Doug McGowan
New member
Been getting a tailwheel shimmy on touchdown. Any ideas on trouble shooting?
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Decathlon Tailwheel Shimmy
- Thread starter Doug McGowan
- Start date
acrodisiac
New member
Doug,
I think we need a file section somewhere dedicated to this topic! It comes up every week it seems.
Here is my 2 cents worth.
1. Slow the landing speed down :wink:
2. Grease the heck out the pivot.
3. Tighten the U bolt at the forward end of the leaf spring
4. Replace the rubber cushion betwen the fusealage and the leaf spring (it eventually crushes to a hard unidentifiable piece of some organic compound :!:
5. Inflate the tire correctly. I use 45 lbs
5. tighten the pivot bolt (just a little ) the tailwheel should pivot freely but not loosely. I am sure that here is some secific turning friction spec for it but my subjective method always works. When the "feel" gets weird it's time to dissassemble the whole thing.
6. Check the leaf springs. You need to remove them entirely to check for broken springs. Quite common.
7. When all the above are deemed good for flight it's time to overhaul the puppy. Inside the pivot are thrust plates , springs . spins , pawls and lots of grease. Any worn bits in there may cause the shimmy . It really is easy to overhaul the tailwheel pivot, so don't be too shy about this job unless your intimidated by figuring out which end of a nail the hammer strikes.
I think we need a file section somewhere dedicated to this topic! It comes up every week it seems.
Here is my 2 cents worth.
1. Slow the landing speed down :wink:
2. Grease the heck out the pivot.
3. Tighten the U bolt at the forward end of the leaf spring
4. Replace the rubber cushion betwen the fusealage and the leaf spring (it eventually crushes to a hard unidentifiable piece of some organic compound :!:
5. Inflate the tire correctly. I use 45 lbs
5. tighten the pivot bolt (just a little ) the tailwheel should pivot freely but not loosely. I am sure that here is some secific turning friction spec for it but my subjective method always works. When the "feel" gets weird it's time to dissassemble the whole thing.
6. Check the leaf springs. You need to remove them entirely to check for broken springs. Quite common.
7. When all the above are deemed good for flight it's time to overhaul the puppy. Inside the pivot are thrust plates , springs . spins , pawls and lots of grease. Any worn bits in there may cause the shimmy . It really is easy to overhaul the tailwheel pivot, so don't be too shy about this job unless your intimidated by figuring out which end of a nail the hammer strikes.
It is very easy and cheap to rebuild it yourself. The cost of all the parts except the castings is in the order of $250 from spruce.
I replaced everything, including the cam, pawl, friction washer, all coil springs, the flat locking spring, center bushing, etc... for that amount. It only took me about 2-3 hours, and my A&P checked it at various points along the way and signed it off.
No shimmy since then!
I will be replacing the rear leaf spring though, it is very cheap, and I noticed the new ECA I flew turned easier, I think when the spring is newer the pivot sits at an angle that is easier to turn.
I replaced everything, including the cam, pawl, friction washer, all coil springs, the flat locking spring, center bushing, etc... for that amount. It only took me about 2-3 hours, and my A&P checked it at various points along the way and signed it off.
No shimmy since then!
I will be replacing the rear leaf spring though, it is very cheap, and I noticed the new ECA I flew turned easier, I think when the spring is newer the pivot sits at an angle that is easier to turn.
Doug McGowan
New member
Thanks for the input guys! The shimmy has given me a chance to become proficient at wheels landing and I have greased the pivot substantially. I will let you know what solved the problem.
Doug,
I had the shimmy problem for a couple of years (Citabria 7KCAB with Scott 3200) and tried most of the received wisdom on the subject without result. Finally saw an article in Vintage Aeroplane Magazine by a gentleman called Gilbert Pierce, who had the following advice for Scott tailwheel users:
"get the geometry right"
In my own case the original tailwheel main leaf spring had become tired and was allowing the whole rear end to sag causing the incorrect geometry. Remedy was to buy a new tailwheel leaf spring (around $70 from ACA) which solved the problem completely.
I was surprised to find that the aircraft was sitting so much higher at the rear following installation -to the extent that the view over the nose when taxying was considerably better.
There has been no repeat of the shimmy problem since installation 12 months ago -either on grass or tarmac- regardless of the load in the rear seat/baggage compartment.
ed Wild
I had the shimmy problem for a couple of years (Citabria 7KCAB with Scott 3200) and tried most of the received wisdom on the subject without result. Finally saw an article in Vintage Aeroplane Magazine by a gentleman called Gilbert Pierce, who had the following advice for Scott tailwheel users:
"get the geometry right"
In my own case the original tailwheel main leaf spring had become tired and was allowing the whole rear end to sag causing the incorrect geometry. Remedy was to buy a new tailwheel leaf spring (around $70 from ACA) which solved the problem completely.
I was surprised to find that the aircraft was sitting so much higher at the rear following installation -to the extent that the view over the nose when taxying was considerably better.
There has been no repeat of the shimmy problem since installation 12 months ago -either on grass or tarmac- regardless of the load in the rear seat/baggage compartment.
ed Wild
wjwilson2000
New member
You might want to look at yahoo groups Citabriapilots. There's a lot of discussion on the group about the taiwheel shimmy and some files there also. It dosen't cost anything to join and there's a lot of info to be obtained there. You can search the archives for info on the topic also. Not trying to cut this site just think it might help.
Woodie
N29763
Woodie
N29763
Doug,
Ed Wild refered to the Pierce Aero arcticle about tail wheel shimmy. You can access it at http://www.pierceaero.net/tws.php
With the airplane loaded the tailwheel pivot bolt should be either vertical or tilted with the bolt head slightly aft of vertical.
The condition of the leaf springs will have the most effect on that. If they are weak the pivot will be tilted forward...just what you DON'T want. You may need to get new leaf srings or have yours re-arched.
Good luck,
Rareblair
Ed Wild refered to the Pierce Aero arcticle about tail wheel shimmy. You can access it at http://www.pierceaero.net/tws.php
With the airplane loaded the tailwheel pivot bolt should be either vertical or tilted with the bolt head slightly aft of vertical.
The condition of the leaf springs will have the most effect on that. If they are weak the pivot will be tilted forward...just what you DON'T want. You may need to get new leaf srings or have yours re-arched.
Good luck,
Rareblair
Doug McGowan
New member
Thanks for the info all. An ample amount of grease seems to fixed the shimmy!!
charlie hensley
New member
Our 7KCAB tailwheel shimmied and all we did was tighten the pivot bolt. It took two tries the first try had it a little too tight and ground steering got sticky. We loosened the castle nut one slot and now, no shimmy and solid steering.