Tailwheel switch from Haigh to Steerable.

Discussion in 'Pitts' started by x2flycrj, Feb 14, 2018.

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  1. Feb 14, 2018 #1

    x2flycrj

    x2flycrj

    x2flycrj

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    Hello, I am finally getting around to switching over my Haigh (sp?) to an API Steerable tailwheel with my rod type stinger. A couple of questions.. (and please forgive me if it’s been answered before but I couldn’t find it anywhere), but what is the proper angle relationship should I have on my S1S (exp) for the tailwheel setup sitting on the ground? I have a new rod and I have to bend it to the proper angle and I’m not really sure what that is., I looked in the S1-S plans and didn’t see where it showed it. Second, What spring set up should I use for the springs and clips? Maule? Or try to get them from Aviat or Spruce? Does it matter? Thank you very much in advance!
    Greg
     
  2. Feb 14, 2018 #2

    lanceav8r

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    I have been trying to get a similar answer. You will get multiple opinions and no clear answers. The best I can figure out is that the angle changes the shimmy characteristics and the speed at which it happens but the proper angle doesn't make it disappear.

    If you find some good answers can you repost them here?
     
  3. Feb 14, 2018 #3

    IanJ

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    Whatever angle you choose, make sure that at max gross weight, the tailwheel steering pivot is either exactly vertical, or very slightly tilted away from the plane, to avoid shimmy.

    tailwheel-angle.png
     
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  4. Feb 14, 2018 #4

    IanJ

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    Sorry Lance, I didn't see your post before I posted my illustration above.

    What I discovered in my research is that the only thing that affects tailwheel shimmy, practically speaking, is the angle of the steering pivot. I'm sure that worn bearings and the like can contribute, but as long as the bits and pieces are reasonably sound, the pivot angle is what matters.

    To understand why, it's probably best to consider how the front end of a motorcycle or bicycle is set up. They have two related measurements: rake and trail. The one we care about is rake, or the angle of the steering pivot. The steeper the (positive) rake, the more resistant to turning the wheel will be, up to a point. That is, with positive rake (the tailwheel steering pivot axis tilted away from the airplane), the tailwheel will actually resist turning. At zero rake, it will follow where it's pointed, which is the ideal behavior for a trailing wheel like this. At negative rake, with the pivot axis tilting toward the fuselage, the wheel will tend to veer off and be sharply corrected back when it veers faster than the mass of the fuselage will follow, and that's exactly what shimmy is. If you find yourself with a shopping cart with bad wheel shimmy, take a moment to look at the pivot axis, and I bet you'll find it's been banged around, and is tilted forward, creating negative rake.

    Wikipedia provides a little bit more information: https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry

    Other factors, like wheel inflation, chain tension, etc. don't seem to matter much in a high-speed shimmy, which was most clearly demonstrated to me in a video like this one:

    [ame]https://www.youtube.com/watch?v=aGxXNZVKh08[/ame]

    You can clearly see that the chains are just along for the ride, and no amount of spring tension is going to stop the energy that's being generated by that shimmy.

    My Champ had bad shimmy last year, and it seems to have been caused by the tiniest negative rake. Adding a quarter inch spacer to the rear attachment point appears to have completely cured it. At the time of the shimmy, it had perfectly tensioned springs, and afterwards (presumably from compression of the springs during the shimmy incident; there was no damage evident), there's about a quarter inch of slack in the chains. Both before and after the shimmy incident, the wheel was and has been shimmy-free, so chain tension doesn't appear to be a factor.
     
    Last edited: Feb 14, 2018
  5. Feb 14, 2018 #5

    Chris McMillin

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    The only tailwheels that didn't shimmy on my parents airplanes were new ones. I knew it was the spring (replaced a nice old original tailwheel on the Chief to an oversized Scott complete with new spring) but for a long time didn't equate it to the pivot angle. But that is it.

    I installed a steerable Aviation Products tailwheel on my S-1D on a round spring and it was very nice. I did adjust the length and angle once I'd flown it for a while but it was all by eye. I'd had it bent too much and it was too long, so I reduced angle and length. Sorry I don't have the dimensions.
    Chris...
     
    Last edited: Feb 14, 2018
  6. Feb 14, 2018 #6

    bpulaski

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  7. Feb 14, 2018 #7

    Freex808

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    Greg, I've got a similar set up on my S-1 I keep at BDU. I've no idea what the angle is but you're welcome to come check it out if you like.

