Tailwheel shimmy, any ideas?
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Tailwheel shimmy, any ideas?
My last 3 landings ( 2 on grass/1 on tar) the tail wheel rattles and feels like its gone flat, from about touchdown thru to about 20mph, the tire, wheel and leaf spring look fine.
Is this the side springs or the bearing that could be causing this
Is this the side springs or the bearing that could be causing this
I would start with re greasing the swivel, there should be a grease nipple.
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Might be a silly question, but did you try "unloading" it briefly with a bit of forward stick to take the weight off, and then resetting?
the most dramatic rattle and shakes i get is when on tar, it feels really rough right after touch down
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I would start with re greasing the swivel, there should be a grease nipple
I am always a bit gun shy about putting to much grease in bearings after talking to the local Ag equipment sales guy who said the majority of the problems he gets with equipment like seed drills etc is actually over greasing that i suppose pushes out the seals then when the grease dissipated lets crap get back in.
But i will try greasing it as a first step, again thank you.
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On our Jodel we lift the tail, put a stand under the fuselage, and check. The bolts holding the spring to the fuselage have needed tightening. Or the movement might be in the wheel assembly to spring. Or in the wheel.
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I had this and the subsequent discussion this week on the citabiria I sometimes fly.
A few things worth checking - tyre pressure being an easy one, also the castoring (inclination of the pivotal axis) angle affects the shimmy or otherwise, so if that were out due to say a heavy tailfirst landing or a loose mount it could be a factor. On the citabria, I noticed that the springs that form part of the linkage from the rudder to the tailwheel are different either side, I presume that is so they have different resonant frequencies, if they both resonated together at the same frequency that would be bad and likely provoke bad shimmy at the resonant frequency. This citabria has a well greased pivot, but I had shimmy on at least one landing last session.
I expect someone with many more tailwheel hours than me can advise the best way to safely stop this during the rollout or comment if there is a landing bad habit such as being in the threepoint attitude too early that tends to bring it on. I'd be interested to know more myself.
A few things worth checking - tyre pressure being an easy one, also the castoring (inclination of the pivotal axis) angle affects the shimmy or otherwise, so if that were out due to say a heavy tailfirst landing or a loose mount it could be a factor. On the citabria, I noticed that the springs that form part of the linkage from the rudder to the tailwheel are different either side, I presume that is so they have different resonant frequencies, if they both resonated together at the same frequency that would be bad and likely provoke bad shimmy at the resonant frequency. This citabria has a well greased pivot, but I had shimmy on at least one landing last session.
I expect someone with many more tailwheel hours than me can advise the best way to safely stop this during the rollout or comment if there is a landing bad habit such as being in the threepoint attitude too early that tends to bring it on. I'd be interested to know more myself.
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Banditb6,
Interesting article, i will throw a set square on it to check but i would be very surprised if that is the issue as the aircraft has about 200 hours on it since a factory rebuild, granted its had some rough landings. Does anyone know what the usual lifecycle/landings is for the tail-wheel componants like leafspring etc, or is that like asking how long is a piece of string?
The leafspring looks pretty robust I would think other parts would give before the leaf did.
Interesting article, i will throw a set square on it to check but i would be very surprised if that is the issue as the aircraft has about 200 hours on it since a factory rebuild, granted its had some rough landings. Does anyone know what the usual lifecycle/landings is for the tail-wheel componants like leafspring etc, or is that like asking how long is a piece of string?
The leafspring looks pretty robust I would think other parts would give before the leaf did.
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Are the "chains" between rudder & anti shimmy springs tight, On the Maule I think you can shorten them a link at a time. If I remember right I think they go slacker as the weight comes on & the leaf spring moves up? Them springs do have to be different to not be in harmony.
Last edited by Crash one; 4th Jun 2013 at 22:44.
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It depends on the tailwheel design. Some have friction plates in them and tightening the through bolts will increase the dampening as will greasing as stated before.
Yes, need to tighten the bolt every now and then as the internal bits wear.
If it is a Scott (now Alaskan Bushwheels) 3200 or similar I can provide their shimmy checklist of things to check and rectify.
Items #1 and #2 are tire tread wear and tire pressure.
If it is a Scott (now Alaskan Bushwheels) 3200 or similar I can provide their shimmy checklist of things to check and rectify.
Items #1 and #2 are tire tread wear and tire pressure.
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The plot thickens !
So i have a good look at the tailwheel this morning and yes the axle did need some grease and putting the tyre pressure right as it was down a few PSI then checked all the bolts on the leafspring were tight, then as i was checking it out i looked at the locking pin and thought to myself "wait a minute i can't remember the last time i felt the lock pin engage while turning during taxiing" upon further inspection the tail-wheel was totally free castoring and was not locking in to the rudder steer, i had a spare lockpin which was duly installed and upon hand testing the unit functioned properly BUT when i went to try it while taxxing it was back to totally free castoring and not locking in to the rudder steering range.
