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Thread: Undercarriage rattle on pavement

  1. #1

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    Undercarriage rattle on pavement

    Having finished the major jobs on my PA-12 Iím now flying it and venturing further afield!!! Iíve noticed since Iíve had it a twitchiness at the end of the landing run and on grass, thatís all Iíve noticed, but today I did 3 circuits on pavement and I noticed something else, which happened at pretty much exactly the same point on the runway:

    With about 75 yards to go until the end of the runway, at a jogging sort of pace, there was suddenly a lot of metallic type rattling from below me, pretty much where the undercarriage is. This happened on the last 2 of the 3 circuits (possibly on the first too) and the first time it happened I thought the gear had failed!! On each occasion the rudder steering suddenly felt very sensitive and I switched instantly to the heel brakes, which immediately settled things down.

    I checked the undercarriage and tailwheel immediately after and nothing obvious and on arrival back at home base on grass, no clanking at all, but still the sensitivity and hunting toward the end of the roll out hmmmmmmmmmmm

    Any ideas please?!!


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  2. #2

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    tailwheel shimmy
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  3. #3

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    Undercarriage rattle on pavement

    Quote Originally Posted by Randyk View Post
    tailwheel shimmy
    Thanks for this and I suspected this might be one of the possible causes............ along with main wheel bearings possibly

    I fitted Maule anti shimmy springs right at the start and an API 6131 tailwheel recently. API donít recommend the Anti Shimmy springs and Iím not completely closed to this, but I had great success with them on my Pitts S-1S, so I have continued to give them a go. Iím looking forward to hearing the collected opinion on this
    Last edited by Philly5G; 05-05-2019 at 02:31 PM.

  4. #4
    BC12D-4-85's Avatar
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    Unweigh the tail with a support under the forward spring mount and move things. Look for loose parts - cable springs, tail spring, tailwheel body, and tire.

    Read this: http://www.airframesalaska.com/v/vsp...Prevention.pdf

    If broke then fix. If ok move to main landing gear and do same. Lift plane and inspect the main gear and fuselage.

    Also look for loose items under the floor and in the aft fuselage.

    Gary
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  5. #5

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    Quote Originally Posted by BC12D-4-85 View Post
    Unweigh the tail with a support under the forward spring mount and move things. Look for loose parts - cable springs, tail spring, tailwheel body, and tire.

    Read this: http://www.airframesalaska.com/v/vsp...Prevention.pdf

    If broke then fix. If ok move to main landing gear and do same. Lift plane and inspect the main gear and fuselage.

    Also look for loose items under the floor and in the aft fuselage.

    Gary
    I have noticed the top spring of the 3 tailwheel springs moving laterally if I waggle the back end of the aircraft from side to side and it can obviously move maybe half an inch laterally at its back pointy end. From looking at this in detail I canít see a way to stop it so assumed this is Click image for larger version. 

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  6. #6
    BC12D-4-85's Avatar
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    I'm a bit confused and the picture doesn't do justice to your post. The spring assembly is bolted to the frame forward of the tail post. At the tail post there's typically a U-shaped clamp or flat or U-channel bar with two bolts to hold it tight to the frame. Any side clearance between the springs and hardware should be removed at that location with shims...rubber or metal. If the top spring is floating at the rear end over the springs below then sometimes a clamp is placed nearby still to prevent side motion but allow vertical movement. If that tailwheel can easily be turned by hand then follow the manufacturer's recommendations regarding preloading the vertical shaft to discourage shimmy (thus more resistance to turning). And note the link above regarding a positive angle of castor.

    Gary
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  7. #7

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    Quote Originally Posted by BC12D-4-85 View Post
    I'm a bit confused and the picture doesn't do justice to your post. The spring assembly is bolted to the frame forward of the tail post. At the tail post there's typically a U-shaped clamp or flat or U-channel bar with two bolts to hold it tight to the frame. Any side clearance between the springs and hardware should be removed at that location with shims...rubber or metal. If the top spring is floating at the rear end over the springs below then sometimes a clamp is placed nearby still to prevent side motion but allow vertical movement. If that tailwheel can easily be turned by hand then follow the manufacturer's recommendations regarding preloading the vertical shaft to discourage shimmy (thus more resistance to turning). And note the link above regarding a positive angle of castor.

