Upper wing rear attach bracket

Discussion in 'Pitts Wings' started by airattack330, Mar 15, 2018.

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  1. Mar 15, 2018 #1

    airattack330

    airattack330

    airattack330

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    I'm putting together our new to us (homebuilt) Pitts S-1S project after recovering the wings, and ran into a slight issue mounting the upper wing. Once the upper wing was aligned, we noticed that the trailing edge appeared to be much higher than any Pitts we've seen. The rear spar attach bracket is also larger than the other Pitts S-1S' on the field. No mention of any modifications in the logbooks, but this aircraft was a former Reno racer as well as IAC advanced competitor. Can anyone help shed some light on this modification? *note the middle holes do not align with the forward attach bracket. Not sure what they were used for.[​IMG][​IMG][​IMG]

    IMG_4973.PNG

    IMG_4971.PNG

    IMG_4972.PNG
     
  2. Mar 15, 2018 #2

    PittsBird ZB

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    With the airplane level, what does the angle of incidence measure on the top wing vs. S-1S plans?
     
  3. Mar 15, 2018 #3

    cgzro

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    Reduced drag by separating the wings slightly?
     
  4. Mar 15, 2018 #4

    biplanebob

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    also check the lower wing incidence, and how do they compare to a stock Pitts.

    Bob
     
  5. Mar 15, 2018 #5

    cwilliamrose

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    Looks like they tried to remove the 1.5° of incidence in the top wing but got he math wrong -- should be a .5" change but that looks more like 1".

    And you have to wonder what they did on the bottom wings. Do you have a photo of that area you could post? If you wanted to remove the incidence from the bottom wings you could modify the fuselage or the root of the spars (or both I suppose). It is difficult to imagine someone going to -1.5° incidence on the top wing and doing nothing on the bottom wings.

    And the other obvious question is what they did in the front upper wing fittings to account for the angle change. You can't raise the rear spar 1" without doing something to allow the front bolt holes to be in alignment.

    If it were mine I'd want to return it to a stock configuration. This looks totally screwed up and not well thought out IMHO.
     
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  6. Mar 15, 2018 #6

    ndlakesdreamer

    ndlakesdreamer

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    Forgive me but I"m from the old school where when problems appear the first thing to look at is the last thing you worked on.
    Any chance the front and rear spar fittings are swapped? I'm visualizing the angle with them swapped and it's more in line with my mental image of "normal". Just my first thought
     
  7. Mar 15, 2018 #7

    cwilliamrose

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    Those fittings are very different front and rear even though the distance from the top holes to the attach hole is the same on both. No way they could be swapped. If you somehow did swap them there would be no place to attach the landing wires since there are holes in the rear fittings (only) for those. This looks to have been done on purpose and the disturbing thing is that rear fitting also seems to also have the stock attach hole which makes me wonder if the hole used was 'optional'. If that's the case, oh those poor front fittings! Snap, crackle, pop!!
     
  8. Mar 15, 2018 #8

    garyg

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    Not that I know a lot but this would make the I-struts different, the wires a different length. On top of every other angle. I had two fuselages that both seem to get the angle wrong and most of that started from the cabanes and how they were built. Is this plate compensating for cabanes or attempting to change the angle of incidence. I would get a set a drawings, levels, angle meters, tape measures and figure out what exactly what this bracket is meant to change or compensate for before proceeding any further. The rear bracket would seem to be a obvious change that would be hard to miss.

    my (almost useless) 2 cents.
     
    Last edited: Mar 16, 2018
  9. Mar 15, 2018 #9

    jrs14855

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    Changing the angle of incidence does change the I struts but not the wires. There should be enough extra length in the wires for the relatively small change.
     
  10. Mar 15, 2018 #10

    airattack330

    airattack330

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    I measured the lower wing incidence to be 2.5 degrees, but do not have the upper wing in place to measure it. I will mount it later today and see what it reads. We did note that it was 1 degree different than the lower wings the other day.

    I have attached photos of the lower wing attach points and a closer view of the rear bracket.

