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Thread: Wing Incidence

  1. #1

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    Wing Incidence

    Does anyone know what the wing incidence (at the root) of a super cub is in relation to the waterline or centerline of the fuselage? The reason I ask is we are in that stage of building a Breezy fuselage and it calls for 0 incidence at the wing root and I had thought that a cub had 1.5 degrees... but I can't remember. Thanks Joe

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    incidence

    Joe, From the horizontal reference line on the fuselage to the bottom of the wing at the first rib the magic number is 1.84 degrees. Most seem to measure somewhere between 1.2 and 1.7 unless they have been rebuilt in a jig recently that puts it all in. You might want to check with Mark E from Thrustline on all this. He has all these numbers in his head.
    Dave

  3. #3
    jgerard's Avatar
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    Since the Breezy is a pusher with the engine so close to the wing you have to consider all the important numbers that affect the flight characteristics. Wing and tail AOI, TL, method of trim (tab vs' moveable stab) and C.G.

    Jason

  4. #4
    skywagon8a's Avatar
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    The L-21 maintenance and erection manual says that the angle of incidence is 2 degrees 2". at the root. This corresponds closely with the bottom of the wing being level.

    If you can find a copy of 'Handbook of Airfoil Sections for Light Aircraft' by M.S. Rice you will find on page 74 all of the specs for the U.S.A. 35B airfoil which is on the Cub. That should accurately answer your question.
    N1PA

  5. #5
    S2D's Avatar
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    Has anyone come up with the angle between the bottom of the rib and the chord line of the supercub rib yet?

    I measured the straightest butt rib I had and came up with 1.9 deg.
    that measures closely with what skywagon says about the botton of the rib should be level when the plane is level.
    I may be wrong but that probably won't stop me from arguing about it.

  6. #6
    cubflier's Avatar
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    There should be 2.5 inch difference between the front and rear attach fitting. After you attach the wings the incidence at the root is the 1.84 degrees. The best measure of this is using the bottom spar caps and not the ribs since things can vary.

    At least this is how the great one (that is currently chasing Kipper Snacks) told me how to do it.

    Here is another thread on the subject.

    http://www.supercub.org/phpbb2/viewt...wing+incidence

    A link to rigging instructions.

    http://www.faa.gov/aircraft/safety/a...7_08_Alert.pdf

    Here is the drawing of the attach points. Note the attach points are not at 1.84 degrees and I forgot the angle. You can use the drawing to solve the math if you need that figure.

    [/url]

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

  7. #7
    S2D's Avatar
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    thanks Jerry. I've got those figures from the same source.
    that figure is great for determining the proper alignment of the fuselage , but it doesn't give me the angle if incidence of the wing at the butt rib. The actual rib determines that.

    I just want to know what my true angle of incidence is.

    both my butt ribs are new dakota cub ribs and both measure exactly the same at the bottom, so the angle of incidence should be that figure plus the difference between the rib and the chord.

    X + 1.9 = AOI (if 1.9 is the correct angle between the bottom of the rib and the chordline)
    I may be wrong but that probably won't stop me from arguing about it.

  8. #8
    cubflier's Avatar
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    Quote Originally Posted by S2D
    I just want to know what my true angle of incidence is.

    both my butt ribs are new dakota cub ribs and both measure exactly the same at the bottom, so the angle of incidence should be that figure plus the difference between the rib and the chord.

    X + 1.9 = AOI (if 1.9 is the correct angle between the bottom of the rib and the chordline)
    Wouldn't the answer to your question be 1.843 ( the wing incidence measured from the bottom of the inboard rib) + 1.9 (your measurement)= 3.743.

    Is there a practical reason for knowing this figure or are you just pontificating?

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

  9. #9
    S2D's Avatar
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    Quote Originally Posted by cubflier
    Wouldn't the answer to your question be 1.843 ( the wing incidence measured from the bottom of the inboard rib) + 1.9 (your measurement)= 3.743.