    Gary
    Thanks for this Gary! Hopefully these pics are better. Itís not easy to see still, but the U shaped bracket is definitely wider than the springs and I guess hence the lateral movement, as the top spring isnít bolted to the lower 2 at the rear and the wider U channel obviously isnít holding it either

    Iíll get some shims organised as a starting point!!

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  8. #8
    BC12D-4-85's Avatar
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    Better pics and thanks for the info. Some will put a shim (like aluminum or reinforced rubber from a tire sidewall) between the tail post mount and top spring to lower the spring away form the fuselage and adjust the castor angle per the Airframes link. But obviously if there's side play in the springs at the clamp that needs to be corrected. Again I've seen (not a mechanic just an owner of many) rubber placed there to take up the side play. If there's still lateral play in the spring assembly then fabricate a reinforcing clamp over the springs near the tailwheel. From there on I don't know what the manufacturer recommends for pre-loading the tailwheel to reduce shimmy.

    Gary
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  9. #9

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    Undercarriage rattle on pavement

    Quote Originally Posted by BC12D-4-85 View Post
    Better pics and thanks for the info. Some will put a shim (like aluminum or reinforced rubber from a tire sidewall) between the tail post mount and top spring to lower the spring away form the fuselage and adjust the castor angle per the Airframes link. But obviously if there's side play in the springs at the clamp that needs to be corrected. Again I've seen (not a mechanic just an owner of many) rubber placed there to take up the side play. If there's still lateral play in the spring assembly then fabricate a reinforcing clamp over the springs near the tailwheel. From there on I don't know what the manufacturer recommends for pre-loading the tailwheel to reduce shimmy.

    Gary
    Thatís a great idea to use a shim to increase the castor angle!! Iíve got some really solid Viton rubber sheet that I used to make the brake master cylinder diaphragm and I could use some of that, or use some metal too. THANKS Gary, youíre really really helping me on this


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  10. #10
    BC12D-4-85's Avatar
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    If adjusting the castor angle requires an upper shim, and the U-bracket isn't tall enough to accommodate that, then shim both the top spring and clamp away from the frame with a full width piece. Bellanca did that on the Citabria series from a piece of aluminum with the ends notched to fit around the inside of the two attaching bolts. You have to play some with all that but I'd want to end up with no appreciable vertical and and lateral movement of the springs at the tail post which a shimmying tailwheel could magnify.

    I'd also maintain a slack-free compression spring link from the rudder to tailwheel. That can help stabilize the tire and reduce its free movement beyond rudder control.

    Gary
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  11. #11

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    Quote Originally Posted by BC12D-4-85 View Post
    If adjusting the castor angle requires an upper shim, and the U-bracket isn't tall enough to accommodate that, then shim both the top spring and clamp away from the frame with a full width piece. Bellanca did that on the Citabria series from a piece of aluminum with the ends notched to fit around the inside of the two attaching bolts. You have to play some with all that but I'd want to end up with no appreciable vertical and and lateral movement of the springs at the tail post which a shimmying tailwheel could magnify.

    I'd also maintain a slack-free compression spring link from the rudder to tailwheel. That can help stabilize the tire and reduce its free movement beyond rudder control.

    Gary
    After a few unexpected weeks off the job, Iíll be back on this on Monday!

    Having had a while to think about this, the compression springs and chains are definitely a bit slacker than Iíd like and this will be first job, along with reducing the play in the U channel clamp and shimming up to get a slightly positive camber angle.

    The adjustment of length is basically by adding/removing chain links, but going to the next chain link will make them pretty tight, so I really need a finer adjustment method. Iíve considered turnbuckles or similar and wonder what others have had success with?


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  12. #12
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    Check out the local hardware stores, t/w steering chain (aka sash chain) comes in different pitches.
    5 links of pitch #A is different length than 5 links of pitch #B.
    You can also slightly stretch a chain by squeezing each link with pliers.
    Not too difficult to end up with the equivalent of an extra half link.
    Cessna Skywagon-- accept no substitute!
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  13. #13

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    Quote Originally Posted by hotrod180 View Post
    Check out the local hardware stores, t/w steering chain (aka sash chain) comes in different pitches.
    5 links of pitch #A is different length than 5 links of pitch #B.
    You can also slightly stretch a chain by squeezing each link with pliers.
    Not too difficult to end up with the equivalent of an extra half link.
    GREAT idea!!!!!! Thanks!!