    Jim
     
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  8. Feb 15, 2018 #8

    bf92

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    My API is tilted forward at the top, about five degrees, in empty configuration. I have never had a shimmy on this tailwheel.

    Danny
     
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  9. Feb 16, 2018 #9

    lanceav8r

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    Any guesses how this tailwheel will act? Im told by some it will shimmy and others it will be fine.

    AA33B5A7-0D37-417A-BE5D-4A708ADE5BA3.jpg

    5E52C6C0-F9C7-417C-BD49-E0422F24EAC0.jpg
     
    Last edited: Feb 16, 2018
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  10. Feb 16, 2018 #10

    lanceav8r

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  11. Feb 16, 2018 #11

    f18shack

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    Reading Drew Fidoe's write up from the link in post #6, his conclusion is that the steering pivot angle should be "positive", as show in Lance's photo above. This is 180 out from Ian's picture in post #3!! Which one is correct?!?
     
  12. Feb 16, 2018 #12

    Acrobum

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    I have a API 6 inch with a J-3 flat spring. Full take off weight at 1035 pounds the tailwheel has a slight forward or slightly tilted towards the plane at the top. Never had a shimmy issue. I’ve ran my chains tight and loose. The API just doesn’t seem to care. The only real shimmy experiences I’ve ever had were Scott 3200s. I wouldn’t put too much thought into it unless it’s a huge noticeable tilt.
     
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  13. Feb 16, 2018 #13

    Neil

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    My bet is that it will be just fine for a couple of reasons. King pin is forward, lots of stagger between the axle and kingpin as the wheel is small so the ratio is high, small diameter wheel making the inertia in the wheel low.

    That lightening hole in the side of the fork does concern me a little though. Seems a bit much.
     
  14. Feb 16, 2018 #14

    lanceav8r

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    The lightening hole is huge leaving very little meat. It is made out of stainless. I’m not sure what the thought process on that is. Doug Dodge of Acro Specialties made it up.

     
  15. Feb 16, 2018 #15

    IanJ

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    I'd love to find out how that tailwheel will work. I'm not married to my theory, that's just what I discovered in my research, and seemed to be true in my case on the Champ. I'm perfectly willing to believe I'm wrong.

    I've thought about making up a testing device that would have a standard $4 caster from the hardware store mounted on a pivoting arm and using the belt sander as a treadmill, so I could test positive and negative tilt for the pivot axis to see if there's an obvious shimmy point. I still think that's an interesting thing to play with. I'll certainly post results here if I do that.
     
  16. Feb 16, 2018 #16

    bf92

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    That is about the loaded angle on the BF9-2, and I've had no shimmy problems at all. I'm using an API 6" dual-fork tailwheel.

    Danny
     
  17. Feb 16, 2018 #17

    Lotahp1

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    Ian I think your theory is ok also because if the tailwheel is slightly forward when empty it should be vertical when loaded with people. I do believe you will get a shimmy if it’s forward after loaded. If you had to be off any from vertical after it’s loaded I think it’s safer to be to like BF92 and the original poster has. That’s my experiences also. Now once you get that rake to far it will shimmy raked back just like it will raked forward. There’s a little bit of wiggle room on both sides of vertical I think.

    The Champ I fly has a shimmy sometimes too. I simply unload the tailwheel and it stops. It’s a rent plane and after I mentioned it to the owner he told me to do exactly that...unload it when it does it. It’s alright.
     
  18. Feb 16, 2018 #18

    x2flycrj

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    Wow, great replies all! Well it sounds like the API is pretty robust, so I'm glad I'm going with that. Ian's theory seems pretty sound. Thank you for the info- I really appreciate it. I did a little researching on this forum and it seems like between 11-14 degrees on the angle of the airplane when its sitting.. and I have the Wolf gear installed which is about 2'' taller maybe so I will try to bend that Stinger to give me that with the 10 degree API 4 inch Tail-wheel.. With a bit of positive rake to be on the safe side.. Thanks again for the responses, and Jim I might take ya up on that.
    Cheers, Greg
     
  19. Feb 17, 2018 #19

    Dennis5678

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    Why are changing it in the first place?
     
  20. Feb 17, 2018 #20

    x2flycrj

    x2flycrj

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    I didn’t like having to ride or rather tap the brakes to S turn while taxiing, and below around 30 kts didn’t have a lot of rudder authority .. and in my case I had a left brake failure on landing and by the time I realized it, was heading to the right side of the runway without a whole lot of rudder authority to straighten me out.. (with the Haigh locking tail wheel) . So.. switching to the steerable.
     

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