Results 1 to 31 of 31

Thread: Setting Wing Dihedral Using a Digital Level

  1. #1
    KJC's Avatar
    Join Date
    Jul 2007
    Location
    Clear Lake, MN
    Posts
    182
    Post Thanks / Like

    Setting Wing Dihedral Using a Digital Level

    My PA-12 is slightly left wing heavy and I would like to start the entire rigging process over to make sure it's correct. I'm familiar with both the L-21 Erection Manual and ac 43-16. The dihedral was set using the string method and both wings are symmetrical. I used the fish line method and measured 3 1/8 from the top of the spar near the the wing attach fitting as instructed. Because the aircraft has extended wings with booster tips, I'm not sure this method will produce entirely accurate results.

    I understand my heavy wing is probably a a washout issue and a turn in on the left rear lift strut will probably fix it. What I'm asking about is DIHEDRAL not washout (or twist if you prefer).

    With the the aircraft leveled, is there a proper number (or at least agreed to by most) for DIHEDRAL when measured under the front spar using a digital level? Both wings are currently set at .81 degrees with an equal number of threads showing on each fork.
    PA-12 N418BS

  2. #2

    Join Date
    Nov 2002
    Location
    Fargo
    Posts
    651
    Post Thanks / Like
    On a PA-18 when you set dihedral at 3-1/8 to the top of the attach fitting a digital level will read 1.1-1.3

  3. #3
    BC12D-4-85's Avatar
    Join Date
    Feb 2016
    Location
    Fairbanks, AK.
    Posts
    1,674
    Post Thanks / Like
    http://www.univair.com/content/PIP_SM008.pdf Says 3" down from the string. I've seen 0.75* dihedral on one I owned and measured with a digital level. Not an A&P.

    Gary

  4. #4

    Join Date
    Nov 2002
    Location
    Fargo
    Posts
    651
    Post Thanks / Like
    3-1/8 is from Piper service memo 19 for rigging instructions

  5. #5
    cubpilot2's Avatar
    Join Date
    May 2003
    Location
    Anchorage Alaska
    Posts
    734
    Post Thanks / Like
    [QUOTE=KJC;691921]My PA-12 is slightly left wing heavy and I would like to start the entire rigging process over to make sure it's correct. I'm familiar with both the L-21 Erection Manual and ac 43-16. The dihedral was set using the string method and both wings are symmetrical. I used the fish line method and measured 3 1/8 from the top of the spar near the the wing attach fitting as instructed. Because the aircraft has extended wings with booster tips, I'm not sure this method will produce entirely accurate results.

    The 3 1/8 " number is for stock wings.
    If you are running the string from the top of the last full rib just out board of the original aileron then you would get good results.

    If going from the extended tip then you will not have enough dihedral .
    Ed

  6. #6
    www.SkupTech.com mike mcs repair's Avatar
    Join Date
    Aug 2004
    Location
    chugiak AK
    Posts
    10,479
    Post Thanks / Like
    you need to TAPE the string DOWN to top of wing above front spar, at ORIGINAL end of aileron rib location, then measure.... simple

  7. #7
    PerryB's Avatar
    Join Date
    Aug 2007
    Location
    Northern Calif.
    Posts
    1,820
    Post Thanks / Like
    As long as the dihedral is symmetrical, 1 or 2-10ths of a degree above or below spec will have no perceptible effect on how it flys. Arguments can and have been made for adding it, and reducing it. I advocate setting dihedral with the string method because it eliminates the guesswork of determining the true and correct level reference. Just be sure to measure very precisely. Once you've done that, twist can be easily set with the smart level. Twist usually has to be tweaked a little anyway, no matter how theoretically perfect it is. Take a little twist out of the heavy wing.
    After Monday and Tuesday, even the calendar says WTF !

  8. #8

    Join Date
    Sep 2003
    Location
    Valdez, Alaska
    Posts
    640
    Post Thanks / Like
    Quote Originally Posted by PerryB View Post
    As long as the dihedral is symmetrical, 1 or 2-10ths of a degree above or below spec will have no perceptible effect on how it flys. Arguments can and have been made for adding it, and reducing it. I advocate setting dihedral with the string method because it eliminates the guesswork of determining the true and correct level reference. Just be sure to measure very precisely. Once you've done that, twist can be easily set with the smart level. Twist usually has to be tweaked a little anyway, no matter how theoretically perfect it is. Take a little twist out of the heavy wing.
    Take a twist out of the heavy wing....my exp cub rolls to the right at cruise. Does that mean the right is heavy...

