Tail Wheel Control Arm Wear

[Ironflight]

Something that hasn't been talked about much recently is the tendency for RV tail wheels to break out easily, or refuse to lock into tail wheel steering mode. This used to get a fair amount of press on the various forums, and was usually blamed on burs developing on the corners of the spring-loaded plunger. Other reported causes include poor lubrication, dirt - the usual suspects when a mechanism refuses to operate properly. I finally have some photographic evidence of what I believe to be at least a major (if only one of several) cause of this behavior - wear in the steering arm itself.

Louise's RV-6 has exhibited a lack of steering lock in the "right" direction several times. I had it happen on my -8 as well. Each time this happened, we did the ritual cleaning and deburring of the plunger. I have also found that flipping the steering arm helps, although the "second side" doesn't last as long as the first. Replacing steering arms usually fixes the problem.

Several months ago, We replaced the Silver Bullet steering arm on Louise's plane with a spare that we had on hand, because I saw that the "U"-shaped cutout for the plunger was wearing out the sides. We had a spare chrome link on hand, and put that into service. Tonight, I had to change it again - this time to a spare Van's unit we had, as we don't have any more from Full Throttle (we have a couple on order, but we all know how that goes with Full throttle - not holding our breath...). Anyway, the following two pictures shows pretty conclusively the wear that we are seeing:





See the "shoulder" in the "U"-shaped cutout? That shouldn't be there! In fact, the shape of the shoulder is the original dimension of th cut-out - the open legs of the "U" should be parallel. in the worn out portions of the channel, they are pointing outwards - allowing the plunger to easily slip out,and the tail wheel to go into full-swivel mode. With a Van's arm, you can flip the arm and get a little more use out of it - but not for long, as the remaining amount of material wears very quickly. With the single-armed, chrome unit from Full Throttle, you don't have that choice - it can't be easily flipped. The fact that the spare unit we had from Full throttle wore out in less than half a year tells me that there might be a materials hardness problem, but since Louise's original lasted several years, I hate to call the problem generic.

At any rate, I was able to prove that with the worn unit, it took almost no effort to break the tail wheel loose by hand. With the new unit, I couldn't do it without the typical sharp push on the fuselage. If you're tail wheel isn't locking in to place, remove and inspect the arm very carefully - you might find the problem right there, before digging deeper into the spring plunger (and sending it shooting across the hangar.....)

Paul

[Tom Martin]

Paul
This problem is more evident in those aircraft that have a heavier tail weight. I did not have a problem with this item on my HRIIs but with the F1 the tail weight is greater and this part had to be replaced after about 75 hours. As long as you are aware of the issue it just becomes a maintenance issue. However when I started flying my EVO, with a heavier tail weight, I was only getting about 20 hours on the control arm before it needed to be replaced. This was not acceptable and so I took one to a lock metal hardening shop and had it heat treated. So far I have about 100 hours on the current arm and it is showing no wear. On top of that the pin seems to be keeping it?s shape a bit better and I do not seem to get that ?pin stuck? issue like I used to. It is possible that what I thought was dirt causing the pin to stick was bits of metal worn from the control arm.
I have discussed this with Wayne Hadath, who markets the Steering Link, and he is considering heat treating some units. At least it could be an option for those that have problems as some aircraft can go hundreds of hours with no issues.
.

[Ironflight]

Interesting thought on tail weight Tom. Unfortunately, my sample set of (two) data points sort of conflicts with it - Louise has worn out a couple of control arms in a few hundred hours, and I've only worn out one in 900 hours - and the -8 is much heavier on the tail than the -6. But I definitely like your idea of heat treating the control arm to make it harder - the wear I see clearly shows that the pin is harder than the arm!

BTW, since I switched to an API tail wheel assembly fairly early in my plane's life, I can't use the stock Van's arm - the hole diameter is different. I'm curious if your 20 hour wear was with the stock van's arm, or an aftermarket one.

Paul

[Tom Martin]

Paul
I had the same experience with both the Van's arm and the aftermarket one. Tail weight does seem to be an issue with rockets but perhaps there is a geometry factor as well that makes the RV8 somewhat different. Either way heat treating did solve my particular issue.

[odlee]

Tom,

Did you see any evidence of movement post heat treat. Sometimes we get some undesirable results when heat treating flat parts in our shop. They warp.

I do not know the control arm alloy used. What specifications did you ask the heat treater to meet? Was any adverse wear noticed on the pin?

Thanks, this sound like a good idea. Sorry for the "20 questions".

David

[Mike S]

I agree, the arm needs to be harder.

If you dont have issues with the inner section (hub) of this assembly wearing, then it may be possible to shift the wear to the pin.

If you can do this, sure does make carrying a spare part easier.

[vfrazier]

I have spare parts for the Van's type tailwheels in stock. I make my control arms from 4130 steel.

They are not heat treated, but you can certainly do that if you like. Consider though that something is gonna take the force and if you heat treat the control arm, then the pin needs it too. Ugh. Where do you stop? Replacement parts are cheap.

The photos above show some pretty substantial wear! Please don't use valve grinding compound to lubricate your tailwheel! ;-) Regular cleaning and greasing should suffice. If you're getting that much wear, there might be other issues involved like vertical misalignment, bad tire, loose parts, etc...

For those who are having trouble with tailwheel wear, consider upgrading to our tailwheel (or Doug Bell's tw) which have less arm working on the steering linkages, i.e. less force is needed to steer which also means less force is applied to the control arm and locking pin.

As Tom Martin mentioned, the heavier tail weight tandem RVs (RV-4, 8, and Rocket) really benefit from the reduced steering forces when you've got a rear seat passenger. Obstacle clearance is improved too.

Our tailwheel is available as a drop in retrofit for the Van's type tailwheels. See the web store for details.

FWIW, we are featured in the May issue of Sport Aviation on page 26 in the "On the flightline" section.

[Ironflight]

What I find most interesting is that the wear is very even - and the pin/plunger has virtually no wear at all. I don't see any way that it can be loaded unevenly, or with oscillatory inputs that would be driving this. The reason you can see the "shoulder" in the groove is because the Full Throttle arm is just slightly thicker than the plunger is tall - the area that is worn is where the plunger has contact, and the shoulder is just above it in the stack.

Since I have seen similar wear in two different "fancy" arms (from Full Throttle), and much less wear in the Van's arms - coupled with the even nature of the wear - I am still suspecting that the arm's are just softer than they should be. Fortunately, this is an easy spot to watch - especially with a Tail Mate in the hangar.

(BTW, Louise has a Bell fork, and mine is an API - I won't use a Van's due to the low clearance!)

[Garage Guy]

Originally Posted by vfrazier

...if you heat treat the control arm, then the pin needs it too.

Is that necessarily true? I know that sometimes having dissimilar hardness means less wear, where similar hardness leads to spalling. Looks kind of like spalling to me on that control arm picture above.

--Paul

[rv9aviator]

You do want a different heat treatment or hardness on the parts to stop the galling effect. Also you normally make the part that is the least expensive the softest.