    Is there a practical reason for knowing this figure or are you just pontificating?

    Jerry
    Kinda just pontificating, but if it was 3.743 (which I didn't say it was) , that would be quite a bit over factory specs


    Plus I want to know the relative angle compared to the trim range of the horizontal stabilizer before I have it covered.


    incidently, both my ribs measured .3 deg less than the bottom of the spar measurement.
    I may be wrong but that probably won't stop me from arguing about it.

  10. #10
    skywagon8a's Avatar
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    I checked on the Northland CD. Piper drawing 13814 is the rib outline which gives all of the coordinates. If my math is correct the angle of incidence to a straight edge along the lower surface is 1.54 degrees.
    N1PA

  11. #11
    S2D's Avatar
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    I come up with 1.61 deg, but I used an online angle calculator

    thanks 8A that will give me what I need.
    I may be wrong but that probably won't stop me from arguing about it.

  12. #12
    skywagon8a's Avatar
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    S2D
    You got me thinking about our different answers of .07 degrees. I found it. Dwg. 13814 shows that the trailing edge is 0.164" thick. The chord line goes through the center of the trailing edge. If you use my center line the answer is 1.54 degrees. If you have the trailing edge come to a point then the answer is 1.61 degrees.
    N1PA

  13. #13
    S2D's Avatar
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    sounds good to me, I'll use yours

    found out why I'm .3 off from the spar too. bottom of rib wasn't flat.
    so cubflier was right on that part, unless I put a straight edge on the bottom of the rib.
    I may be wrong but that probably won't stop me from arguing about it.

  14. #14
    S2D's Avatar
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    Re: incidence

    Quote Originally Posted by ag-pilot
    From the horizontal reference line on the fuselage to the bottom of the wing at the first rib the magic number is 1.84 degrees. Dave
    Dave can you tell me how that number was arrived at?

    Seems too exact to be a figure that Mark just decided works best.

    (I could understand 1.8 or 1.85 but to come up with 1.84, he must have calculated the difference between the bottom of the spars and the true chord)

    If it is the measurement at the bottom of the spars, what is the true AOI of the wing on a supercub.
    I may be wrong but that probably won't stop me from arguing about it.

  15. #15
    SuperCub MD's Avatar
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    Brian,

    Airworthiness Alerts
    Service memo #19

    The numbers are all in there, if you do the math and cook it all down it comes to this.


    PA18
    Level at door bottom
    Butt rib incidence +1.843
    Last full rib incidence -.717
    Wing washout 2.56

    PA12
    Level at floor between gear fittings +1.665
    Butt rib incidence -.060
    Last full rib incidence -2.627
    Wing washout 2.567

    Tolerance is +/- 1/4 degree.

    Use a 30" straight edge on the rib bottoms between the spars to account for the curve in the ribs. I've got the numbers written on the shop wall so I don't have to keep looking them up.

  16. #16
    S2D's Avatar
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    Quote Originally Posted by SuperCub MD

    Butt rib incidence +1.843
    Thanks Mark, I'll look up that Service memo,

    but for posterity's sake (pun intended)

    please define "Butt rib incidence".

    So far I've seen it measured three different ways, none of which give the same reading.

    1. Bottom of spars
    2. Bottom of rib
    3. Actual cord of wing
    4. None of the above

    (all compared to the longitudinal plane of the A/C)

    only one is correct.


    Edited to include 4. none of the above


    I may be wrong but that probably won't stop me from arguing about it.

  17. #17
    SuperCub MD's Avatar
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    Quote Originally Posted by SuperCub MD
    Brian,


    Use a 30" straight edge on the rib bottoms between the spars to account for the curve in the ribs.

    This should be the same as the bottom of the spars. Everyones favorite fish watcher likes to use the spar bottoms to check the frame, and I agree this is more accurate for the frame, ribs can be tweaked. I use the 30" straight edge between the spars when rigging. Can't check the spar bottoms on the outboard rib with fabric on.

