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Thread: Therapy Project

  1. #321
    RVBottomly's Avatar
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    Still plodding along….

    Lately I've been mocking up flap ideas. I've decided to not go with the double-fixed-position-slot I was playing with (and discussed on this thread: https://www.supercub.org/forum/showt...-on-kits/page4 )

    I've decided to stick to a simple slotted flap design. I was getting bogged down in stress calculations and permutations--and I was sort of getting scared of going too far astray.

    So, given that I don't want to take the time to learn CAD and model things the modern way, I rigged up a 3d mockup to look at things.



    I was moderately please with myself, noting the gap to fall within the .01-.02 chord length I see stated in various tomes, but then I looked at that thread above an saw Pete (skywagon8a) mentioning how the slot isn't really needed until higher deflections of the flap deployment. It sounds right to me. I'm thinking I have the gap opening too early in the video.

    The plan at this point is to go with longer than stock flaps (84 inches vs 63 inches or thereabouts), and increase the chord to 15.5 inches from the 13.25 inches. I chickened out on doing anything too radical because I'm working with a wooden wing and I'm not quite accomplished enough to push the envelope very far. I'm pretty sure I'll never enter a serious STOL contest anyway.

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  2. #322
    BC12D-4-85's Avatar
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    I wonder if there's any advantage or downside to forming a flap like a small wing with an upper (and/or maybe lower) upward camber? There are constraints at the rear wing cove that determine the frontal shape, but after that to the trailing edge would more of a wing shape be good? Or at least maintain the wings upper camber to the training edge. Another consideration might be to extend the cove shape and incorporate the Piper add-on fairing into the main wing like Maule and Cessna did. This probably has already been tried and decided.

    Another tome: http://naca.central.cranfield.ac.uk/...rc/rm/3681.pdf

    Gary
    Last edited by BC12D-4-85; 01-14-2020 at 12:55 AM.
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  3. #323
    Gordon Misch's Avatar
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    more of a wing shape
    Near the trailing edge the top and bottom contours of the wing are very nearly linear. So there's really no shape to play with there. If I'm understanding you correctly - - -
    Gordon

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  4. #324
    BC12D-4-85's Avatar
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    Yes Gordon there's probably not much to play with and I suspect the final shape is often dictated by ease of fabrication. But look at a Cessna flap (similar to the classic 2-h in A&V-D; p. 212) and unlike Piper the forward shape is more of an airfoil and the wing trailing edge more of a shaped cove that extends rearward to both accommodate the seal in cruise and then the upper energizing flow when extended. Still....an under camber somewhat paralleling the upper might be an interesting experiment. Might act similar to Gurney flaps by redirecting the trailing edge flow down more than a normal flat flap bottom (but at the expense of added drag in cruise).

    Gary

  5. #325
    Gordon Misch's Avatar
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    Yeah, could be. Unfortunately I don't know enough to offer an opinion on that! Oh wait - "fortunately". Ha!
    Gordon

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  6. #326

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    Personally I would model the flaps shape following what Cessna has done as Vic is leaning to. Their sliding pivots are nice but in Vic's view not easily achieved in his build such that a simple pivot is the way to go. Heck even I am utilizing simple pivots in my considerably more complex build.

    The 15.5 cord is right in line with the general feeling that .25c is right for the flaps. This is conservative and safe engineering.

    I myself would not utilize a Gurney lip but would prefer a blunt trailing edge should the effect the lip offers be desired. The blunt trailing edge would just be cropping off the the aft end leaving something like a ¼" or less flat end with sharp edges. This does not have the drag the lip has.

    But that said another personal opinion, I would stay close to the original airfoil unless you are ready for the can of worms that altering the TE can lead into.

    I would spend time working with "paper dolls", as in cut out the parts in cardboard and pivot them on a push pin. This work all done on a table allows to very fast alterations with quick measurements. This allows fast visualizing of the changes.
    I would not go to wood mockups till it really feels right.

    My wife marvels at how much I scrounge thin cardboard like what shirts used to get folded over and what pizza boxes were once made from. Rosin paper works well too.
    Yah the CAD program lets you see allot but I also model in cardboard as well as cut out my "dolls" in Masonite on a Gerber router. These parts are impressively accurate but do "move" with humidity.
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  7. #327
    skywagon8a's Avatar
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    Food for thought.
    No flaps:
    https://www.youtube.com/watch?v=E19812NHEmE
    https://www.youtube.com/watch?v=6UlsArvbTeo
    With hinged flap: Slotted flap starts at 7.38

    Notice what happens to the flow along the bottom of the wing as it moves up through the slot. At 8.04 notice the dead air space directly behind the wing in the slot. That is created by the shape of the curve transition from the bottom of the airfoil up into the slot. The radius of the curve in this video is too tight as is yours. It is actually creating a small aerodynamic dam reducing the smooth flow up through the slot opening. The whole idea is to have high velocity smooth flow through the slot opening. This high velocity maintains flow attachment on the top surface of the flap through higher deflection angles. There are some good views of leading edge slat flow at the end of this video. The principles are the same.

    Look at the bottom of a Cessna wing. Notice the shape of the surface as it transitions up into the slot. Years ago there was an STC kit which closed this opening when the flaps were up with the claim of higher cruise speeds. Cessna marketing wanted some speed changes for the new model year on the 206. So they closed the gap. This decreased the cruise drag by the amount that allowed the airspeed needle to move from one side of the marking line on the indicator to the other. It also increased the stall speed by the same amount. The test pilot who did these tests told me of this. Your Cub will not be anywhere near the speed of a 206 where the drag component has more effect.

