Tailwheel shimmy

[Lee]

Hi ... flew my '54 -170 early this evening and had one landing where the tailwheel came down very firmly (after the mains ... flying off asphalt runway) ... enough so that we had a really pronounced shimmy during the takeoff roll during a touch and go. The next landing the shimmy was not really as noticeable but still a little noticeable ... the airplane appeared to respond to rudder inputs in that taxi was not a problem with little brake use. (calm winds) I noticed when we parked the airplane that the connections (light chain) between the tailwheel steering arms and the tailwheel control springs were sagging a bit ... much looser than on preflight where they have been snug. Also, when I moved the steering arms on the tailwheel, there was pronounced play ... ie: move the arms and no movement in the tailwheel. The rudder is fine ... no damage. The tailwheel spring set attaching the tailwheel to the fuselage show no damage. Any ideas? And I apologize for the terms but, I do not as yet have an illustrated parts book for the Scott 3200 or the fuselage and engine for that matter. I had a Pacer years ago that developed the same problem and a Pawnee I fly on occasion for tow has had a similar problem. Can't remember what I did with the Pacer (30 years ago) ... the Pawnee still will occasionally get a shimmy. I did put a little grease in both grease fittings a couple of days ago ... flew it, no problems. Thanks ... Lee (N1967C)

[davevramp]

Try this link and click on" Click here for Tail Wheel Geometry Drawing"
for the pictures.

http://merlin.alleg.edu/group/eaa1194/twshimmy.php

[sphillips]

Lee.

Go to http://www.micro-tools.com/pdf/cessna/ for free 170 parts manual.

Steve

[Bruce Fenstermacher]

Lee

The grease is the problem.

Seriously there is a lot of information here on tail wheel shimmy and there doesn't seem to be any magic bullet solution.

Your tail wheel assembly prior to greasing probably had enough friction to stop shimmy. Now you have changed that with the new grease.

Otherwise condition and geometry are the key.

Your description of your tail wheel and chains doesn't seem that out of line.

If you study the geometry of the tail wheel spring, tail wheel control horn and the control chain angle from the fuselage you will see that with no weight (in the air) the chains will be tighter than with weight on the tail wheel. Your plane was probably sitting with the weight shifted differently at preflight that it was after the flight.

The angel of the main pin is important and needs to be right. If the angle is right and you know your tail wheel to be in good repair you could just try tightening the nut on the underside which holds the assembly together just one flat.

Some people like to use grease sparingly while others will grease their tail wheel daily. It's a Ford vs Chevy thing.

BTW the main tail spring usually breaks just below the second spring and usually just snaps. But perhaps yours is ready to go and is just bent which would change the main pin angle and loosen the control chains. This is a stretch but worth looking at.

If you don't know the history of your main spring it's cheap insurance to change it. By doing so should set the main pin of the tail wheel at the correct angle.

[Lee]

Thanks for the information Bruce ... I think I will replace both the leaf springs and the chains/springs as suggested. I think Iwill also take the tailweel apart, clean and remove any excess grease. I thought I used just a little but, I probably should have left well enough alone. Two other comments: 1) I put the airplane in the hangar and in pushing it back, the tailwheel swivels 180 degrees and that is how she sits ... does this put an undue strain on the tailwheel casting or components in any way? 2) What is the safest way to jack up the tail? Bottle jack or floor jack? Is there a location that a padded rest can be placed so that the fuselage is not totally resting on the jack? Thanks ... Lee

[Lee]

One other thought Bruce, is it possible the play I feel at the tailwheel with the steering arms could be due to a broken pawl? Or again, as you suggested, just a slightly loose nut? (Not the one flying the airplane! ) Thanks! Regards, Lee

[Bruce Fenstermacher]

Lee

I don't think you need to change the chains and control springs unless you don't have the right springs. I and many here at the forum don't like the compression springs sold by many and I think developed by Maule.

As for parking the plane with the wheel reversed I don't think that puts any more stress on the assembly. I don't park my plane with the wheel perpendicular to the airframe because of the twist imposed on the main spring and other assemblies it's attached to.

[Lee]

Hi ... my airplane came with compression springs ... I will change the springs out if this is a "better" ( ) idea! Curious ... why the preference or should I say "non-preference" for the compression springs? Thanks, Lee

[n3833v]

For jacking purposes, I use a scissors car jack that has a rounded grove for a frame and block to catch the frame bracket forward of the spring at the one side. I found this to be my best way. When I use the tailwheel puller, my wheel is always in relaxed position in normal position.

[GAHorn]

Hi ... my airplane came with compression springs ... I will change the springs out if this is a "better" ( ) idea! Curious ... why the preference or should I say "non-preference" for the compression springs? Thanks, Lee

Lee, the original type tension springs will stretch and will even over-stretch, if necessary or if an obstruction/malfunction should occur.
Compression springs will compress...until they bottom out...and then further movement will cause damage and overstress the associated components, bulkheads, chains, cables, steering arm, etc etc.

Another place here on this site this was discussed:
http://www.cessna170.org/phpBB2/viewtop ... ning+right

[Lee]

I am plowing through the posts ... good info and I think I am going to go with the original tension springs and replce the tailwheel springs set ... George said to make a slight radius to prevent a crack riser and that sounds like good advice. The comments make sense regarding the tension springs vs compression. I am just a bit puzzled why I now have some slack in the springs where I did not before ... that will be interesting running that down! Question: where can we find torque values? (thining of the attach point of the springs to the fuselage and to the tailwheel bracket) Regards, Lee

[Bruce Fenstermacher]

Lee

Here is one thought although I think a stretch.

If your compression springs bottomed out then the chain could have stretched. In fact a technique I've used to make fine adjustments to the chain length is to use a pliers to squeeze several links slightly making them longer.

Probably not what happened but it could.

[GAHorn]

What concerns me about the "slack" chains is that something may have stretched/bent/broke...
Other causes might be a change in weight/C.G. (to aft).
Torque values are found in the 100 Series service manual and at:
http://www.cessna170.org/phpBB2/viewtop ... 9741#29741

[Robert Eilers]

N3833V,

I'd like tp know more about how you jack the tail up. I am considering changing out my main tail wheel spring and am pondering how best to raise the tail. At this point, it looks like I ned to build some sort of frame to sit atop my jack and placed just forward of the tail wheel bracket on the empennage. Thanks.

[bradbrady]

N3833V,

I'd like tp know more about how you jack the tail up. I am considering changing out my main tail wheel spring and am pondering how best to raise the tail. At this point, it looks like I ned to build some sort of frame to sit atop my jack and placed just forward of the tail wheel bracket on the empennage. Thanks.

Robert,
Not a bad idea, but in the past we just set the A/C on a short stool with a pillow on the stool under the last bulkhead works fine.
brad