    I'd say 1 and 2 are both correct. The cord line wouldn't even be close.

    Here is the AC. Read it and work it out. I'd bet a beer or 2 that my numbers are right....
    http://www.faa.gov/aircraft/safety/a...7_08_Alert.pdf

  18. #18
    cubflier's Avatar
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    Quote Originally Posted by SuperCub MD
    Here is the AC. Read it and work it out. I'd bet a beer or 2 that my numbers are right....
    http://www.faa.gov/aircraft/safety/a...7_08_Alert.pdf
    I was in agreement with your numbers until (very recently) I broke out the plans and did the math. I also measured the spars on a set of wings I have in my hangar and came up with an angle substantially less than 1.843 and very similar to my math calculations using the plans.

    Then I looked at the methodology behind the 1.843 number. The man (Gordon Mandell) that wrote the AC did not use the bottom of the spars in his calculation of the 1.843 number. It's an average of two methods taking the angular difference in the horizontal reference line of the wing bottom and the horizontal reference line of the fuselage. At least this is my interpretation of his notes.

    I have his notes and methodology and would be glad to send them via email if you like. I'm interested to see if you draw the same conclusions that I have and if I'm wrong will be happy to buy you that beer or two.


    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

  19. #19
    SuperCub MD's Avatar
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    Jerry,

    I'd like to read the info you have. My e-mail hasn't been working well lately, but give it a try if you want to,

    drath@lakefield.net

    If it takes much more than elementary math, I'll probably just drink beer.

  20. #20
    Crash's Avatar
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    Quote Originally Posted by cubflier
    Quote Originally Posted by SuperCub MD
    Here is the AC. Read it and work it out. I'd bet a beer or 2 that my numbers are right....
    http://www.faa.gov/aircraft/safety/a...7_08_Alert.pdf
    I was in agreement with your numbers until (very recently) I broke out the plans and did the math. I also measured the spars on a set of wings I have in my hangar and came up with an angle substantially less than 1.843 and very similar to my math calculations using the plans.

    Then I looked at the methodology behind the 1.843 number. The man (Gordon Mandell) that wrote the AC did not use the bottom of the spars in his calculation of the 1.843 number. It's an average of two methods taking the angular difference in the horizontal reference line of the wing bottom and the horizontal reference line of the fuselage. At least this is my interpretation of his notes.

    I have his notes and methodology and would be glad to send them via email if you like. I'm interested to see if you draw the same conclusions that I have and if I'm wrong will be happy to buy you that beer or two.


    Jerry
    Jerry; you're now seeing and maybe fighting the same problem I had with the numbers last year when you and I went round and round over this.

    They're less in actual practice and I don't know why. I pulled my hair out over this!

    Did the measurements on a set of un-covered wings and a fuselage with wings mounted. The math on the fuselage was right on the money but that is where everything ended.....

    Crash

  21. #21
    S2D's Avatar
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    [quote="SuperCub MD"]
    Quote Originally Posted by SuperCub MD
    Brian,



    Here is the AC. Read it and work it out. I'd bet a beer or 2 that my numbers are right....
    you may need to load that cub up with beer when you come out this winter Mark, but I'll digress until I get home and study Jerry's document again.

    I've also edited my original multiple choice above
    I may be wrong but that probably won't stop me from arguing about it.

  22. #22
    cubflier's Avatar
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    Quote Originally Posted by SuperCub MD
    Jerry,

    I'd like to read the info you have. My e-mail hasn't been working well lately, but give it a try if you want to,

    drath@lakefield.net

    If it takes much more than elementary math, I'll probably just drink beer.
    Mark,

    I emailed the document to you last night so let me know if you did not receive it. I look forward to your opinion on this.

