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Thread: rigging Cub wings

  1. #41
    Gordon Misch's Avatar
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    Agreed Pete. Yes, I'm aware of the stall scenario, but I also know that Bob Turner reported his Decathalon has zero washout and "stalls just fine". Certainly other means can be used to help out the ailerons near the stall - maybe VGs, maybe inboard stall strips, etc. That's why I'm soliciting experience (not necessarily just opinions) Thanks - -
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  2. #42
    skywagon8a's Avatar
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    Bob's Decathalon is aerobatic so has different requirements. Thus should not be used as an example in your application.
    N1PA
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  3. #43
    BC12D-4-85's Avatar
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    Besides having done it that way (the 2.5* washout) since the 1930's, is there some performance requirement via CAR 4a (J-3; see Sec. 04.7 for performance requirements) then later CAR 3 (PA-18; also see Sec. 3.81 on) that determined that value? Or was it just done and left alone? I prefer 1* on Cub and Taylorcraft but my Champs have been flat by factory design.

    Gary

    Edit: There's also wing heaviness - elevator and trim deflection/drag to adjust for and wash has an effect. Edit: Meant nose or tail heaviness as affected by washout which affects lift from both wings.
    Last edited by BC12D-4-85; 12-07-2019 at 05:38 PM.
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  4. #44
    Gordon Misch's Avatar
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    Couldn't really find much in CAR 3. Speaking of wing heaviness, I assume you're referring to differential wing wash, rather than nominal.
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  5. #45
    BC12D-4-85's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    Couldn't really find much in CAR 3. Speaking of wing heaviness, I assume you're referring to differential wing wash, rather than nominal.
    CAR 4 and 3 list performance requirements. How wash affected the results during certification is a question that may never get answered - I don't have an answer - but I assume it's related to meeting those requirements or at least being benign enough to permit design acceptance.

    By heaviness I meant the relationship between wing wash and resulting overall wing lift versus the requirement for down force on the tail in all flight regimes for desired flight. I guess nominal applies if that means close to equal wash for both wings rather than one wing heaviness versus the other. Down force required on the tail has consequences. Edit: nose versus tail heaviness is a better term in this case. My winter Solstice fumbling.

    Wing devices change the behavior and washout may be less of a concern unless personal testing proves otherwise.

    Gary
    Last edited by BC12D-4-85; 12-07-2019 at 04:58 PM.

  6. #46
    AkPA/18's Avatar
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    Gordon I really don't mean to hijack This Thread into another Direction. I just glanced through all the posts on this thread and wanted to comment on the washout portion. The goal is not to have identical twist in each wing. The goal whether using a digital level or a bubble level is to have the same angle at one aileron Bay rib as the opposite side.
    http://thrustline.com/

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  7. #47
    Gordon Misch's Avatar
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    Quote Originally Posted by AkPA/18 View Post
    Gordon I really don't mean to hijack This Thread into another Direction. I just glanced through all the posts on this thread and wanted to comment on the washout portion. The goal is not to have identical twist in each wing. The goal whether using a digital level or a bubble level is to have the same angle at one aileron Bay rib as the opposite side.
    Well, sorta, I think. Let's assume that the incidence at the fuselage fittings differs between the two sides. Then, if the wing tips are at the same angle in reference to something, say the Horiz Ref Line, the wing roots will differ, and lift will differ because the average incidence across the span will differ. Same result if the wings each have the same twist.

    If the fuselage is "right", then either approach will yield the same, and proper, result. Piper's Service Memo is consistent with what you're saying.

    Please correct me if you think I'm wrong or missing something here.

    This is a new, jig-built fuselage by Steve Furjesi, and I'll bet that the wing incidence is very close to identical on each side. Probably worth double-checking though.

    Thanks for your input!
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  8. #48
    Gordon Misch's Avatar
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    Good stuff so far; thanks and please keep it up. Back to my original question - I know how to rig the wings to a given spec. What I don't know is what the preferred washout spec is among those who have flown different than the factory spec, and why.
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  9. #49
    AkPA/18's Avatar
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    It can be confusing Gordon and I wish I could explain stuff better in text. I will try with an extreme example. Suppose you have a fuselage with an 18 angle of incidence on one side and a 12 angle of incidence on the other side. If you put in the same wash on each wing no way will the aircraft Fly level. The 18 side will have considerably more lift than the 12 side. If you match the wing tips there will be considerably more wash taking away lift on the 18 side. This will give you a chance for the airplane to Fly level. That is all they are doing in the original Piper rigging instruction. They completely disregarded angle of incidence and Wing wash in the instructions and went right to matching the tips. I rarely come across an airplane with an identical angle of incidence on each side. This is probably confusing also but the best I've got at the moment. while the wash you have on your Cub is very important to know, if you match the tips you have a very good chance of a level flying cub.
    http://thrustline.com/

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  10. #50
    BC12D-4-85's Avatar
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    2nd hand personal comment to me from an experienced pilot that flew at altitude and lots on skis...flatter wing = less "mush" and uncertainty on takeoff and landing. He wanted lift that could be expected and depended upon when needed. Cruise speed was not a concern. Stock PA-18 wings and engine with factory flat prop.