  9. #9
    BC12D-4-85's Avatar
    Join Date
    Feb 2016
    Location
    Fairbanks, AK.
    Posts
    1,674
    Post Thanks / Like
    On floats I'd want enough dihedral to add to longitudinal stability. I suggest 1* at least and test from there. Extended wings require some more than stock in my experience especially when floats require added rudder springs or ventral fins. Dihedral is our friend.

    Gary

  10. #10

    Join Date
    Sep 2003
    Posts
    6,783
    Post Thanks / Like
    Do the digital level three different places on each wing. You will be surprised at the variance.

    Before you do that, take your bird aloft, trim it for stable flight, let go of the stick, and take a good look at both ailerons. Both should look the same - if one is slightly up and the other slightly down you have an aileron mis-match. It takes a lot of differential wash to fix that. Better way is matched ailerons.

  11. #11
    skywagon8a's Avatar
    Join Date
    Mar 2008
    Location
    SE Mass
    Posts
    9,444
    Post Thanks / Like
    Quote Originally Posted by J5Ron View Post
    Take a twist out of the heavy wing....my exp cub rolls to the right at cruise. Does that mean the right is heavy...
    Are you holding the ball centered with your feet when the wing rolls right? If so then, yes the wing is heavy.
    N1PA
    Thanks J5Ron thanked for this post

  12. #12
    skywagon8a's Avatar
    Join Date
    Mar 2008
    Location
    SE Mass
    Posts
    9,444
    Post Thanks / Like
    Quote Originally Posted by KJC View Post
    The dihedral was set using the string method and both wings are symmetrical. I used the fish line method and measured 3 1/8 from the top of the spar near the the wing attach fitting as instructed.
    It is important to measure each wing spar (not just one side) to ensure that the fuselage is symmetrical to the wings. The plane can fly straight with the fuselage cocked at an angle. If the fuselage is not symmetrical the ball will not be centered and you will be pulling your hair out wondering why you are having difficulty with the rigging. Be sure both wings are the same with the fuselage first.
    N1PA

  13. #13
    Steve Pierce's Avatar
    Join Date
    Mar 2002
    Location
    Graham, TX
    Posts
    18,812
    Post Thanks / Like
    In my notes I have .7 but I can tell you when you start putting a digital level along the wing your measurements will be all over the place. I use a long straight edge and measure it inboard of the strut attach fittings.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

  14. #14
    PerryB's Avatar
    Join Date
    Aug 2007
    Location
    Northern Calif.
    Posts
    1,820
    Post Thanks / Like
    Quote Originally Posted by J5Ron View Post
    Take a twist out of the heavy wing....my exp cub rolls to the right at cruise. Does that mean the right is heavy...
    Its a figure of speech. If you let go of the stick and the aircraft begins to bank right, the right wing is said to be "heavy". Either way, shortening the rear strut on the heavy wing cures the problem, assuming everything else (such as dihedral, and flap/aileron rigging) is true and symmetrical.
    After Monday and Tuesday, even the calendar says WTF !
    Thanks J5Ron thanked for this post

  15. #15

    Join Date
    Nov 2002
    Location
    Fargo
    Posts
    651
    Post Thanks / Like
    Here's a link to Piper service memo 19 it would have most of your answers

    http://www.fadodge.com/fad_pdfs/Corr...a8ztqFncHAgdU4
    Thanks Pete Schoeninger thanked for this post

  16. #16
    BC12D-4-85's Avatar
    Join Date
    Feb 2016
    Location
    Fairbanks, AK.
    Posts
    1,674
    Post Thanks / Like
    This is Piper's Service Memo No. 8 for rigging PA-12's: http://www.univair.com/content/PIP_SM008.pdf
    This is Piper's Service Memo No. 19 for rigging PA-18's: http://www.univair.com/content/PIP_SM019.pdf

    Gary
    Last edited by BC12D-4-85; 07-08-2017 at 08:49 PM.
    Likes Hardtailjohn, Jim Hann liked this post

  17. #17
    cubflier's Avatar
    Join Date
    Nov 2003
    Location
    Palmer, AK
    Posts
    1,299
    Post Thanks / Like
    Quote Originally Posted by J5Ron View Post
    Take a twist out of the heavy wing....my exp cub rolls to the right at cruise. Does that mean the right is heavy...
    Ron,

    People tend to fixate on fixing the so called heavy wing. My approach is to ask the question "do I want more or less washout after the rigging". The reason this is important is if you look at cub rigging in general the amount of washout is all over the place ( this is just my observation over time). I have seen cubs with flat wings and cubs twisted to 2.5 degrees at the outboard measuring point. Both of these scenarios are incorrect (per the instructions) but sometimes this done intentionally to make tweeks in flight characteristics. In the case of the flat wing you would do the opposite of what is being suggested and you would remove lift(add twist) from the "light wing". In the case of the over twisted wing you add lift(remove twist) to the "heavy wing".