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

  23. #23
    S2D's Avatar
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    to further muddy the waters:

    in the Alerts Mr Mandell states that the measurement of washout should be -.7 deg at the outer aileron rib when the aft point of the level is under the rear spar.

    using that calculation and assuming he got the 2.5 deg washout from some Piper source, not his own calculations, the bottom of the spars should read 1.8 (or thereabouts)

    however, he says the angle of incidence measured at the centerline of the spar hing bolts should be 1.84 deg I just don't get that statement.

    thirdly in his calculations that cubflier sent, he uses the full length bottom of the rib for his final figure to get 1.84 and says that the rear spar is .28 above this line. He admits there is a .2+ discrepancy and this is the more accurate measurement.

    so even he uses three different lines he calls AOI, and only the one referring to the centerline of the bolt is close to the true AOI, but nowhere close to 1.84.



    all I do know , is that when I measure an airplane that has 1.8 deg at the bottom of the spars, I get 2.75 inch diff between the heigth of the front and rear spar bolt C/L (Max allowed by Piper), and when I measure an airplane that has 1.5 deg at the bottom of the spars I get the factory spec 2.5" measurement
    I may be wrong but that probably won't stop me from arguing about it.

  24. #24
    SuperCub MD's Avatar
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    Quote Originally Posted by S2D

    however, he says the angle of incidence measured at the centerline of the spar hing bolts should be 1.84 deg I just don't get that statement.


    so even he uses three different lines he calls AOI, and only the one referring to the centerline of the bolt is close to the true AOI, but nowhere close to 1.84.
    I'm sure I'm stating something you all ready know, but this gave me some confusion when I first looked at this. Maybe it will help someone trying to measure a bear frame.

    The AOI line is parallel to each individual front and rear spar attach bolt hole, not a line between the front and rear spar attach fitting bolt holes.

    I haven't gotten the e-mail yet, but my computer has been very constipated. I'll give it a while to see if anything develops.

    Glad I only bet some beers and not a bottle of Jack. It seems there has been evidence withheld, and the question altered after I answered it? I think I have grounds for a mistrial, certainly an appeal to a higher Court. I'd hate to think I can't even measure my own butt. And I will not divulge the measurements of my own butt. I'll plead ignorance, most times I can't even find it with both hands and a flashlight.

  25. #25
    FdxLou's Avatar
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    Boz, you crack me up!

    Congratulations on the new addition to your Family.

    See you in N'olstein.

    Lou

  26. #26
    S2D's Avatar
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    Can anyone verify or refute these figures


    distance from center of spar attach bolt to top of spar:

    Front spar: .531"
    Rear Spar .614"

    I got the front one from the Northland drawing and had to figure the rear one using the Piper drawing and trig.

    If these are correct, I come up with the same figures cubflier does, but not positive they are correct.
    I may be wrong but that probably won't stop me from arguing about it.

  27. #27
    cubflier's Avatar
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    I solved from the bottom but if I subtract from the spar heights I get

    Bolt CL diff from top of spar

    Front = .531

    Rear = .618

    Bolt CL diff from bottom of spar

    Front = 5.156

    Rear = 3.382

    These measurements are in agreement with Gordon's calculation #2 and very close to what you have.

    I could not find enough information to solve the equation from the top of spar so I had to work my way up from the bottom and subtract the spar height.

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

  28. #28
    cubflier's Avatar
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    Mark,

    I posted the info here

    Good luck.

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

  29. #29
    S2D's Avatar
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    Quote Originally Posted by SuperCub MD

    PA18
    Level at door bottom
    Butt rib incidence +1.843
    Last full rib incidence -.717
    Wing washout 2.56






    I'd bet a beer or 2 that my numbers are right....

    Guess you won't have to bring the beer Mark. Your numbers are close enough to count. Just not for the reason Gordon Mandell gave.

    (as long as you define rib incidence as the measurement taken directly under the spars) on an airplane with maximum tolerance at the spar hinge fittings
    I may be wrong but that probably won't stop me from arguing about it.

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