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Thread: Drilling into spar

  1. #1

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    Drilling into spar

    As I turn to messing with the wings in my project, I'm faced with potentially putting some holes in the spar (e.g. pitot tube bracket secured to rear spar and maybe magnetometer mount). This makes me a little nervous!

    The best discussion I found in this area was a series of posts (beginning here) in Bill Rusk's Javron build thread.

    It sounds like I'm ok with generally drilling into spar in the center of the web (equidistant from top and bottom)

    Is there anything else I should be concerned about here?

    Thanks!
    Sam

  2. #2
    stewartb's Avatar
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    I appreciate your caution and don’t have any “rules” to offer but I drilled my spars in a few places. Every Cub with Atlee Hurricane ties has, too. A few holes won’t threaten the integrity.
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  3. #3

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    Make sure to deburr any holes made.
    LiteCub
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  4. #4

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    I might be a bit different but prefer my pitot tube up front in the jury strut.
    Regards, Charlie
    Super Coupe E-AB build in process
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  5. #5
    skywagon8a's Avatar
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    This is mounted on a rib centered between the two spars. I have flight tested it throughout the speed range from 35 up to 155 and it is 100% accurate. Notice the yarn tied to the pitot tube at the high angle of attack. It is straight back at all Angles of attack. remaining within the recommended +/- 10 degrees to the relative wind. I recommend this location for accuracy.



    Edit: It is also midway between the jury struts and the lift strut wing attachments.
    Last edited by skywagon8a; 09-25-2021 at 09:21 AM.
    N1PA
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  6. #6

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    Quote Originally Posted by Litecub View Post
    Make sure to deburr any holes made.
    LiteCub
    Ream them, most drill bits leave ragged holes, I used to work in a fabrication shop. That built parts for boeing. Tim

  7. #7
    Steve Pierce's Avatar
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    Outboard of the lift strut is not a high stress area.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers
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  8. #8

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    Quote Originally Posted by Steve Pierce View Post
    Outboard of the lift strut is not a high stress area.
    How far outboard, anywhere near the strut attachment, inboard or outboard is the highest loaded area on the wing.
    Regards, Charlie
    Super Coupe E-AB build in process

  9. #9
    Steve Pierce's Avatar
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    Piper's own documentation says you can only splice outboard of the struts. Legend and Dakota Cub wings were tested in DC's wiffle tree with lightening holes outboard the fittings but I don't know how far. Experimental folks should do some testing.
    Steve Pierce

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  10. #10
    skywagon8a's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    Piper's own documentation says you can only splice outboard of the struts. .
    This only means you and I are only allowed to repair outboard of the struts using Piper's instructions. Piper Service Memo 3a is supposed to cover this.
    A DER would be able to develop repairs in other locations.
    N1PA

  11. #11
    Steve Pierce's Avatar
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    Correct, as an A&P/IA I am not going to go drilling inboard from what I see Piper published and what I have learned from talking to Mark at Dakota Cub on the testing of the Super 18 wing and the Legend Cub wing. If I was building an experimental Cub and wanted to go drilling holes I would do some testing. I don't want to be that guy that broke a spar and spiraled down. That sounded very scary to me and I have already survived a midair fall straight down from 100 feet.
    Steve Pierce

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    Will Rogers

  12. #12
    stewartb's Avatar
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    I remember buying my Cessna’s safety net from Atlee and balking at the idea of drilling the spar carry-through for the Ancra hook bases. He told me not to worry about it, the carry-through was in compression. The same is true of the wing spar. I also remember pics of the lightened early Carbon Cub spars. Do they still do the lightening holes?

    Steve Kracke told me about a wing test for Atlee tanks and how -12 wings flexed up and down inboard of the strut. Not being an engineer type I found that surprising. Further proof that what I don’t know is far greater than what I do know.
    Last edited by stewartb; 10-01-2021 at 09:08 AM.

  13. #13
    skywagon8a's Avatar
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    True stewart, but the wing spars also have bending loads which the carry-through does not. When the spar is both flexed in bending and loaded in compression at the same time it becomes weaker because it is not as strong in compression while it is bending. I know it doesn't look like a lot, but if it is not perfectly straight the column strength is less.
    What Steve Kracke was referring to was the flexing taking place about the hinge where the struts attach to the wing.
    N1PA

  14. #14

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    Quote Originally Posted by stewartb View Post
    I also remember pics of the lightened early Carbon Cub spars. Do they still do the lightening holes?
    None in my FX-3 main spar. I don't know what was done in the lower gross weight Carbon Cubs.

  15. #15
    Steve Pierce's Avatar
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    Quote Originally Posted by frequent_flyer View Post
    None in my FX-3 main spar. I don't know what was done in the lower gross weight Carbon Cubs.
    I can't remember on the SS either. Here is an FX3 wing.
    Click image for larger version. 

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    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

  16. #16
    stewartb's Avatar
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    Steve described the flex as a wave that moved full length of the wing. I thought the strut would reduce it. Apparently not.

  17. #17
    Gordon Misch's Avatar
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    See figure 18. And superimpose the compressive load inboard of the strut attachment onto this. From https://engineering.purdue.edu/~ce47...amFormulas.pdf


    Gordon

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  18. #18

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    Quote Originally Posted by Steve Pierce View Post
    Outboard of the lift strut is not a high stress area.
    Are we talking tiny holes for mounting a pitot tube as the OP was asking that are no more than the rib mounting or are you talking about BIG lightening holes through the web?
    Regards, Charlie
    Super Coupe E-AB build in process

  19. #19
    skywagon8a's Avatar
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    As I recall from an engineer I used to work for, small holes in and near the center of the web are not critical. Big ones like the lightning holes are a different story.
    N1PA
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  20. #20

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    Quote Originally Posted by CharlieN View Post
    Are we talking tiny holes for mounting a pitot tube as the OP was asking that are no more than the rib mounting or are you talking about BIG lightening holes through the web?
    Good question. Also, I *think* I am ok with the pitot bracket because it appears that it is installed inside the lift strut in the parts manual as shown in the attachment (figure 5 from the parts manual)
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  21. #21
    Gordon Misch's Avatar
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    Quote Originally Posted by skywagon8a View Post
    As I recall from an engineer I used to work for, small holes in and near the center of the web are not critical. Big ones like the lightning holes are a different story.
    Yes. Note the transverse shear graph referenced above, then add to that the internal shear imputed from the moment diagram. For me, I wouldn't be making big holes without careful analysis or proven experience.
    Gordon

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  22. #22
    RVBottomly's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    See figure 18. And superimpose the compressive load inboard of the strut attachment onto this. From https://engineering.purdue.edu/~ce47...amFormulas.pdf

    Thanks, Gordon. Something added to my library!

  23. #23
    Steve Pierce's Avatar
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    The screw holes for the pitot bracket are in the middle of the web and should be no issue. I tend toi look at how Piper did things and figure it was for a reason and stick to their engineering unless a reputable source proves otherwise. I am not smart enough to engineer on my own
    Steve Pierce

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  24. #24
    RaisedByWolves's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    I can't remember on the SS either. Here is an FX3 wing.
    Click image for larger version. 

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    Sport cubs had lightning holes everywhere. Front and rear spar. Carbon cubs do not.


    Sent from my iPhone using Tapatalk

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