    Gary
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  11. #51
    BC12D-4-85's Avatar
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    "...I'm looking for some guidance on an EAB -12 build I'm working on...I'd like to learn what the consensus might be for the "best" washout setting..."

    Gordon it's your turn. What's your design goal(s) for the new plane? That might help with any replies.

    Gary

  12. #52
    Gordon Misch's Avatar
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    Quote Originally Posted by AkPA/18 View Post
    It can be confusing Gordon and I wish I could explain stuff better in text. I will try with an extreme example. Suppose you have a fuselage with an 18 angle of incidence on one side and a 12 angle of incidence on the other side. If you put in the same wash on each wing no way will the aircraft Fly level. The 18 side will have considerably more lift than the 12 side. If you match the wing tips there will be considerably more wash taking away lift on the 18 side. This will give you a chance for the airplane to Fly level. That is all they are doing in the original Piper rigging instruction. They completely disregarded angle of incidence and Wing wash in the instructions and went right to matching the tips. I rarely come across an airplane with an identical angle of incidence on each side. This is probably confusing also but the best I've got at the moment. while the wash you have on your Cub is very important to know, if you match the tips you have a very good chance of a level flying cub.
    That makes perfect sense. Thank you.
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  13. #53
    Gordon Misch's Avatar
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    Gordon it's your turn. What's your design goal(s) for the new plane? That might help with any replies.
    Good question - This plane will spend time on floats as well as wheels. Likely more on floats. So we're thinking that with reduced washout it might come off the water (and ground) a little quicker and with a little more authority, and maybe even cruise slightly faster and more nose-down. Naturally we wonder if slow flight stability could be adversely affected. It will have Dakota squared off wings, with extended flaps. No slots. There's no intent for the crazy-short gravel bar stuff, however high density altitude strips (Idaho, Utah, etc) would be in the picture. The plane will have increased gross weight, probably 2200.

    Thanks for asking, and lemme know if this doesn't answer.
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  14. #54
    BC12D-4-85's Avatar
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    That's a good plan Gordon. I guess once built you'll have an opportunity to experiment with washout versus tail control effectiveness and trim over the range of airspeeds and CG.

    Edit: Also consider having adjustment for the rear strut length on floats for tuning to the fuselage. Depending on strut type and fitting block size there's potentially room for change (http://www.stoneylake.org/pipcom/frey.htm). Atlee's rear fittings with a solid cross drilled strut to fitting interblock allow for variable length. Straps can change rear wire length.

    Edit: Will you have an option for building a partial slot into the Dakota Cub wing? I'm thinking Stinson-like in front of the aileron and then a flatter wing would have less loss of aileron in a stall.

    Here's the plot of Cl, L/D, and Cd to play with. Varying washout might be a good tool to fine tune the performance.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by BC12D-4-85; 12-08-2019 at 01:12 AM.
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  15. #55
    RVBottomly's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    It will have Dakota squared off wings, with extended flaps. No slots.
    I can't remember where I read it, but I remember a discussion that squared-off wings don't really need washout. Something about diagonal flow across the trailing edge.

  16. #56
    RVBottomly's Avatar
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    Found one place discussing rectangular wings not needing washout:

    https://aviation.stackexchange.com/q...st-at-the-root
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  17. #57
    BC12D-4-85's Avatar
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  18. #58
    BC12D-4-85's Avatar
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    Here's a great read about aerodynamics. See pages 61 on for planform effects and 79-80 for pics of tufted rectangular wings during a stall. I keep one in the airplane shelf.

    Naval Aviators. https://www.faa.gov/regulations_poli.../00-80t-80.pdf

    Same here referenced above: https://ntrs.nasa.gov/archive/nasa/c...9930083818.pdf

    The airfoil used is a symmetric NACA 0012 section, or a 23012 like Taylorcraft, Caravan, Beech w/o the drooped leading edge. A Cub wing may differ.

    Gary
    Last edited by BC12D-4-85; 12-08-2019 at 03:00 AM.
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  19. #59

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    One of the Cubs I get to fly on a very regular basis was bought with the wings rigged the way piper wanted them rigged. The previous owners were that kind of crowd.

    When we got a hold of it, we thought it was under powered, lacked fuel, or something to that effect. It took at least 2500 rpm (at sea level) to get the tail up in flight. Less than that, and it always felt like it was off step, behind the curve, and dogged. We thought we had a lemon.

    Turns out we’d never flown a cub rigged the piper way, only the Alaska way. Took left and right rear struts in two turns and WHAMMO! new cub. Flies like the rest of them now. I surmise that because the entire wing is has a little more AOA in flight, it takes less power (airspeed) to get the required lift. (The ol’ lift equation comes in handy to explain that one).

    Now it’s true that nothing is free. When/if a stall occurs, the wing falls off all at once. Properly acknowledged, this is not a major threat in my opinion, though it might be for others.

    Short story is I much prefer no washout for the type of operations we do.


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