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    Update! Having watched this video a couple of times https://www.youtube.com/watch?v=Qtok...&index=19&t=0s I realised my steering chains were too loose, as I could break out the tailwheel with very little rudder applied. I removed a link from one of the steering chains and then switched the chains over right to left and vice versa to get the tailwheel more lined up with the rudder as the chains are different lengths, this reduced the overall slack in the chains to a point where it wasn't TOO tight and more rudder is now needed before the TW will break out. It's not exactly as good as the video, but I'll need to fine tune the chains less than one link's length to get it better.

    I test flew on Saturday and the TW definitely feels more "locked" and I can really feel when the steering is engaged and when I need to use the brakes. I did 2 tarmac circuits, on a more broken tarmac surface than when I noticed the shimmy and had to use the brakes towards the end of the landing run as a shorter strip, but no signs of shimmy at all. So I'd say we're on the right road and will see what happens when I'm back at the longer and smoother tarmac runway I'll keep you in the loop!!

  15. #15

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    If it isn’t the tailwheel shimmying, the “rattle” from the undercarriage might just be the brake pucks rattling around in their housing.
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  16. #16

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    Quote Originally Posted by Paul Heinrich View Post
    If it isnít the tailwheel shimmying, the ďrattleĒ from the undercarriage might just be the brake pucks rattling around in their housing.
    Sounds like a possibility, but What are brake pucks? Itís still got the bag brakes. Iím currently researching the Grove disk conversion kit, which is the cue for another thread


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  17. #17

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    Quote Originally Posted by Philly5G View Post
    What are brake pucks?
    The pucks are the moving part of the brakes that push up against the brake disk or brake drum when you step on the brake pedal causing friction that stops the wheels from turning. They rattle around in their housing when there is no brake pressure applied. It’s normal and a sign that the brakes are fully released.

    One way to know if the brake pads are causing the rattle is to taxi fast enough to hear the rattle and then gently press on the brakes while keeping the speed up. If the noise goes away, you know it’s normal brake rattle.
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  18. #18

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    Quote Originally Posted by Paul Heinrich View Post
    The pucks are the moving part of the brakes that push up against the brake disk or brake drum when you step on the brake pedal causing friction that stops the wheels from turning. They rattle around in their housing when there is no brake pressure applied. Itís normal and a sign that the brakes are fully released.

    One way to know if the brake pads are causing the rattle is to taxi fast enough to hear the rattle and then gently press on the brakes while keeping the speed up. If the noise goes away, you know itís normal brake rattle.
    Thanks!! The rattle certainly went away when I used the heel brakes, but I hadnít heard the rattle before or since!! It was the first time Iíd landed on such smooth tarmac though AND let the landing run continue so far on tarmac before using the brakes........... hmmmmmmmm


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  19. #19

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    If you get a chance, post a couple pictures of your main landing gear that shows your brakes and let us know if my suggestion works.
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  20. #20

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    Quote Originally Posted by Paul Heinrich View Post
    If you get a chance, post a couple pictures of your main landing gear that shows your brakes and let us know if my suggestion works.
    Will do and thank you


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  21. #21

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    Update on the update

    We made a shim to go between the tailwheel spring and the bottom of the fuselage, where held on by the U bolt. About 1/4" thick and this increased the spring height which adjusted the castor angle to just slightly positive.

    This also allowed me to fine tune the chain lengths and use the Maule anti shimmy instructions to pre tension the springs before fitting to the steering arms.

    I went back to the smooth tarmac (pavement) runway yesterday and did 3 circuits (and proud of my landings!!) and 2 things:

    There was no shimmy
    The steering was so precise on landing that I had to consciously relax my foot pressure on the rudder pedals to avoid slightly oversteering and getting the slight nose bob from side to side

    So all in all, I'm thinking we're pretty much there
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