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Thread: Dihedral question

  1. #1

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    Dihedral question

    Firstly I am a new member and thank you for all of the great info! I have a stock PA-12 other than droop tips. After a few flights I noticed that the right wing was low. I assumed it needed bungee cords and ordered the 1280hd from Univair. After I got things opened up for bungees there really wasn’t
    Much difference in the gear position or bungee cord condition. Once leveled following the alternate method ( I couldn’t find the plumb bob markings) there is a 4 inch difference floor to wingtip measurement. So I looked at the dihedral and it is off. Digital level 0.2 right and 1.5 degrees left.
    when I looked at the strut attachments the threads showing on the right ( the low wing) was 6
    And on the left about one. My question is if this a normal discrepancy and do I have room to adjust the dihedral....or what other info might help establish the problem. Airframe TT about 10,000 Thanks
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  2. #2
    mike mcs repair's Avatar
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    Dihedral question

    Rerig per book. I use front carry through spar as sideways level point.

    Also once you have fuselage level sideways, if you have a turn and bank or just a ball, center the ball now by rotating instrument

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    Last edited by mike mcs repair; 01-11-2021 at 11:30 AM.
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  3. #3
    mvivion's Avatar
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    Yes, definitely re-rig the plane. Piper designed in dihedral for a reason.

    MTV

  4. #4

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    Roger Thank you. I did see that you can have as many as 15 threads showing which gives me plenty of flexibility on the right wing but I am wondering if I can adjust the left wing with only a couple of threads available.
    I will also check the rear strut attachment to tail wheel bolt dimensions as another post recommended and tackle the bungee cords as well. If I need a guru is there one near Nashville?
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  5. #5
    mvivion's Avatar
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    It might be easier to start from scratch, rather than chase the rigging you have, so to speak.

    It isn't rocket science.

    MTV
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  6. #6
    Gordon Misch's Avatar
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    15 threads showing seems like a lot? Bare minimum thread engagement would be 1.5 diameters, depending on class of thread fit and considering the two different materials. Personally, I'd opt for at least 2 diameters (1 1/4") engagement. Using the standard 1.5 factor of safety times the 1.5Xdia, would result in 1.4" engagement.
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  7. #7

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    Thank you for the feedback I'll follow those recommendations. Hope to connect with the A&P next week. Is there a step by step giude of measurements to confirm symetry? I see that i am on a steep learning curve.

  8. #8
    mike mcs repair's Avatar
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    Quote Originally Posted by Jrsmith View Post
    . Is there a step by step giude of measurements to confirm symetry? I see that i am on a steep learning curve.
    Yes the original rigging instructions Service bulletin. It think Univair has it online?.


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  9. #9
    Gordon Misch's Avatar
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  10. #10
    mike mcs repair's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    EXCEPT, that leave out the VERY IMPORTANT FIRST STEP, level plane SIDE TO SIDE using front spar carry through spar(or floor if plane was NEVER wrecked.....) before doing the dihedral measurements with the level under wings...

  11. #11
    Gordon Misch's Avatar
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    Mike's the expert, but I'd add to pick "something" to define as level, and then stick with it. Pick a tube that is pretty straight, so that your level tells you the same thing every time. And then, mark which end of the level goes where, and mark the portion of the tube span you're using. That's the only way I know to be consistent.
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  12. #12
    skywagon8a's Avatar
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    Quote Originally Posted by Jrsmith View Post
    ...the right wing was low.
    ... there is a 4 inch difference floor to wingtip measurement. So I looked at the dihedral and it is off. Digital level 0.2 right and 1.5 degrees left.
    ...when I looked at the strut attachments the threads showing on the right ( the low wing) was 6
    ...And on the left about one.


    Airframe TT about 10,000 Thanks
    Do like the others said. First use front carry through spar behind the windshield as sideways level point. Then check the dihedral. You don't have to pick up the tail for an initial investigation.

