Use of canned expanding foam to support aluminum hard lines

Discussion in 'General Experimental Aviation Questions' started by Wayne, Feb 22, 2018.

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  1. Feb 22, 2018 #1

    Wayne

    Wayne

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    Folks,
    I'm running the aluminium hard lines for my Zenith Cruzer, with a UL Power FADEC engine. The engine requires a supply and return for each tank.

    There is a nice, but tight, channel that makes up the rear sill of the pilot and passenger door and I'd like to run the lines inside that for protection and visuals.

    Given that the channel is structural, and has a cover that gets riveted on, I am wondering if I can secure the fuel lines at the top and bottom of the approx 3 foot run with adel clamps and then use expanding foam to hold them in-situ as they run down the channel. There are a couple of areas that have rivet tails so I could also make sure the lines won't touch the rivets by making sure they are bent properly and then foaming them.

    I'd foam then after the foam cures would trim the excess foam off and rivet the lid on.

    If I don't foam I'll have to secure the lines with adel clamps inside the channel which means I'll have to drill holes in the channel and probably wrap the lines in fuel hose to avoid any possibility of chafing.

    I can see some cons - foam degradation would leave the lines vulnerable and perhaps moisture could get trapped under the foam and increase the risk of corrosion.

    What do you think? Maybe I'm missing a better way to do this?

    Wayne
     
  2. Feb 22, 2018 #2

    recmob

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    I don't have an answer for a better way to do it, but I would not using expanding foam. Expanding foam absorbs humidity and against the aluminum I'm sure would cause corrosion from the inside.
     
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  3. Feb 22, 2018 #3

    cheapracer

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    Wayne, I have used expanding foam for numbers of things since I was a kid, and you would be advised against it.

    It's surprisingly heavy, it holds moisture out of the can, and even fuel fumes will eat at it. Fuel will disintegrate it instantly into a horrible muck.

    Cutting and manually fitting some fuel proof hard or soft foam would be a better solution.

    If you want to inject a liquid, why not corrosion proof silicone rubber?
     
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  4. Feb 22, 2018 #4

    Toobuilder

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    Agree with all. It's great stuff in general, but as a permanent part of structure it is a mess for all the reasons listed so far.
     
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  5. Feb 22, 2018 #5

    Victor Bravo

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    Use several small blocks of EPP (expanded polypropylene) semi-rigid foam. This stuff is very easy to cut, and you can poke a small hole in one side of each block so the rivet tail holds the block in place against sliding up or down the channel. Cut a slot or gouge a channel through the middle of the block to hold the fuel lines. The foam blocks will prevent the fuel lines from abrasion by the rivet tails. The blocks are removable for inspection or replacement. EPP does not absorb much moisture and I believe it is non-corrosive.

    Another option is to put an outer layer of rubber on the aluminum fuel lines themselves. Automotive generic fuel line (rubber with embedded fiber) is cheap and will take a lot of abrasion. If you use thick enough rubber tube, the rivets can eat through the entire tubber tube at one location, and the rubber tube between the rivets will still prevent the fuel line from moving far enough to contact the rivet tails.

    EDIT: Rubber donuts made from the automotive rubber hose. Cut a slit down one side of the hose, then cut donuts off of the hose with a band saw. Size the donuts so that they hold the hose away from ever touching the rivets, but you don't have to carry around a pound or two of hose.
     
    Last edited: Feb 22, 2018
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  6. Feb 22, 2018 #6

    BoKu

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    One other thing that both Stan Hall and I independently discovered about squirt foam: The inconsistency with which it expands is exceeded only by that of the elapsed time before the expansion stops. I've had some set firm after five minutes. I've had other batches that continued slowly swelling for a week, popping open the structures they were intended to stiffen. For that reason and the others raised in this thread, I never use it in flight articles, and use it only sparingly for other shop applications.

    I believe Stan used some of the stuff as core foam for his Ibex (beautiful glider, BTW!) ailerons. As I recall, he made conventional plywood upper skins and ribs, filled the bays with squirt foam, waited for the foam to stop expanding, then sanded the foam flush with the bottoms of the ribs before gluing on the lower skin. It's in the _Collected Works_, and I recall the tone being not necessarily negative, but at least ambivalent about the whole adventure.

    --Bob K.
     
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  7. Feb 22, 2018 #7

    narfi

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    There must be different types of expanding foam?

    I haven't messed with any of it, but will this spring on the boat I am building.
    I feel like there must be a difference between the cheap squirt can of window sealing foam off the shelf at the corner store, and two part mix yourself closed cell polyurethane foam?

    I haven't studied anything with the canned stuff, but do know you can get the polyurethane in different densities, and that you really need both of the two chemical parts, the structure applied to, and the air temperature to all be above 80f for uniform expansion.
     
  8. Feb 22, 2018 #8

    Kyle Boatright

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    I've never used the 2 part stuff, but others who have seem to complain about the same problems exhibited by the canned stuff.
     
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  9. Feb 22, 2018 #9

    narfi

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    3ft, inside a channel. Do you really need to secure it?
    Foam or any other stuff will add weight even if not much.
    Could you just wrap the line in firesleeve and secure it at each end? It seems this would prevent any chafing issues, and would make any maintenance or replacement easier down the road.
     