    With regards to dihedral - I have never been able to warm up the digital level and pull out the string.

    Have fun - Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!
    Thanks J5Ron thanked for this post

  18. #18

    Join Date
    Dec 2004
    Location
    Inkom, Idaho
    Posts
    1,364
    Post Thanks / Like
    Am I the only one to still use a adjustable bubble protractor? I shy away from the digital levels.... yeah I know it's old school, but I've used the same Starrett protractor for 30+ years, and am still slow to get on board with the digital stuff. I certainly don't think they are better, just because they are digital. My technique when setting washout and other things that I want as accurate as possible, is to "crowd the line", I don't try to center the bubble in the middle of the lines, I go for the bubble just barely touching one line, the important thing being the opposite side is also set using the same technique but opposite. Digital users will laugh, users of the old school adjustable protractor will know what I mean, I hope. I get more consistent results using the whiskey bubble, the digital stuff seems to give different results a few minutes later, when nothing has moved.....
    Likes Southern Aero liked this post

  19. #19
    BC12D-4-85's Avatar
    Join Date
    Feb 2016
    Location
    Fairbanks, AK.
    Posts
    1,674
    Post Thanks / Like
    Lots of chatter here and elsewhere regarding the relationship between wing rigging and flying performance. Flying performance being takeoff, landing, and even flight between those events.

    I suggest some experimentation if the pilot's curiosity is sufficient. If not then the manufacturer's design will suffice for most. But for some the improvement is worth the time to explore.

    It has been for me at least with several planes and gear configurations.

    Gary

  20. #20
    Steve Pierce's Avatar
    Join Date
    Mar 2002
    Location
    Graham, TX
    Posts
    18,812
    Post Thanks / Like
    Quote Originally Posted by courierguy View Post
    Am I the only one to still use a adjustable bubble protractor? I shy away from the digital levels.... yeah I know it's old school, but I've used the same Starrett protractor for 30+ years, and am still slow to get on board with the digital stuff. I certainly don't think they are better, just because they are digital. My technique when setting washout and other things that I want as accurate as possible, is to "crowd the line", I don't try to center the bubble in the middle of the lines, I go for the bubble just barely touching one line, the important thing being the opposite side is also set using the same technique but opposite. Digital users will laugh, users of the old school adjustable protractor will know what I mean, I hope. I get more consistent results using the whiskey bubble, the digital stuff seems to give different results a few minutes later, when nothing has moved.....
    I have found the rigging process is much faster after I switched to a quality digital level. The airplane either flies wings level the first flight or after a half turn of a rear strut.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers
    Likes Southern Aero liked this post

  21. #21
    Eddie Foy's Avatar
    Join Date
    Dec 2014
    Location
    South Florida
    Posts
    3,426
    Post Thanks / Like
    Quote Originally Posted by Steve Pierce View Post
    I have found the rigging process is much faster after I switched to a quality digital level. The airplane either flies wings level the first flight or after a half turn of a rear strut.
    What level do you use?
    "Put out my hand and touched the face of God!"

  22. #22
    Southern Aero's Avatar
    Join Date
    Jan 2008
    Location
    kcco/1ga2
    Posts
    180
    Post Thanks / Like
    FWIW ........... I find the digital level is "more convenient" but a good "spirit level" is more accurate if you need to split hairs. Any good level advertises accuracy withing .005 per foot. If you use that dimension, then a good bubble level is almost 4 times as accurate as a digital level that uses a tenth of a degree increments. One degree in one foot is about .210. .... so .1 of that would be .021. There are a few good levels that have both the bubble and digital scale in the same frame. That way you can have the convenience of the digital and accuracy of the bubble.............. but the accuracy of the bubble is also in the "reading" of the vial. Either way I have found, like Steve, that you are usually within a half turn or so of a rear fork to get it to fly level. One thing that did make me chase it once was a drooped tip that had been "repaired" at some point and was slightly different from the other side. That did throw a little sheeee-at in the game but finally found it......... doesn't take much difference in a tip shape to make it roll.

    As for brand, I don't have any strong preference......... I use Husky, Johnson and Stanleys. If using a digital you just gotta make sure it's adjusted to zero. And check occasionally, both digital and bubble types. If a bubble level ever gets off for any reason, toss it.
    ......... It doesn't cost any more to go first class! You just can't stay as long.

