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Thread: Engine mount tube thickness?

  1. #1

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    Default Engine mount tube thickness?

    I have been looking for info on the early model kitfox motor mounts and i know the tube diameter is 1/2" but don't know what the thickness is? Anybody have that info?

  2. #2
    Senior Member HighWing's Avatar
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    Default Re: Engine mount tube thickness?

    Not sure on the original mount, but I made my own mount on my latest Model IV and used .049" from Aircraft Spruce.
    Lowell Fitt
    Cameron Park, CA
    Model IV-1200 912 UL

    My You Tube Channel

  3. #3
    fathom's Avatar
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    Default Re: Engine mount tube thickness?

    Where did you find plans to fabricate your mount? Or did you reverse engineer based on an existing one?
    -David
    Building Classic IV
    https://carrieraero.wordpress.com/

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    Default Re: Engine mount tube thickness?

    Quote Originally Posted by HighWing View Post
    Not sure on the original mount, but I made my own mount on my latest Model IV and used .049" from Aircraft Spruce.
    Thanks for the reply. I made an assumption perhaps incorrect that since the spools are 3/8" and most of the tubing in the mount is half inch that a wall thickness of .065 would allow the tubes to be welded to the spools without any gaps.

    By spools i mean the little 1" long 3/8" outer diameter .26 inner diameter tubes that the bolts go through to bolt the mount to the airframe.

  5. #5
    Senior Member HighWing's Avatar
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    Default Re: Engine mount tube thickness?

    First, My mount was a custom mount to correct some W/B issues because of the gap closures and fairings on the elevator and rudder. I had about 7 lbs back there that at eleven feet seriously compromised my baggage weight allowance. I wanted the engine a couple of inches forward of where it normally is. I understand the thinking behind the thickness - welding issue. I used TIG and it helps a lot getting penetration thin to thick.

    The fabrication was pretty much fit and weld. Using an engine stand, I positioned the engine where I wanted it relative to the fuselage and essentially closed the gap. For posoitioning, I used a high lift transmission jack from Harbor Freight with some modifications for adjusting alignment to support the engine and wood supports to position the fuselage. I made my own doughnuts for dampening and used a ring mount on the engine. With the extra spacing it worked, but with coolant hoses and other items needed behind the engine, a ring mount would be very difficult if not impossible, using the standard Model IV dimensions.

    Searching through my build photo collection, I can't find anything that might help design wise. I did find one picture that I used to get a close alignment with regard to vertical placement and longitudinal alignment. That picture is attached. Center Line copy.jpg
    Last edited by HighWing; 03-07-2019 at 01:30 PM.
    Lowell Fitt
    Cameron Park, CA
    Model IV-1200 912 UL

    My You Tube Channel

  6. #6

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    Default Re: Engine mount tube thickness?

    How did you make your own donuts? What material etc?

    Also have some questions about annealed vs normalized 4130. From what i can see ACS 4130 steel plate is annealed, and i haven't looked into it for sure but I think the round tubing might be normalized. I'm not going to weld the mount personally i'll have someone else do that but if some of it is normalized and some is annealed what then? Even if it is all normalized do you have to heat treat it after?

  7. #7
    Senior Member jrevens's Avatar
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    Default Re: Engine mount tube thickness?

    Hi Will... FWIW, normalized 4130 is considerably stronger than the annealed condition. Many welded steel aircraft structural components are fabricated from 4130 N, but it is not normally re-heat treated after welding. It is usually good practice to stress-relieve a structure that has been arc welded (TIG or MIG) after welding, usually with an oxy-acetylene flame. This seems to be ignored by some fabricators. The need to do that varies with the design of the structure.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE

  8. #8
    Senior Member HighWing's Avatar
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    Default Re: Engine mount tube thickness?

    Quote Originally Posted by FoxbuilderWill View Post
    How did you make your own donuts? What material etc?
    I used urethane rubber - the type used for making rubber molds. It comes as a two part liquid that cures after mixing. I had to make molds for getting the shape I needed.
    I dont remember the shore hardness I used.
    Lowell Fitt
    Cameron Park, CA
    Model IV-1200 912 UL

    My You Tube Channel

  9. #9
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    Default Re: Engine mount tube thickness?

    Quote Originally Posted by jrevens View Post
    Hi Will... FWIW, normalized 4130 is considerably stronger than the annealed condition...
    Agree with everything John says above. Any good welder who knows his stuff will also be able to stress relieve by flame heating the welded area and then using chalks/colour charts to determine temperature levels required to stress relieve. In 2016 while at Oshkosh, I asked one of the demonstrators at one of the weld workshops whether they stress relieve after welding. Simple answer was no, which was unfortunate because that was a conversation stopper. I don't understand why some don't do this. The inbuilt stresses resulting from the welding process can lead to grief later on if you don't stress relieve. I only deal with a particular qualified aircraft welder at my airfield because I know he does this as standard practice.FWIW.
    David
    SS7 Builder

  10. #10

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    Default Re: Engine mount tube thickness?

    Ok thanks for the replies.

    New question, similar topic. I have the motor off my bird and have a nice flat leveled table that I set the motor mount on and put a framing square with the short side flat on the table with the long side sticking up see pic so that i could measure the thrust line angle. According to my slightly rough measurements the thrust line is angled up .5 degrees. Does this sound right to you guys? Mount came off a model 1 kitfox that as far as i know was factory standard for all rotax equipped aircraft. There was a 532 on this mount when i bought the plane. 20190313_184709.jpg

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