    Vic, change the shape of the bottom corner of your wing rib to a much larger radius. Eliminate that dead air pocket when the flaps are down.
    N1PA
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  8. #328
    BC12D-4-85's Avatar
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    Here's some examples of a flap on the Arctic Tern or earlier Interstate. Note the cove and hinge forming an extension and gap. This is very close to the flap 2-h design on a 23012 airfoil in A&V-D I noted above.

    Gary
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  9. #329

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    Quote Originally Posted by BC12D-4-85 View Post
    Here's some examples of a flap on the Arctic Tern or earlier Interstate.

    Gary
    I had forgotten they had a good flap system. Vic I consider what Gary is showing here a great guideline for you to study. The brackets and cove shape are pretty darn easy to fabricate.
    The brackets could be steel or aluminum.
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  10. #330
    BC12D-4-85's Avatar
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    Here's a screen grab of the Cessna flap and cove cross section. Note the upper overlap lip extending aft over the flap and the design to the forward curved intake to the cove.

    Gary
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  11. #331
    skywagon8a's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    Here's a screen grab of the Cessna flap and cove cross section. Note the upper overlap lip extending aft over the flap and the design to the forward curved intake to the cove.

    Gary
    Keep in mind that this flap moves aft as well as down. If it were just hinged down, that top overlap would be further forward. In this case when the Cessna flap is down, the open gap size is the same as the Cub's hinged opening.
    N1PA
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  12. #332

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    Even with a simple pivot you can get similar travel to the Cessna system. Just that you have head banging pivots well down under the wing as mine will have.

  13. #333
    RVBottomly's Avatar
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    Wow. I've been off-line all day driving through snow storms and having hearings in remote courts--and I come back to see a lot of good ideas.

    Thanks everyone. Gary, before I saw your Tern photos I was thinking of drawing something like that. It's great to see it's been done before. I see it has the bottom of the cove radius just as Pete described.

    Charlie, I printed a wing profile and am cutting out shapes this evening. The head-banging hinge is something that I've been trying to avoid, but I might live with it and wear a hard hat. I know I'll bang my head at least once. I've done it on a Cessna twice.

    While I was driving, I was imagining a modified track system that mimicked a pivot like a low hanging hinge. Not as complex as the Cessna's, but it might be more complicated than I want. There's always that balance between the elegant, the outlandish, and the "just get it done."

    Vic

  14. #334
    RVBottomly's Avatar
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    Snip snip, measure with ruler and compass. I haven't had so much fun since grade school:



    Sorry for bad quality lighting and composition. Cell phone camera on a makeshift platform, but I think there is promise here.

    The hinge point is only 3.2" below the flap surface, which seems just enough to scalp, but not render unconscious.

    In the photos, the hinge point was adjusted ever so slightly forward compared to the video.

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  15. #335
    BC12D-4-85's Avatar
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    Husky put some rubber baby buggy bumpers on their flap hinge linkage = MTV might have some scar based feedback. I can see a couple of soft rubber oversize discs clamped by the hinge hardware dampening the experience.

    Gary
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  16. #336
    skywagon8a's Avatar
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    Quote Originally Posted by RVBottomly View Post
    Vic, You are getting there. As shown your gap between the flap and wing is closed until the flap is nearly fully deflected. This can initiate flow separation on top of the flap until the gap opens when it suddenly reattaches. This will effect your intermediate positions. Try moving your pivot point around until the gap starts to open as the flap begins it movement. Perhaps forward a bit? The gap wants to gradually open as the flap moves down.
    N1PA
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  17. #337
    RVBottomly's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Vic, You are getting there. As shown your gap between the flap and wing is closed until the flap is nearly fully deflected. This can initiate flow separation on top of the flap until the gap opens when it suddenly reattaches. This will effect your intermediate positions. Try moving your pivot point around until the gap starts to open as the flap begins it movement. Perhaps forward a bit? The gap wants to gradually open as the flap moves down.
    Moving the pivot forward is going the right direction:



    With these dimensions I think I'll mock up a full size model out of scrap ply.
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  18. #338
    skywagon8a's Avatar
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    Quote Originally Posted by RVBottomly View Post
    Moving the pivot forward is going the right direction:



    With these dimensions I think I'll mock up a full size model out of scrap ply.
    Don't forget to allow for the thickness of two layers (wing TE & flap LE) of fabric finished.
    N1PA

  19. #339
    RVBottomly's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Don't forget to allow for the thickness of two layers (wing TE & flap LE) of fabric finished.
    Right!

  20. #340
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by RVBottomly View Post
    Right!
    and on a cove shaped like that, the fabric tends to try to pull off, once the glue dries up and releases.

    so you might want a metal strip over fabric like the -18 uses in aileron bay, and cover that with a tape...

  21. #341
    skywagon8a's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    and on a cove shaped like that, the fabric tends to try to pull off, once the glue dries up and releases.

    so you might want a metal strip over fabric like the -18 uses in aileron bay, and cover that with a tape...
    mike, That is true with dope, more so with butyrate than nitrate. I haven't seen any evidence of that with the Stewarts glue.
    N1PA
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