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Thread: Tail Shakes

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    BES's Avatar
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    Tail Shakes

    A question for those of you who fly a PA-12 with an O-320 and flaps: Do your tailfeathers shake a lot when you select full flaps on final?

    Being a relative virgin on my flapped and O-320'ed -12, I experience a pronounced shaking of the stabilizers and elevators on final approach. It might be me doing something wrong - I try to approach at 45 knots - and the landings (three point) are quite controlable and soft (and short).
    I am less than impressed with the tail shakes, though.

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    Seaworthy's Avatar
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    Not on landing, but I am told that at an extreme AOA on take off, folks on the ground state that there is significant movement in the tail feathers that I cannot feel through the rudder pedals or stick. I checked the tension on the tail stabilizer wires and tension appears within normal limits. Still---somewhat disconcerting knowing that the tail is doing a dance out there. How much causes or may cause structural failure---I have no idea. I believe Charlie center has an STC for a tail section H brace--- but some times making things too rigid causes more issues ie the wing on the Lockheed Electra (turbo prop) circa 1957. Harmonics in aircraft are not a good thing.
    Marine Corps Aviation since 1966

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    Well, don't look back when departing at full power in a 150 Super Cub, either. I have been scared to look with the 180 SC. It does go straight up.

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    jgerard's Avatar
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    A little shake is normal from time to time on cubs and seems to vary based on load, speed, AOA, and power setting. Tail shake is one thing, but the stick pumping in your hand as the elevators hunt for neutral is something different. Change CG or adjust trim to see if you can lessen the effect.

    Is the Jack Screw worn out? Do you have a lot of up and down movement?

    Are your flaps extended inboard all the way to the fuse or do you have the normal wing root fairing?

    Do you have the standard long mount or the short mount engine installation?



    Jason

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    i can feel a little bit of buffeting in both of mt 12's when i go to full flap on short final, i don't think it's a big deal but on mine it is pretty mild. Both have ext flaps one is a 6' flap and the other has 8' both go right to the fuse. to me it just feels like a bit of turbulent air going over the tail it's not an osculation. how many degrees is full flap on your 12?

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    Steve's Aircraft (Brian)'s Avatar
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    On our Pacer with the flap extended to the fuselage gets a bit of tail shake at full extension..... Our Tri Pacer with stock flaps does not... That air going through the gap between the fuselage and flap makes a difference....

    Brian

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    BES's Avatar
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    The flaps are std. PA-18 flaps. Std. long engine mount. Vortex generators. The shaking is not so violent as to have the stick 'pumping', it's more like a shake, like driving down a gravel road at speed.
    The fuselage has all the recommended reinforcements, so I am not worried that something might break there. It is more the tailfeathers themselves I am concerned about.

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    skywagon8a's Avatar
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    I always thought that was a characteristic of a Super Cub. My Cub with extra long flaps does not shake the tail. So, this leads me to believe that the turbulent air that shakes the tail, is actually the air which is flowing around the ends of the flap creating a vortex. Has anyone tried placing a fence on the end or ends of the flap in an attempt at straightening the vortex?
    N1PA

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    Quote Originally Posted by BES View Post
    A question for those of you who fly a PA-12 with an O-320 and flaps: Do your tailfeathers shake a lot when you select full flaps on final?

    Being a relative virgin on my flapped and O-320'ed -12, I experience a pronounced shaking of the stabilizers and elevators on final approach. It might be me doing something wrong - I try to approach at 45 knots - and the landings (three point) are quite controlable and soft (and short).
    I am less than impressed with the tail shakes, though.
    On our SC rebuild, the flap gap seals were inadvertently left off. Result was tail shake with full flaps. Put them back on---no shake! Wind comes through that gap and shakes the tailfeathers.

  10. #10
    windy's Avatar
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    I had the 150hp O-320 on my PA-12, now the 180hp O-360. No tail shake or buffeting on landing with either, although it shakes some during the static runup. BTW knots are for boats.

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    BES's Avatar
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    Quote Originally Posted by Scruffdog View Post
    On our SC rebuild, the flap gap seals were inadvertently left off. Result was tail shake with full flaps. Put them back on---no shake! Wind comes through that gap and shakes the tailfeathers.
    Mine are scratch built wings, so I don't have any gap seals. Could you possibly post a picture of yours? What are they made of and how are they attached to the wing?