So not sure where to go from here? not sure if its a new tailwheel assembly to be bought, as i see no other parts that i could adjust or replace that would allow the caster lock to function properly
So i have a good look at the tailwheel this morning and yes the axle did need some grease and putting the tyre pressure right as it was down a few PSI then checked all the bolts on the leafspring were tight, then as i was checking it out i looked at the locking pin and thought to myself "wait a minute i can't remember the last time i felt the lock pin engage while turning during taxiing" upon further inspection the tail-wheel was totally free castoring and was not locking in to the rudder steer, i had a spare lockpin which was duly installed and upon hand testing the unit functioned properly BUT when i went to try it while taxxing it was back to totally free castoring and not locking in to the rudder steering range.
So not sure where to go from here? not sure if its a new tailwheel assembly to be bought, as i see no other parts that i could adjust or replace that would allow the caster lock to function properly
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Locking pin or lack of doesn't cause shimmy but can cause poor ground handling and greater risk or groundloops. The geometry of the tailwheel can be a huge factor though, and I'd advise checking that. I've had two aircraft that suffered shimmy. Both were cured by "straightening" up the angle of the pivot pin by altering the tailspring angle.
SS
Edited to add ... What sort of tailwheel is it? I had a share in a Falconar with a Prestige tailwheel. The locking pin on that disintergrated and ended up fully castoring too (didn't shimmy mind).
SS
Edited to add ... What sort of tailwheel is it? I had a share in a Falconar with a Prestige tailwheel. The locking pin on that disintergrated and ended up fully castoring too (didn't shimmy mind).
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I've been in the same position with several types of tailwheel.
Tyre pressure or wear will often set if off. My Terrier was paticularly sensitive to the pressure dropping even a little.
Next thing is the general condition. I've been surprised by the amounto f wear that occurs, usually in the vertical bearing, but sometimes also on the vertical shaft. The assymetric loading on the assembly is pretty high and naturally elongates the bearing. Not a lot of play is necessary for the shimmy to start. My Zodiac needed a complete rebuild after 500 hrs.
Greasing - well always necessary, if only to delay the situation above.
Next biggy is whether the pivot axle is vertical to the ground when in the three point position. The more it trails due to the spring settling the more likely shimmy is. My Emeraude and Zodiac both suffered from this - the Zodiac needed the spring 'resetting' every annual!
Finally if all this is still not working you might try the Maule springs - they delayed this all happening on the Emeraude - but as you fly a Maule perhaps they are already fitted?
Tyre pressure or wear will often set if off. My Terrier was paticularly sensitive to the pressure dropping even a little.
Next thing is the general condition. I've been surprised by the amounto f wear that occurs, usually in the vertical bearing, but sometimes also on the vertical shaft. The assymetric loading on the assembly is pretty high and naturally elongates the bearing. Not a lot of play is necessary for the shimmy to start. My Zodiac needed a complete rebuild after 500 hrs.
Greasing - well always necessary, if only to delay the situation above.
Next biggy is whether the pivot axle is vertical to the ground when in the three point position. The more it trails due to the spring settling the more likely shimmy is. My Emeraude and Zodiac both suffered from this - the Zodiac needed the spring 'resetting' every annual!
Finally if all this is still not working you might try the Maule springs - they delayed this all happening on the Emeraude - but as you fly a Maule perhaps they are already fitted?
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Anyone who regularly "rides" the supermarket shopping trolleys, knows all about castor-shimmy 
The trail-angle of the pivot (clue, VERTICAL PIVOT ) is all-important.
Likewise, sidecar-outfit drivers know all about "tank-slappers" caused by the wrong trail-angle......some bikes had the axle offset from the fork-stanchion centreline....you removed it. turned the stanchions 180* and refitted the wheel with the altered trail-angle....it was only a couple of inches, but enough to eliminate the shimmy, otherwise, one had a steering-damper (friction-plate "sandwich" with a large knob on the top f the steering-head) amazing what half a turn could do!
The trail-angle of the pivot (clue, VERTICAL PIVOT ) is all-important.
Likewise, sidecar-outfit drivers know all about "tank-slappers" caused by the wrong trail-angle......some bikes had the axle offset from the fork-stanchion centreline....you removed it. turned the stanchions 180* and refitted the wheel with the altered trail-angle....it was only a couple of inches, but enough to eliminate the shimmy, otherwise, one had a steering-damper (friction-plate "sandwich" with a large knob on the top f the steering-head) amazing what half a turn could do!
