Tailwheel caster and shimmy?

Discussion in 'Pitts' started by lanceav8r, Feb 16, 2018.

Help Support Biplane Forum by donating using the link above.
  1. Feb 16, 2018 #1

    lanceav8r

    lanceav8r

    lanceav8r

    Well-Known Member

    Joined:
    Mar 7, 2012
    Messages:
    546
    Likes Received:
    72
    Alright guys. After doing some reading and hearing varying opinions im worried sbout my tailwheel setup. Can some of you guys that are in the know take a look at this! This is on a One Design.

    43EFE1ED-2B40-466E-8FC5-65B15986B339.jpg

    B4C7C3EE-289E-4741-989B-181CDBA39B02.jpg
     
    Last edited: Feb 16, 2018
  2. Feb 16, 2018 #2

    EAABipe40FF

    EAABipe40FF

    EAABipe40FF

    Registered Users Supporting Member

    Joined:
    May 30, 2009
    Messages:
    4,857
    Likes Received:
    920
    NOT what you want.
     
  3. Feb 16, 2018 #3

    navybipe

    navybipe

    navybipe

    Well-Known Member

    Joined:
    Apr 19, 2013
    Messages:
    543
    Likes Received:
    173
    and when you load it up, going to get worse. boy, that is one big lightening hole in the forks.
     
  4. Feb 16, 2018 #4

    PittsDriver68

    PittsDriver68

    PittsDriver68

    Well-Known Member

    Joined:
    Jan 9, 2013
    Messages:
    4,055
    Likes Received:
    1,110
    First, how does it perform?

    Does it shimmy at all?

    Does it steer properly?

    Who made it?

    From the picture, it appears that when the spring flexes the tailwheel pivot will still be at an angle that should keep the steering axis (extended pivot line) ahead of the tire contact patch. But there is theory and there is mission adequate. So the question is what does it really do?

    Best of luck,

    Wes
     
    Last edited: Feb 16, 2018
    curtis likes this.
  5. Feb 16, 2018 #5

    cgzro

    cgzro

    cgzro

    Peter Ashwood-Smith P

    Joined:
    Nov 18, 2012
    Messages:
    1,053
    Likes Received:
    283
    The contact point wheel/ground relative to the pivot point is important. Drag acts at the contact point so if its in front of the pivot axis drag will rotate the wheel out of trail. If the contact point is behind the pivot drag will keep the wheel in trail. The latter is better for tracking and takes an
    energy source away from
    shimmy. It may however work find as is if its low mass and locking.
     
  6. Feb 16, 2018 #6

    Neil

    Neil

    Neil

    Well-Known Member Lifetime Supporter

    Joined:
    Nov 10, 2006
    Messages:
    6,048
    Likes Received:
    860
    Gender:
    Male
    Location:
    Monroe, Louisiana
    The kingpin angle is arguable for both ways ways far as tilt. You can tilt it forward a good bit on a lightweight airplane and it will become very stable. I had a 6" Maule on my Sonerai tilted forward about 15 degrees and it would track so straight on it's own you had to push a lot of rudder to steer it off track. It had a single leaf spring and when loaded the angle was more like 20 deg. plus. If it had been on an airplane with 100 pounds on the tail you would have to use a lot of brake just to steer.

    From my observations and information collected over the years, there is a range of angle where "shimmy" is likely to occur and moving beyond that in either direction seems to counteract that tendency.

    Lean the kingpin back too far and it gets closer to being over the wheel causing the airplane to want to flop away from straight. Leaning the kingpin forward causes the airplane to want to sort of pendulum toward the center line. Finding what works best on a particular airplane is the challenge.

    If the tail wheel is simply worn out, all bets are off.

    Also, Castor is an oil, caster is geometrical.:)
     
  7. Feb 16, 2018 #7

    lanceav8r

    lanceav8r

    lanceav8r

    Well-Known Member

    Joined:
    Mar 7, 2012
    Messages:
    546
    Likes Received:
    72
    Thanks guys. Keep the opinions coming. Doug Dodge made it for me. I too thought the lightening hole is a bit large. I don’t know why he put such an angle on the king Pin. He doesn’t do technology and he is hard to get on the phone.

    It’s on a one Design and I think the tail weight will be similar to a Pitts S1. As far as the rest I have no idea.
     