  12. #12
    BES's Avatar
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    Quote Originally Posted by windy View Post
    I had the 150hp O-320 on my PA-12, now the 180hp O-360. No tail shake or buffeting on landing with either, although it shakes some during the static runup. BTW knots are for boats.
    Sorry 'bout the knots thing - I have an old ex-mil helicopter ASI, which indicates knots.
    What would be your normal approach speed to a 2000' grass strip??

  13. #13
    skywagon8a's Avatar
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    BES, See if this works for you. It is Piper's flap fairing drawing.
    12795.PDF

    Piper Drawing #12795
    N1PA

  14. #14
    aviationinfo's Avatar
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    My tail shakes on takeoff at high angles of attack and low speed. I don't do those much anymore, and I have also learned to stop looking back there... on approach with the flaps down it shakes a bit. I've heard it's quite common.
    Aviationinfo

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    A little tail feather shake ain't a bad thing

    http://m.youtube.com/#/watch?v=rN5V-...%3DrN5V-6yCbpg

    Sorry just couldn't resist
    Remember, These are the Good old Days!

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    I have the 18 tail feathers on my -12 with a 160 horse 0320 and big/long flaps. I'm heavy enough that I won't set any takeoff records, but have managed to land fairly short a time or two. I've not noticed any tail-feather shake on either takeoff or landing, including full-flap landings. I don't fly very long on the final with full-flaps and would estimate that most of the time I'm using full-flaps, my speed is indicated around 35 mph or lower...but the gradations and the variability of the ASI at the low speeds mean that the readings are neither accurate nor what I rely on.

    FWIW, I have VG's on both the wings and on the underside of the tailfeathers. When the -12 was rebuilt, there were many things done to it, so I can't be exact about which change affected which flight characteristic, but prior to rebuild, even with the 18 tailfeathers, there were some specific landing situations (not that I recall the specifics) where the elevators ran out of authority...not often, and not that it was a bad thing. With the VG's on the underside, it seems that there is control effect all the way to tie-down.

  17. #17
    BES's Avatar
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    I agree about the VGs - they are great!
    Besides, the Micro Aero VG kit is the most complete and easy-to-install kit I ever saw!

  18. #18
    BES's Avatar
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    Quote Originally Posted by skywagon8a View Post
    BES, See if this works for you. It is Piper's flap fairing drawing.
    12795.PDF

    Piper Drawing #12795
    Thanks very much for the drawing. How (where) are the flap fairings installed - I cannot seem to find them in the PA-18 parts list..?!?

  19. #19
    Steve's Aircraft (Brian)'s Avatar
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    Here are a couple photos from the last cub I built... With and without the flap installed..

    Brian

  20. #20
    BES's Avatar
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    Thanks very much Brian. I guess it's true what they say:"A picture is worth a thousand words".


    Are these flap fairings always installed, or are they somestimes omitted?

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    I built and fly an experimental Super Cub. I flew the first year with out the wing flap gap strip. The shaking was exactly as you describe. I installed a pair of factory flap trim pieces and the shaking stopped. Full flaps was the worse. The photo above shows it well. Bought mine from Dakota.

  22. #22
    Steve's Aircraft (Brian)'s Avatar
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    The flap gap fairings are factory installed on every Cub and Pacer that I know of that have flaps...

    Brian

  23. #23
    BES's Avatar
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    I have ordered a set from Univair today. Looking forward to installing them and seeing what they will do.

    Thanks for all the helpful info.

  24. #24
    BES's Avatar
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    Update: After ordering and installing the flap fairings from Univair, the tail doesn't shake anymore. Thanks very much to all of you who took the time to help. Gotta love Supercub.org

  25. #25
    Steve's Aircraft (Brian)'s Avatar
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    Good to know that it solved your problem... Seems that ole Mr. Piper had it figured out....

    Brian
    Thanks JDWilliams thanked for this post

  26. #26

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    Are the gap seals different for use on a 14 wing? The Univair Catalog only list ones for an 18.

    Do they help clean up airflow on ailerons, too, or is that just a waste of money? Recommendations and videos, please.

  27. #27
    skukum12's Avatar
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    And while we are at it, how much gap should there be between the flap and seal?
    "Always looking up"

  28. #28
    skywagon8a's Avatar
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    Quote Originally Posted by skukum12 View Post
    And while we are at it, how much gap should there be between the flap and seal?
    None when up. 1/2" +/- when down.
    N1PA
    Thanks skukum12 thanked for this post

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