Tailwheel Spring Question

[CraigH]

Can anyone tell me if the F21B uses the same tailwheel spring as the BC12? I've got a little shimmy, and the A&P thinks the spring needs to be re-arched. Wanted to order a replacement from Univair so as to avoid any down time (and have a spare).

[jrzangger]

Don't know about the spring size between the 12-D and the F-21, but my initial reaction to your shimmy would be to suggest using the small Maule hard rubber tailwheel. Any of the larger, air filled tailwheels are much more prone to shimmy than the small Maule. I can't imagine the spring would be the culprit unless it's about ready to fall off! Make sure it's on tight and then put on the small Maule!

That's my opinion!

[CraigH]

Originally posted by jrzangger
Make sure it's on tight and then put on the small Maule!

It's tight. Don't think the small Maule woud fare well on some of the rough sod strips I fly into.

[dennispip]

Mine has the Maule Tundra Pneumatic tire and the Maule anti-shimmy springs. The springs are not identical, one side is slightly stiffer than the other, I guess, and that prevents the shimmy according to a very knowledgeable tailwheel guy I know. He told me that if they were just alike, shimmy would be guaranteed and that my springs were good. I have no shimmy at all. The other suggestions would probably work fine as well. Does the smaller Scott wheel stick in the soft turf? And what about on hard surface?

[CraigH]

I've got the larger Scott 3200 pneumatic. Had the solid Scott 2000 on another plane. It was better on asphalt, but got swallowed up by chug holes on rough turf strips.

The springs seem to be OK and my A&P who is quite knowledgeable concerning tailwheels feels it is the arch of the spring. I don't want to be without an airplane while we send the spring off to be re-arched, so I wanted to buy extra spring. However, I can't find anyone who can tell me if the BC12 spring stocked by Univair is the same as the F21B spring.

[M Jones]

I've never had a problem with my Scott 2000. It sounds like your A&P may be on to something Craig. I remember something about a year ago or so in the EAA Vintage magazine that illustrated the geometry of the tailwheel post, and how that was the primary cause of shimmy. Bottom line was that the post should be angled so that it's farther back on top than the centerline of the wheel hub would be in the three-point attitude.

If you can find that, it's got a nice graphic. I'm traveling right now, but if you need me to scan it and send or post it, I'll be home this weekend. I should be able to dig it up. Your A&P knows anyway I'm sure.

[Robert Lees]

Tailwheel shimmy

A lot of shimmy is caused by, in order of commonality:

A) castor angle incorrect;
B) Loose attachment (allowing the natural resonant frequency to diverge);
C) Worn "break-out" mechanism or slack steering springs (again allowing the resonant frequency to diverge);
D) underinflated or overinflated tyres;
E) Other things I can't think of.

Maybe (A) is your problem, Craig. The castor angle must allow the tailwheel to present a "positive angle-of-attack" to the ground, not drag along like some add-on. Re-arching [tightening] the leaf springs would help you, if this is the case.

Your local blacksmith (try Farriers in Yellow Pages, or ask any horsey people ) will correct them. Owner maintenance, you know.

Certainly the Anti-Shimmy steering springs help (they upset the natural resonant (shimmy) frequency of the tailwheel assembly), we use them on all our tailwheels here in the UK.

[Marv]

Craig I have never had trouble with a Scott tailwheel shimmying, but shimmy seems to be common with the Maule. To correct the problem on my airplanes that had maule tailwheels I made a tapered shim to fit between the spring and tailwheel mount. This will change the caster angle. The shim I made was 1/4" thick to nothing spring width and 2" long . Drill the mounting hole to match the tailwheel mount bolt. Install the thick part of the shim to the front. Hope this helps. Marv

[CraigH]

Thanks for everyones help and input! Forrest just sent me an email that the spring is common between the two models. That will make getting on much easier. Just a wild guess, but I'm wondering if the larger tires I put on changed the angle of the spring to the ground contributing to the shimmy in crosswinds.

I flew again yesterday but with much less crosswind component. No shimmy today.

[Forrest Barber]

Larger tires? Caster angle? Hmmmmm maybe so.

I have many Tcrafts with springs that were bent backquite a bit. Craig has a new set from here on the way today. Lets get his feed back. We talked on the phone about tension & compressions springs. tires pressures etc..... bye Forrest

[CraigH]

It will take me awhile, but I will post the results once the new spring is arched and installed. One other thing that didn't hit me until this evening. . . . the shimmy problem only happens when there is a passenger (extra weight) in the plane.

[CraigH]

Got the new spring put on today. Problem appears to be solved. The old spring had lost it's arch.