  8. Feb 17, 2018 #8

    lanceav8r

    lanceav8r

    lanceav8r

    Well-Known Member

    Joined:
    Mar 7, 2012
    Messages:
    546
    Likes Received:
    72
    I had a conversation with Doug Dodge today. He said he designed/built it with an expected 5 degree angle and said it should be fine. But then he told me he didn’t like my mounting bracket and attachment point. He said the aluminum may crack and that the attach bracket was too thin. So i guess ill ditch the aluminum part and cut off the attach bracket and make one out of .100”. View attachment IMG_4324.jpg
     
    Dennis Flamini likes this.
  9. Feb 17, 2018 #9

    PittsDriver68

    PittsDriver68

    PittsDriver68

    Well-Known Member

    Joined:
    Jan 9, 2013
    Messages:
    4,055
    Likes Received:
    1,110
    You do not state how thick the steel of the fitting welded to the bottom of the tailpost is, but historically, Pitts have a problem with those cracking off. A friend just went off the runway on landing due to that fitting cracking off. The taller the vertical triangle part of the bracket, the better. Doug Dodge's advice is likely sound.

    Best of luck,

    Wes
     
  10. Feb 17, 2018 #10

    lanceav8r

    lanceav8r

    lanceav8r

    Well-Known Member

    Joined:
    Mar 7, 2012
    Messages:
    546
    Likes Received:
    72
    It is .063 and built to the One Design plans. I see in Aviat’s plans they spec .100”. I was thinking of just welding on a doubler but Doug didn’t recommend that and said i should replace it. Any thoughts on replacing vs doubler? I don’t have any expertise in this area.
     
  11. Feb 17, 2018 #11

    Dave Baxter

    Dave Baxter

    Dave Baxter

    Well-Known Member Lifetime Supporter

    Joined:
    Nov 15, 2007
    Messages:
    1,563
    Likes Received:
    732
    I am with Doug Dodge also, under normal flight loads the .063 bracket would be just fine, the one on the Starduster is .071, and if one has severe tail wheel shimmy it can and will rip off that bracket, honest, it should be made from .090 4130, and also should go farther up the back side of the tail post.

    As for caster I am big on positive caster, but negative caster will also work, where they are the most susceptible to shimmy is in the neutral point as the wheel has no inherent directional stability. There are several detailed threads posted on this site regarding shimmy issues. also tail weight and spring flex can change the angle when loaded. Dave

    tws.jpg
     
    Last edited: Feb 17, 2018
    ilikeitinverted likes this.
  12. Feb 17, 2018 #12

    lanceav8r

    lanceav8r

    lanceav8r

    Well-Known Member

    Joined:
    Mar 7, 2012
    Messages:
    546
    Likes Received:
    72
    Thanks Dave.
     
  13. Feb 17, 2018 #13

    cgzro

    cgzro

    cgzro

    Peter Ashwood-Smith P

    Joined:
    Nov 18, 2012
    Messages:
    1,053
    Likes Received:
    283
    Negative caster means as wheel turns it keeps turning. That may or may not cause shimmy but its not great for directional control. Positive caster means as wheel turns the drag as the contact point rotates forward pulls it back into trail.
     
    lanceav8r and Neil like this.
  14. Feb 21, 2018 #14

    tl_z_bub

    tl_z_bub

    tl_z_bub

    Supporting Member Supporting Member

    Joined:
    Sep 13, 2011
    Messages:
    214
    Likes Received:
    114
    Here's a little video of my old Maule tailwheel.

    What interests me about this is how the wheel bounces to the side when the shimmy is initiated. It's such a violent movement I'm not at all surprised there's a bit of shimmy before it settles again.

    Maybe a little friction in the tailwheel pivot itself might help ? (Mine rotates very easily).

    [ame="https://vimeo.com/70845565"]https://vimeo.com/70845565[/ame]

    Whatever the answer, the poor old tail wheels have a hard life to say the very least !
     
    Last edited: Feb 22, 2018
    philly5g and biplanebob like this.
  15. Feb 21, 2018 #15

    cgzro

    cgzro

    cgzro

    Peter Ashwood-Smith P

    Joined:
    Nov 18, 2012
    Messages:
    1,053
    Likes Received:
    283
    Yeah no kidding, and that was a good landing. I find the Maule pivot needs weekly greasing or the pivot bushing wears very quickly.
     
  16. Feb 24, 2018 #16

    lanceav8r

    lanceav8r

    lanceav8r

    Well-Known Member

    Joined:
    Mar 7, 2012
    Messages:
    546
    Likes Received:
    72
    Moderator: can you fix the spelling in the title?
     

Share This Page