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Thread: Fuselage advice

  1. #1

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    Fuselage advice

    On read that one particular company is building the pa18 fuselage entirely from 3/4 4130 with the reason given that the factory use to build front are rear in to separate area then slide them together. Not sure if this is true however in my area I can source 4130 3/4 .058 easily where .035 would have to be shipped and is 3 times the cost. My question is would it be feasible to build the entire fuselage less firewall forward from 3/4 x .058 ??? I know there are many variables but any advice would be greatly appreciated. Cheers Des

  2. #2

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    Have you figured out how much more it would weigh? That would be the only problem, I would think.

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    www.SkupTech.com mike mcs repair's Avatar
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    Don’t do it!!! That would be way heavy !!


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  4. #4
    RVBottomly's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    Don’t do it!!! That would be way heavy !!


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    I agree. Just a quick calc shows it could be 1.66 times as heavy.

    If a regular bare fuselage is around 100+ pounds, that's a lot of added weight for no benefit.
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    skywagon8a's Avatar
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    DJT, Is this to be a replacement for a certified plane? Or is it to be a homebuilt? If it is to be a certified replacement, it would be difficult to get it approved since it doesn't comply with the approved drawings.
    N1PA

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    RVBottomly's Avatar
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    Quote Originally Posted by DJT View Post
    .035 would have to be shipped and is 3 times the cost.
    What kind of cost? I had tubes for an entire bare fuselage shipped to me by truck for a total cost, including materials, of around $1300.

    Granted, that was three years ago.
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  7. #7

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    Quote Originally Posted by skywagon8a View Post
    DJT, Is this to be a replacement for a certified plane? Or is it to be a homebuilt? If it is to be a certified replacement, it would be difficult to get it approved since it doesn't comply with the approved drawings.
    Would be build as an experiment Aircraft in Brisbane Australia.
    was concerned weight would be the biggest issue. I rough calculated an extra 40kg but justify this in my head that the aircraft would be stronger and using a modern higher HP engine would compensate.

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    $10 per meter locally $21 per meter plus freight from a source 1200km away

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    Why is .035 so expensive? I find it hard to believe that there would be that much of a difference.
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  10. #10
    wireweinie's Avatar
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    40 kg is a lot of extra weight for that airframe.

    Web
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  11. #11
    skywagon8a's Avatar
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    This sounds like a project if you have an engineer friend or have the capability yourself to do tubular truss calculations? Perhaps you could change the layout of the tubing with the result of reducing the tube count and thus some of the excess weight?
    N1PA
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    I think there is a lot more 5/8x.035 than there is 3/4x.035. If you upsize not only the wall, but also the diameter, you are going to have a very heavy and out of balance fuselage.


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  13. #13
    RVBottomly's Avatar
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    Quote Originally Posted by DJT View Post
    $10 per meter locally $21 per meter plus freight from a source 1200km away
    Astounding. You probably have looked all over, but I saw this:

    http://racetechsteel.com.au/4130-chr...9mm-wall-round

  14. #14
    fobjob's Avatar
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    Simplicate and add lightness...Geoffrey DeHaviland
    Although, by starting out too heavy, you could probably avoid a weight spiral, and maybe even run one negative...

    Although, if I was wanting to increase tubing thickness, it would be the lower longerons from the firewall to the baggage compartment area.
    Last edited by fobjob; 01-11-2021 at 10:15 AM.
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  15. #15
    Hardtailjohn's Avatar
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    I thought the biggest strength gain per pound was by going up in diameter, not wall thickness?
    John
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  16. #16
    fobjob's Avatar
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    Yep, but that introduces fit problems....

    Actually, tensile/ yield point depends on cross sectional area of the steel itself, so it can go either way between diameter or wall thickness. Bending or compression strength improves more with diameter than wall thickness.
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  17. #17
    aktango58's Avatar
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    The reality is that there are many 1950's frames flying to this day, many have been wrecked. Other than the x brace over head, box in the tail and maybe an up gross kit they are the original.

    Beefing up just to beef up really makes no sense. Protecting your head, and keeping lift under the wings will do more than using heavy duty steel in the frame. My thoughts only
    I don't know where you've been me lad, but I see you won first Prize!
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  18. #18
    aeroaddict's Avatar
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    Not to be pedantic or a buger, but cross sectional area has no effect on Ultimate tensile strength of the material nor it's yield point where permanent deformation occurs. Ultimate and yield are properties of a particular alloy and how it has been manipulated (rolled, cast, forged, cold working, hardening, tempering,....).

    It is true that a greater cross section will take more force to break, but the material will still break/yield at the alloys metallurgical properties.

    When talking about 'bending' moments on tubes, diameter and wall thickness play the only roles of hoop stresses of a particular metal. It is difficult to say which has the greater effect; diameter or wall as both come into to play with a given alloy, but yes in general, larger diameter will resist bending moments better. This is why aluminum mountain bike frames are usually large tubing with thin walls. But the old steel bike frames were smaller diameter. Again, very different properties between the aluminum and steel.
    Last edited by aeroaddict; 01-13-2021 at 05:02 PM.

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    Gordon Misch's Avatar
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    I agree with paragraphs 1 and 2, but don't understand how hoop stress plays into a tubular steel truss?
    Gordon

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  20. #20
    aeroaddict's Avatar
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    I cannot explain the complex load dynamics of a truss; tension, twist, bending, etc..


    But you are correct, without a pressure gradient, hoop stress is not a factor. I guess the place where it might come into play would be on engine mounts, but again, probably not much thermal exposure there.

  21. #21
    Hardtailjohn's Avatar
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    Ok, so on a longeron, is diameter or wall thickness going to get you the "biggest bang for the buck" or biggest strength gain for the weight gain?
    John

  22. #22
    Gordon Misch's Avatar
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    For any bending, buckling or torsional loads, diameter. Pure tension or non-Euler buckling compression, just the cross sectional area of the metal.
    Gordon

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  23. #23
    PerryB's Avatar
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    A heavy airframe compensated for with a heavier engine equals a heavy airplane. It's power-on performance will obviously equal a stock built machine, but it's low speed, power off performance will suffer. Too many Cubs get turned into sleds.
    After Monday and Tuesday, even the calendar says WTF !

  24. #24
    aeroaddict's Avatar
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    What Gordon and Perry said.

    IMO, I think Perry sums (no pun intended) it up best; light planes will have better 'cub' performance.
    Last edited by aeroaddict; 01-15-2021 at 11:51 AM.
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