Tailwheel Shimmy and Spring

[jiott]

In the last 50 hours or so (out of a total of 235 hrs) I have begun to develop a tailwheel shimmy problem when landing on paved runways. I had the standard Kitfox 3-leaf steel spring and the optional Alaskan Bushwheel tailwheel. I tightened the tailwheel pivot bolt as much as I dared as was suggested on this forum, and it seemed to help a little but by no means solved the problem. The pivot caster angle seemed very near vertical so it appeared the next thing was to improve this angle by rebending or getting a different tailwheel spring. I bit the bullet and ordered the Grove spring aluminum spring kit from Kitfox. It came with all the brackets and hardware and installation was a snap.

It is a much sturdier spring for backcountry flying, has a better caster angle and is somewhat longer so it raised my tail about 1.75" higher. The results are all good: after 8 landings the shimmy is all gone and it has a more stable feel on rollout. My view over the nose on taxi is improved. Another important improvement I feel is on landing; It is easier to get the tailwheel down a little ahead of the mains (Maule style) so when the mains come down the AOA decreases and sticks you to the runway-kind of like landing a tri-gear. Also if you do a standard 3-point landing the higher tail reduces the AOA so there is less tendency to balloon up in gusty wind conditions. In other words I am pleased with the results.

[rosslr]

Nice report Jim - I looked at them at the factory and was tempted but thought I should use the one I have for now - look forward to seeing it Tuesday.

Cheers

ross

[rv9ralph]

Jim, I read your post several times and my understanding is that you replaced the tail wheel spring with a Grove product you purchased from Kitfox. I checked both product lines and could't find any information.
Could you elaborate on the source and specifications on the aluminum tails spring. I have a Kitfox 3 with a single leaf steel spring.
Ralph

[jrevens]

I would suspect that the solid aluminum spring is stiffer than the steel leaf spring. Is that the case? Some other designs have had experience trying solid aluminum tail springs, with a resultant stiffer ride and more stress to the airframe at the attach points. Just curious.

[Paul Z]

I had my SLSA converted to a tail dragger by the Kitfox Factory. They installed the Aluminum tail spring. I do not find it to be a rough landing or a stiff ride. In fact I find it really nice, especially compared to the Cessna 120 I had. I had a picture of the part number on the Spring I'll have to check my desktop to see if I still have it.

[Dusty]

This looks similar to what I have (mounting is slightly different)
Very good ground handling but very stiff over bumps.
I think the tyre does all the work

[airlina]

I had a Grove solid aluminum tailspring on my Series 5 for 8 or 9 years and liked its performance but I started to hear thru the grapevine about failures that made a mess of the rudder after failure, so I replaced mine with a three leaf spring made to the same length and angle as the aluminum spring it replaced. A search on this site will provide some insight. Just a consideration if you go the aluminum route. Now I can sleep at night. Bruce N199CL

[jiott]

First of all, the Kitfox factory has all the info on this spring and you can purchase from them. They can tell you if it would fit a model 3.

Yes it is somewhat stiffer and I did notice a stiffer ride over bumps on my grass airstrip, but surely don't consider that much of a problem.

As far as the extra stiffness causing structural damage, all I can say is that Paul Leadabrand at S&R has been using them on his training aircraft for several years now with no problems that I am aware of. Those planes get beat up pretty bad with all the new students he trains from scratch (I know because I was one of them) and all their hard landings on a daily basis, plus his use of the same aircraft for his backcountry training on rough airstrips. Maybe earlier Kitfox models were not as strong in the tail and have had some problems. If anyone knows details of failures and on which models, we all would like to hear.

I do agree that a failure of the forward bracket bolt would allow the tailwheel to pivot up into the bottom of the rudder and make a mess. This looks to me like its weakest point.

[jrevens]

... If anyone knows details of failures and on which models, we all would like to hear.

I do agree that a failure of the forward bracket bolt would allow the tailwheel to pivot up into the bottom of the rudder and make a mess. This looks to me like its weakest point.

I didn't mean to imply that I knew of any specific KITFOX issues with solid aluminum springs. One design that I'm intimately familiar with is the Thorp T-18, and builders moved away from the original solid aluminum design mainly because of the lesser shock absorbing capability and rougher ride. A stiffer spring there will definitely transfer more shock load to the fuselage than most steel leaf or rod designs that you see on homebuilt aircraft, which will absorb more of those loads in flexing. I was just thinking about the reports of damage to the aft fuselage structure when towed incorrectly, and the possible vulnerability in that area. I'm trying to learn as much as I can about this design that I'm building. In order to help avoid fatigue cracking & failure, the aluminum spring will usually be pretty thick & stiff. I'm glad they're working out well for Paul and company.

[Av8r_Sed]

I've got the Aluminum Grove tail spring on my III. It was purchased many years ago, so I'm not sure if this configuration is still available. It's been giving good service with a little less than 200 hours on it at this point.

The ride is a little stiff going across larger runway cracks, but I hope to upgrade from the solid rubber Maule to a pneumatic tailwheel to resolve the issue.