  23. #23

    Join Date
    Dec 2004
    Location
    Inkom, Idaho
    Posts
    1,364
    Post Thanks / Like
    Thanks for confirming my suspicion that the bubble splits the hair finer, too fine to bother with sounds like! I do like my digital center finder I use when pipe fitting, and my inclometer app on my phone to see how steep a LZ is. But, I'm not getting rid of the Starrett for setting my wing washout, my experimental flies too well to change anything! Thought of this thread while coming back from breakfast today, hands off/feet off for long periods of time, real nice air of course.

  24. #24
    Steve Pierce's Avatar
    Join Date
    Mar 2002
    Location
    Graham, TX
    Posts
    18,812
    Post Thanks / Like
    You guys are way better with a spirit level than me. I can zero my digital level out on the level part of the fuselage and go straight to the outboard aileron bay rib and set to the desired angle. Don't have to level the fuselage etc.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers
    Likes Hardtailjohn liked this post

  25. #25

    Join Date
    Sep 2003
    Location
    Valdez, Alaska
    Posts
    640
    Post Thanks / Like
    This is a great thread...thanks for getting it started KJC. So my experimental PA12 has had this nasty roll to the right...heavy wing right....???. Today went to the hanger and double checked things after stuying this thread. My dihedral was off about a quart inch...I changed that which in turn allowed the washout to match the other side perfectly....end result no roll, hands off. Thanks everyone...who new a quarter inch was so significant.
    Likes Steve Pierce liked this post

  26. #26

    Join Date
    Dec 2004
    Location
    Inkom, Idaho
    Posts
    1,364
    Post Thanks / Like
    Same deal with the protractor, you set it to whatever your datum point is , no need to have anything level. Agreed, that'd be a pain. https://www.amazon.com/Starrett-PR-1...a0c90e0c469879

  27. #27

    Join Date
    Sep 2003
    Posts
    6,783
    Post Thanks / Like
    Smart Level. It does have a bubble. I can check an airplane in ten minutes flat. It takes me around an hour to find the jacks and level a J-3, and I cannot even lift the tail of an 18 without a helper. I am with Steve. I did my first J-3 in 1969, before Smart Levels existed.

  28. #28
    Hardtailjohn's Avatar
    Join Date
    Jan 2007
    Location
    Marion, MT
    Posts
    707
    Post Thanks / Like
    I had a couple friends building a Glastar and getting down to the nitty gritty with the smart level when they set dihedral. I told them to use a string and a bubble and tape...but they wanted to use the "Smart level"....couple days later, I saw the UPS guy at lunch and he asked them if they got the new struts he'd dropped at the back of the shop. Come to find out, they'd gotten the level switched from degrees to %grade..... and drilled the strut holes. After that, they called it a "dumb level" hahaha!
    John

  29. #29
    hotrod180's Avatar
    Join Date
    May 2008
    Location
    Port Townsend, WA
    Posts
    2,686
    Post Thanks / Like
    You gotta be as smart as or smarter than the equipment.
    Cessna Skywagon-- accept no substitute!

  30. #30
    Steve Pierce's Avatar
    Join Date
    Mar 2002
    Location
    Graham, TX
    Posts
    18,812
    Post Thanks / Like
    Quote Originally Posted by courierguy View Post
    Same deal with the protractor, you set it to whatever your datum point is , no need to have anything level. Agreed, that'd be a pain. https://www.amazon.com/Starrett-PR-1...a0c90e0c469879
    I have a couple of those but a little short and cumbersome to me.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

  31. #31
    BC12D-4-85's Avatar
    Join Date
    Feb 2016
    Location
    Fairbanks, AK.
    Posts
    1,674
    Post Thanks / Like
    Great previous thread from here on rigging PA-12/18's: http://www.supercub.org/forum/showth...out-from-where

    Gary

Similar Threads

  1. Wing dihedral
    By Durham66 in forum Website Q & A
    Replies: 1
    Last Post: 02-26-2017, 07:39 AM
  2. Running out of thread setting dihedral
    By d_kandle in forum Experimental Cubs
    Replies: 20
    Last Post: 02-21-2016, 12:11 PM
  3. Supercub wing dihedral
    By Clifford in forum Cafe Supercub
    Replies: 8
    Last Post: 05-04-2014, 10:39 PM
  4. MEASURING WING DIHEDRAL
    By garyh in forum Cafe Supercub
    Replies: 3
    Last Post: 01-04-2009, 07:18 PM

Bookmarks

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •