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  1. #1
    BJ64's Avatar
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    How To Build RC Jet Turbine Exhaust Tubes

    Hi Guys'n'Gals

    I've started this thread in the hope of pooling together ideas and experience from those who have built RC Jet Turbine exhaust tubes - both single and double-walled.

    Any info you have regarding the initial calculation of the tube length, front and rear diameters, offset between the tube and the Turbine's exhaust, and dimensions of the bellmouth etc. would be great.

    And of course, the actual build - what materials to use, how to shape/form them, how to spot-weld/other kinds of welds etc.

    Pics, pointers to other threads/material - all most welcome...

    Regards,

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  2. #2
    jetrichie's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    I seen some details on the GTBA site once on making a spot welder that might help, also 304 spec stainless is ok 316 would be better.

    Got the guys in my workshop to make one once for a 14lb turbine, made the inner diameter the same as the NGV with the front flared out, length was just made to suit model.
    You make the zig zag spacer between the walls with a strip about 10mm wide and fed through a gear mesh, or just fold with pliers..

    Spot welder is important and makes the job fairly easy, check out GTBA or google on making one.

    no doubt someone will correct me and give a more accurate method of calc dia etc.

    Gap from tailcone to tube is important, 15-20mm seem to be the norm


  3. #3
    DelGatoGrande's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    any one know numbers between:

    engine size=pipe inner diameter=inner tube wall thicfkness?

    ..
    Best Regards George
    www.rcjet.gr \"props are for boats\"

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  5. #5
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    Some excellent links and reading there, thanks Gary

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  6. #6

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    RE: How To Build RC Jet Turbine Exhaust Tubes

    A couple of things, and a further question.

    Wren (Mike Murphy?) published a couple of articles on the augmented thrust of slightly divergent pipes (larger end of the cone towards aft) in RCJI a number of years ago, along with tests of the proper distance from turbine tail cone to bell mouth opening. Their pipes for the smaller ('44 and '54 sized) turbines seem to be largely single walled .007 or .008 SS, for the cone (remains cool anyway) and the straight segment towards the rear (that gets hot). They use a 1.5-2 inch 'extractor' double-walled segment at the exit of the aircraft, with 5-8 mm corregated or waffled SS to separate the outer from the inner part, with outer part extending 5-10 mm aftwards of the inner part. Pipes for larger thrust turbines are not usually divergent, as they presumably don't need the augmented thrust from a venturi, maybe run hotter and need perhaps full length double walls.

    Another good question is does anyone know about the design and construction of double exit pipes-eg F4, and the A6. What thrust losses can be expected compared to single straight back pipes, what angles can be used and how heavy must the SS be to tolerate the heat just aft of the turbine? I think only Tam makes them and might have the experience to comment on them.
    L Turner

  7. #7
    grbaker's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    augmented thrust of slightly divergent pipes (larger end of the cone towards aft)
    The divergent section is only in the very front section of the pipe. The rest of the pipe is the same diameter all the way to the exit.

    The thrust augmenting pipes work well for the larger turbines as well, but the pipe size at the exit end gets too large to fit into some scale planes. I have built a couple of augmenting pipes for a Jetcat Titan and a Merlin 160 with excellent results.
    Gary Baker

  8. #8

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    RE: How To Build RC Jet Turbine Exhaust Tubes

    It was me that did the testing and published in RCJI. The effects of the augmentation principles work equally well on all sizes of our engines running from the Lambert size up to P200 size. The great thing is that the augmentation sucks in a large amount of cool air together with the hot gasses. This cool air runs down the pipe against the wall and keeps it cool. If you are careful and do not have hot starts it is possible to keep a single walled pipe from going any hotter than a straw colour. I did some work on bifurcated pipes and managed to make one that had almost no loss. Most standard straight pipes have a significant measurable loss and bifurcated ones can be much worse.

    John
    John Wright

  9. #9
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    How do you guys fix the middle 'rippled' separation stips between the inner and outer tubes? Spot-weld there too?

    And... if running an Aluminium outer tube, would pop-rivets take the interstitial heat between the inner tube and the outer tube?

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  10. #10
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    How much are you guys overseas paying for the Stainless?

    I just got a couple of quotes today for -.010" (0.254mm) 316 grade stainless - 1000mm x 305mm for around AUS$130 a sheet, or 1000mm x 600mm for $140 a sheet but with a $60 cutting fee, which makes it around $200 a sheet.

    I really need to work out what diameter my pipes would need to be on a 15kg (30lb) thrust Turbine before ordering, as a 100m diam pipe is going to need around 314mm of material, plus overlap - so a 305mm sheet isn't going to work...

    BJ"_
    Ahhhh...I love the smell Kero in the mornings....

  11. #11
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    Are there any charts or formulas around to help you calculate what diameter the front and rear of the exhaust tube needs to be?

    What size the bellmouth needs to be?

    What the distance between the Turbines exhaust cone and the inlet of the exhaust tube needs to be?

    And if you need to run a parallel or tapered tube?

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  12. #12
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    Hmmm...

    Gone a bit quiet in here...

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  13. #13
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    This looks interesting. Might need a set of longer tongs for most pipes.

    http://www.harborfreight.com/115-vol...der-45689.html

    PaulD
    If it\'\'s too LOUD - You\'\'re too OLD

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  14. #14
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    BJ,

    Both of the tailpipes I built used the John Wright thrust augmenting pipe design. They were used for a Jetcat Titan and a Merlin 160.

    They were about 86mm @ the inlet, tapering up to 96mm (at about 180mm from the inlet) and then 96mm all the way to the exit. On a test stand I measured about a 5# increase in thrust (over no tailpipe at all) with the Titan but never tried the Merlin on the stand.

    I'm probably going to build a new tailpie for my JMP T-33, but I won't be able to build a thrust augmenting pipe due to the scale exhaust opening being too small. It would be nice if someone published a chart with tailpipe sizes for various engine sizes.

    Paul, the spot welder shwn is the one I used to weld my pipes. Harbour Freight sells longer electrodes for that welder.
    Gary Baker

  15. #15
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    ORIGINAL: PaulD

    This looks interesting. Might need a set of longer tongs for most pipes.

    http://www.harborfreight.com/115-vol...der-45689.html

    PaulD
    I've got a lil spot-welder gun that I can run using my arc welder transformer.

    It's supposed to do the welds from one side only. I haven't tried it out yet.

    What I am finding, though, how unbelievably difficult it is to find stainless shim material. So far, I've only managed to find 2 sources of 316 grade 0.010" stainless in the whole country. And boy, that stuff ain't cheap - around AU$150 for a 3ft x 1ft piece

    BJ
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    Ahhhh...I love the smell Kero in the mornings....

  16. #16
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes


    ORIGINAL: grbaker

    BJ,

    Both of the tailpipes I built used the John Wright thrust augmenting pipe design. They were used for a Jetcat Titan and a Merlin 160.

    They were about 86mm @ the inlet, tapering up to 96mm (at about 180mm from the inlet) and then 96mm all the way to the exit. On a test stand I measured about a 5# increase in thrust (over no tailpipe at all) with the Titan but never tried the Merlin on the stand.

    I'm probably going to build a new tailpie for my JMP T-33, but I won't be able to build a thrust augmenting pipe due to the scale exhaust opening being too small. It would be nice if someone published a chart with tailpipe sizes for various engine sizes.

    Paul, the spot welder shwn is the one I used to weld my pipes. Harbour Freight sells longer electrodes for that welder.
    That's really great info - thanks Gary!

    Would those dimensions work on a 30lb thrust Turbine?

    (I think the Merlin is 38lbs??)

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  17. #17
    grbaker's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    BJ

    I used the same specs on the pipe I used on the Titan and it was only putting out 27# without the pipe and about 321# with. I suspect it would work well for yours..
    Gary Baker

  18. #18
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes


    ORIGINAL: grbaker

    BJ

    I used the same specs on the pipe I used on the Titan and it was only putting out 27# without the pipe and about 321# with. I suspect it would work well for yours..
    Thanks heaps for the info, Gary

    I'll use those dims to build my pipes with then.

    The 96mm diameter is perfect - the only 316 0.010" stainless I can get at a reasonable price is 305mm wide (by 1m long), so 96mm will need 301.4 of sheet. Which leaves 3-4mm overlap to spot-weld on.

    I'd be more comfortable if the sheet was 310mm wide - just to give me a bit more room to move in, but I might just be able to squeeze it using the 305mm stuff.

    There is one other place that I can get 316 0.010 stainless from, and it's 600mm wide - but they have a AU$60 cutting fee on top of the sheet price, which brings it closer to 200 bucks a sheet. And I'd end up with about 290mm left over that would be next to useless.

    Do you have any suggestions on how to make the bellmouth on the tailpipe at the Turbine end? They look like about a 1cm flange affair...

    Oh, one more question: What distance do you space the start of the tailpipe from the tip of the Turbine's exhaust cone? From a lot of pics I've seen, it looks to be around 15-20mm.

    Thanks again for your help on this - much appreciated...

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  19. #19
    grbaker's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    I had a bellmouth from another pipe that worked for one of them and I modified the carbon fiber tailpipe inlet from a Eurosport for the other.

    You will need to experiment with your pipe to turbine spacing to get the most gain. It will probably be more like 30 to 40 mm.
    Gary Baker

  20. #20
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    ORIGINAL: grbaker

    I had a bellmouth from another pipe that worked for one of them and I modified the carbon fiber tailpipe inlet from a Eurosport for the other.

    You will need to experiment with your pipe to turbine spacing to get the most gain. It will probably be more like 30 to 40 mm.
    Any close-up pics of that bellmouth available?

    I'm trying to get my head around how to design/build one.

    I guess it's not much more than turning up a wood/aluminium blank on a lathe, then panel-beating some stainless shim material into shape using the blank, and finally spot-welding that 'trumpet' onto the front of the tailpipe.

    Again - thanks for your input. It is priceless and much appreciated...

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  21. #21
    BJ64's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    Would anyone have any suggestions on how to build the bellmouth... and what rough dimensions that it should be?

    Cheers

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  22. #22
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    Call Tam, He'll make whatever you want !!!!


    D

  23. #23
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    ORIGINAL: rcjetsaok

    Call Tam, He'll make whatever you want !!!!


    D
    Erm....

    The idea is to learn how to build them myself - seein as I have at least 4 tubes I need for various "projects"...

    BJ
    Ahhhh...I love the smell Kero in the mornings....

  24. #24
    DelGatoGrande's Avatar
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    .
    Best Regards George
    www.rcjet.gr \"props are for boats\"

  25. #25
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    RE: How To Build RC Jet Turbine Exhaust Tubes

    I have made one tube so far, and so far I am very happy with it. It is the augmenting type, and I made it from .008" 304 from McMasterCarr.
    Bellmouths: I once found a formula that said the radius should be internal dia/5. This is what I used, and the proportions seem good. Bellmouth was spun over a mdf plug in the lathe. I use full length lithoplate aluminum outer duct, 4mm spacing at the rear, and 6mm extension beyound the inner duct. I am running a JG100 engine, so the dimensions should be ok with 120 class turbines.

    Lars
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