Author Topic: Tail Wheel Shimmey  (Read 144 times)

TreeTopFlyer

Tail Wheel Shimmey
« on: September 07, 2019, 09:16:11 AM »
Just thought I would drop a line about an issue I had...that has been talked about many times, but still wasn't clear to me until I took the tailwheel A frame out and looked at the tail wheel assembly closer. 

TAIL WHEEL SHIMMEY

Off and on over the years I have had tail wheel shimmey...sometimes just a little sometimes quite pronounced...recently I found that it was one of the pawls in the clutch mechanism hanging up and not contacting the shoulder on one side of the cam...(a couple of discussions with Jim Metzler were very informative in regards to HOW MUCH grease is introduced into this mechanism and not overdoing it with the grease fittings).

The first time I took the tailwheel apart (which I recommend everyone do) My mechanic and I realized as per Jim observations that there was too much grease in the pawl mechanism and one of the springs seemed a little weak.  The grease can work like glue and hang up one side or the other up. We used minimal grease and replaced one of the springs.

Within a couple of flights one of the pawls would hang up again....so we took it apart again.  We closely inspected it and found minor burrs on the mechanism, smoothed the burrs and tested it...it would work correctly about half the time.

So we used a bit more emery cloth to ensure everything was smooth, VERY LITTLE GREASE mostly on the brass bushing on the shaft and TRI FLOW Lubrication on the pawls.  This time it worked correctly all of the time and was noticeably smoother.

SO I believe the answer is use very little grease...tri-flow works great...and those pesky little pawls HAVE to contact BOTH shoulders of the cam or it WILL shimmey.   Smoothing out those pawls helps tremendously. You should hear a positive click on both pawls when you raise the tailwheel off the ground and slowly turn the TW back and forth to the stop to test it. If as you turn the TW back and forth one side has resistance until you push it hard to get the pawl to open on that side and the other side rotates easily around - the side that rotated easily has a pawl that is NOT engaging the cam shoulder, and IT WILL shimmey.  A locking tail wheel pin Will NOT stop the shimmey if both pawls do not have contact.

On a test flight with normal landings no shimmey so it worked...when I intentionally landed a little fast and let it roll a bit without brakes I could introduce a slight amount of shimmey....but we also tested my original type shimmey damper and it is very weak...pretty much on its last leg...my next step is to put on a LORD shimmey dampener which I think will take care of the issue once and for all as long as the pawls are both working correctly.

On another note inside the cap where the pawls fit and rub up against had a lot of machining marks, little ridges etc...I would think making this area smooth (Im not saying remove a lot of metal, just removing machining marks etc.) and using one of the coatings available today (such as NP3) on the pawls would help this situation.  When metal parts are coated in NP3 they are very smooth and have natural lubricity.  I use NP3 on a few rifles...when the internals are coated in NP3 the carbon literally wipes off. Just a thought.

Barry Dechert

Re: Tail Wheel Shimmey
« Reply #1 on: September 17, 2019, 07:30:41 AM »
Thanks for the input, realized the same intermittent pawl action. If you wouldn’t mind what the part # of the shimmy dampener and the best price. Thanks.