    IMG_4985.jpg

    IMG_4986.jpg

    IMG_4988.jpg

    IMG_4989.jpg
     
  11. Mar 15, 2018 #11

    cwilliamrose

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    They changed the front lower spar attach point to reduce the incidence. Hopefully the fuselage was built with these lower wing mods planned in advance otherwise the rear fitting could have some alignment issues.

    When you get the airplane assembled again to the point where you can put the rigging boards on it please measure the incidences relative the the upper longeron in the cockpit. That longeron represents 0.0° and if the wings agree with the longeron your rear cabane is too short or the front one is too tall,,,,,,

    It doesn't look like the 'other' hole in the upper rear attach fittings has been used.
     
  12. Mar 15, 2018 #12

    bf92

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    Upper and lower wings should both be 1.5 degrees.

    Danny
     
  13. Mar 15, 2018 #13

    airattack330

    airattack330

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    Aircraft is leveled at the cockpit longeron. Lower wing shows 2.5 degrees and the upper wing is .5 degrees.
     
  14. Mar 15, 2018 #14

    jrs14855

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    Bill- What are you seeing that I am not?? I just did the front lower wing fittings and the front side of the front fitting is approximately in line with the TOP surface of the longeron/crossmember. The aft side of the front fitting is located to accomodate the 1 1/2 degrees incidence. The rear spar fitting hole is on the longeron centerline. Doing this form memory, I don't have the plans in front of me.
    One additional comment: the upper wing incurs a substantial force trying to move the wing forward. Even with the flanged front fittings the fittings still bend. The tube between the front and rear cabanes allows the rear spar/cabane to share some of this load. The extra long fittings in the pictures negate any benefit from the connector tube.
     
  15. Mar 15, 2018 #15

    PittsBird ZB

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    so was the builder/modifier trying to increase lift on bottom wings and decrease lift on top wing? Is there some theory that this would result in a speed increase for racing?
     
  16. Mar 15, 2018 #16

    airattack330

    airattack330

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    That's what we are trying to figure out. I have reached out to the previous owner who competed in it, but have not heard back yet.
     
  17. Mar 15, 2018 #17

    cwilliamrose

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    The spar fitting has been raised on the spar such that the bolt hole is higher than it should be by around 1/2", maybe more.

    temp.JPG

    temp1.JPG
     
    Last edited: Mar 15, 2018
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  18. Mar 16, 2018 #18

    jrs14855

    jrs14855

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    Bill-
    I know that trying to scale dimensions from a picture is not very accurate but the spar fitting is 1" wide and I measure the inboard hole approx 3/4" from bottom of spar. Should be 1/2" which equates to 3/4 degree reduced incidence. This does not make any sense compared to the approx 1" change in the upper wing which equates to 3 degree change in incidence.
    Also the angle on the end of the lower spar apparently was not changed so the edge distance on the inboard hole is less than specified.
     
  19. Mar 16, 2018 #19

    cwilliamrose

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    It would be nice to see that wing spar without the wing being mounted to the fuselage. I couldn't tell if the end of the spar was changed to allow this mod to be properly done or not.

    I don't understand the math used here either. Maybe the basis for the dimensions comes from the flat wings which have 1.5° more incidence on the upper wing. But that is done with the upper wing fittings' geometry relative to the chord line, the cabanes are the same for either wing set. While I'd love to understand the builder's intent here the bottom line is figuring out what needs to change to at least get the wings at the same angle of incidence.
     
  20. Mar 16, 2018 #20

    airattack330

    airattack330

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    Spoke with the previous owner this morning and got the backstory. The upper wings were factory built and not modified by him. Lower wings were custom built to change the angle of incidence. Hence the movement of the fwd attach hole as cwilliamrose pointed out. The I struts were built special to give the upper wing 0 degree incidence. He said that when rigged properly, both wings will have 0 degrees of incidence. This was purposely done for acro. The Pitts would fly beautifully and have no adverse stall effects with this setup either inverted or right side up. Will have to see what it shows once I get it fully rigged. Will post the results next week.
     
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