way to the experimental part of the forum but with good materials such as certified, documentable aircraft quality tubing, maybe this can help someone out, if only to provide a more cost effective alternative and quite possibly stronger AND lighter to boot.

    Moving along…..
    For comparison purposes (I blame Bill Rusk for this, turning me into a weight geek freak , the Piper stringer material weighs 43.7 and 23.294 grams per foot, large and small respectively. All tubing listed below is 6061.
    28.35 grams = 1 ounce

    .035 x 3/8” ~20 grams/foot Not considered, not strong enough
    .049 x 3/8” 26.66 grams/foot Holds up far better than the small Piper stringer, no deformation at 30#
    .035 x 1/2” 26.3636 grams/foot No deformation at 40#
    .049 x 1/2” 35.44 grams/foot No deformation at 69# and still lighter than the Piper stringer
    .058 x 1/2” 40.35 grams/foot. No deformation at 81#, still lighter…
    .035 x 3/4” 42.53 grams/foot. Didn’t get a weight on this size because I didn’t have any. Safe to say it would hold 100#+. Still lighter than Piper stringer
    .049 x 3/4” 55.66 grams/foot. Over the Piper weight but ROBUST. You could push on this hard and it’ll take it.
    It supported my full body weight (195#) with my feet off the floor, No deformation.

    Maybe this will help someone with something. Seems to work for Wayne, see photos above. Not exactly sure how he attaches it. I’m using 3/8” x .049 and 1/2” x .049 on my build, will be plenty robust.
    Pics included of a mock-up to show how I’m attaching the 1/2” or larger. Pair of #40 holes in the stringer tube for the wire to wrap through. That’s .032 wire, .041 might be better but always harder to work with.

    Regards, Oz
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  30. #30
    nanook's Avatar
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    Thanks OZ! Good stuff…
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