    Your highlighted comments got my attention. The high wing has 1 thread showing and the low wing has 6 threads ???? It should be the other way around. It sounds as though that fuselage is severely out of alignment.
    N1PA
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  13. #13
    PerryB's Avatar
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    Quote Originally Posted by skywagon8a View Post

    Your highlighted comments got my attention. The high wing has 1 thread showing and the low wing has 6 threads ???? It should be the other way around. It sounds as though that fuselage is severely out of alignment.
    Very likely. I didn't want to be the party pooper, but you may learn more than you wanted to know about your airframe.
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  14. #14

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    How did the plane actually fly?
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  15. #15

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    A friend has a J3 with this problem. Fuselage is way out of square. I don’t know why someone would cover an airplane without first checking the geometry of the fuselage, at least pull a tape measure across the diagonals from upper to lower longeron s at the rear carry through!


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  16. #16

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    JRSmith

    I agree that this should remediated by rerigging - but as a point of curiosity...

    Much like J5Ron I wonder...

    Did you fly it before you discovered the issue? If so, how did it fly?

  17. #17
    mvivion's Avatar
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    Quote Originally Posted by RedOwlAirfield View Post
    JRSmith

    I agree that this should remediated by rerigging - but as a point of curiosity...

    Much like J5Ron I wonder...

    Did you fly it before you discovered the issue? If so, how did it fly?

    More to the point: How did it stall?

    MTV

  18. #18
    TurboBeaver's Avatar
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    Sad truth is ALOT of Cubs are crooked as heck, unbalanced dihederal is a common fault in many of them. Out of square cabin sections are more common than square ones, honestly square ones are usually only common in airframe's that have recently just been in a jig......... But 4" of differential from one side to the other, is way out.....
    We recently reriged a Cub that showed 3" of over all differential from one side to other, and it flew like crap! And it was rediculously left wing heavy. After rigging it correctly for dihederal, and setting correct washout, we had to set the wash to a 'slight differential', to offset the out of square of the fuselage. It now flys "like a dream" hands off .....and the stall is evenly. And the big bonus is: its around 2/3 mph slower stall than before it was rigged!
    HUGE difference in the overall feel of Cub before and after. Special thanks to the Campbell family in Millinocket with over 50 years of Cub rigging experience!


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    Last edited by TurboBeaver; 01-13-2021 at 02:23 AM. Reason: Jim
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  19. #19
    Doug Budd's Avatar
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    There is a lot of info on rigging on this sight and ShortWingPipers.Org. Type in rigging in the search box. There is also a cd you can buy on rigging tube and fabric pipers.


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  20. #20

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    Quote Originally Posted by RedOwlAirfield View Post
    JRSmith

    I agree that this should remediated by rerigging - but as a point of curiosity...

    Much like J5Ron I wonder...

    Did you fly it before you discovered the issue? If so, how did it fly?

    The problem sometimes with re-rigging is that these planes are simply old and re-done by in some cases many people...I walked around lake hood one day with a digital level, and I am not sure I measured one PA12 that was by the books...wings with different washout as well as different dihedral in many cases...I helped rig a guys J5 once that had a odd fuse...one wing lower then the other at the attach points...got it rigged nicely, ie flys nice, stalls perfect, turns well...however not per Piper Books.
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  21. #21
    Doug Budd's Avatar
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    As one of the members said, set the difficult wing as close as you can then adjust the other one to match. When it’s time to recover put it in a jig and get it right ( hopefully)


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  22. #22

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    Thank you for the great feed back. I am still enthusiastic . To answer a question or two. Yes I did fly around the patch several times. The right wing was heavy and landings didnt have any surprises but I didnt want to try any stalls based on the issues i had identified. I have a pretty long list of squaks and reading material. LOL

    I'll have my tape measures and levels working next week and will likely have more questions. I do have reservations that the left wing will adjust down to spec.
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  23. #23
    Steve Pierce's Avatar
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    If you have 1.5 degrees dihedral and 1 thread showing on the left wing you are kinda stuck. Fuselage is tweaked, probably a previous repair. You can't go in anymore to take out the excess dihedral. Bring the right wing up to match. Check both outboard aileron bay ribs and rig the rear struts to match the angle on those ribs left to right and test fly. The only other option involves cutting tubing.
    Steve Pierce

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