  10. Feb 22, 2018 #10

    BoKu

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    I really like the idea of running the lines inside the channel to protect them from abuse. But the fact that the channel gets riveted closed makes it kind of awkward.

    If I was to contemplate proceeding with that plan, it would be with the idea that whatever goes into the channel is going to stay there for the service life of the aircraft, without requiring any service or repair. That calls for the use of high quality materials and high standards of workmanship. I would strongly consider:

    * Securing the lines with high-quality red-pad (not hardware store) Adel clamps at relatively close intervals.

    * Arranging the lines so that there are no joints or fittings within the channels, and so that the lines within the channels cannot be damaged or distorted by abusing whatever sticks out of the channels.

    * Making an exception to the rule above to include AN bulkhead fittings for each line at each end of the run. If I did that, I would pressure-test the lines and fittings before closure.

    * Using an epoxy primer on the outside of the lines to inhibit corrosion from moisture trapped within the channels.

    * Adding inspection holes to the channel just large enough to peek in and spot check for corrosion, contamination, or leaks.

    --Bob K.
     
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  11. Feb 22, 2018 #11

    Victor Bravo

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    +1 on Bob's ideas, especially the ones above.
     
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  12. Feb 22, 2018 #12

    Himat

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    Yes, there are different qualities of expanding foam. The squirt can variety seem to vary among other factors between manufacturers, production batch and moon phases.

    The two part industrial kind can be had in reasonable well defined qualities with known physical and aging properties. In some cases the density of the foam can be manipulated if the foam can expand free or if expansion is restricted.
     
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  13. Feb 22, 2018 #13

    proppastie

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    If you can attach the cover with screws and nut plates, it might work out better all around, as regards inspection, maintenance and maybe even installation.
     
  14. Feb 22, 2018 #14

    Victor Bravo

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    Unfortunately, I believe he is referring to a structural skin, which is part of a structural channel that essentially becomes the rear upright member of the cabin and wing attach primary structure. If I am correct that this is a primary load path, it is absolutely possible to convert this to a removable skin, but it will require quite a bit more than just substituting screws for rivets. You're talking structural nut-plates for fine thread machine screws, AN solid rivets in the nut-plates, reamed holes in the skins and channel, precise alignment, etc. Definitely not rocket science, but hours of work.

    I might be 100% wrong about the structural importance of this channel, but the OP would have to know this one way or another for certain before making the shin removable. With no disrespect at all to proppastie, I would bet that a removable cover is it not justified for a fuel line like this.
     
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  15. Feb 23, 2018 #15

    pictsidhe

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    Globs of micro?
     
  16. Feb 23, 2018 #16

    Toobuilder

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    If this hard line is going to be buried forever, make it stainless (not hose, hardline). The slight weight penalty will be offset by much higer abraision and corrosion resistance. Pretty close to an "install and forget" thing.
     
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  17. Feb 23, 2018 #17

    cheapracer

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    Yup, that's what I tried last year, made a test thin skinned and thin ribbed aileron, then filled it with expanding foam for stiffness.

    The weight was laughable, quite the surprise. Then there was an internal section where the foam obviously created an air bubble and didn't fill, things you never know when doing it blind.

    Anyway, if you don't try you never learn.
     
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  18. Feb 23, 2018 #18

    Wayne

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    Holy smokes - all great info guys, and I get your points. So no foam and I'll use adel clamps with corrosion protection. I have been using Cortech thus far so will use that as it is the same as the rest of the plane. The cover is pop riveted on so if I leave a hole to see any corrosion and find any it won't be that hard to remove, fix the issue, and put it back on. I'll grab a picture of the area this weekend. VB knows exactly where it is - he has a lot of knowledge of this aircraft type.

    I'm really grateful for the help!
     
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  19. Feb 23, 2018 #19

    Aesquire

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    I've used the two part foam for packaging equipment in wood boxes. Build box. Foam bottom. Cover with tough plastic film. Place item on plastic. Tuck plastic down into foam at wall of box. Cover item with plastic, up over top of box. Spray foam over item & film. Fold film over foam. Nail lid on box. Sit on box & watch for box to explode. Add labels. Ship. The top foam slab pulls out easy revealing item.

    There's some experience involved in using not too much evil foul smelling foam. We used Devilbis professional spray guns wbich are far better for big applications. ( than a can ) you can see the liquids hit and foam. Still a tricky material to make parts out of because of density variation.

    A buddy filled his hollow core wood front door with Great Stuff brand foam and it burst. Now they have minimally expanding versions that work better for that application.

    However, "donuts" of automotive fuel hose, stainless hard line, and Adel clamps in channel at each end should be bullet proof. You want there to be zero movement of the line possible. Vibration will eat hard & soft metal where in contact. Structural parts, like your idea or engine mounts need to be rub-free.
     
  20. Feb 23, 2018 #20

    TFF

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    Just remember permanent does not nessisarily mean forever in aviation:). I have had to fix plenty of permanent hydraulic lines on airliners. Lines that were never meant to have to be fixed. If possible, I would make it so you can cut the ends off and slide in and out the tubes. If really not possible, build a spare tube in there that you can uncap.
     

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