Tailwheel Internal worn Parts

[MoonlightVFR]

My Scott 3200 tail wheel no longer steers through the rudder cables, suspect worn internal parts. It worked beautifully after a A/P, AI rebuild but stopped the positive crisp steering action after 2-3 months

I know it is almost impossible to diagnose without seeing and disassembling the tail wheel.

My question at large is what are the one or two internal parts most likely to wear? Technicians at Aircraft Spruce and Alaskan Bush wheel are not a source of definitive information. I have heard "we don't have an opinion". If your livelihood depended on selling a lot of parts could vagueness increase quantity of parts sold?

My questions to the forum is What is the single part inside the Scott 3200 assembly most likely to wear that would first effect steer-ability? What is the part number? Is it mfg from Steel, Brass or aluminum? How does this part engage with other internal parts?

[Bruce Fenstermacher]

Pictures. They can say a thousand words. You typed about 20 describing your problem. You've got another 980 to go till you can equal one good picture. Take pictures of your tailwheel installed as it sits on the ground. You can't take to many from all angles. If you already have the wheel off and a part take pictures of all the parts including the bracket and fork castings.

First and foremost you will hear that you really can't steer many 170s with just rudder input via cables to the wheels. Of course you did say it was better and degraded.

As for wear in the wheel I'd say there is nothing in the wheel that would have been good 3 months ago when the wheel worked better than know. The parts just aren't going to wear that fast. With one exception. The steel compression spring AB item 22. this us a steel flat spring bent in a U shape and is connected to the lower fork. The two bent arms engage the steering arm. If both are broken your will my free caster with no control with the cables. If one is broke, you won't have control to one side. Of course the steering arm could be gouged out to the point the arms won't engage but I doubt it.

You can test for a broken compression spring by lifting the wheel off the ground and trying to swivel it 360 degrees in both directions. If it freely swivels in one or both directions, one or both ends of this spring are broke.

Half of the parts inside the tailwheel are a dampening system to reduce shimmy. They have nothing to so with steering. Control comes from the cable to the control arm. The arm is connected to the lower fork with the compression spring which engages in detents. The lower fork rotates about the king pin on bearings. If control is getting to the lower fork but the fork won't spin and seems to have to much drag it is one of two things. The king pin nut is to tight forcing excessive friction on the thrust washers, AB item 9. This nut should only be so tight as to eliminate shimmy and no tighter. Then of course the bearing could be bad. But after that you need to look at the machined surfaces inside the bracket and the fork. These surfaces that the AB item 5 fiber thrust washer rides on must be parallel to each other when assembled. If they are not the fork will not rotate on the bracket without excessive force. I had a tail wheel assembly worn in this area because of a loose king pin which should be press fit squarely in the bracket.

So yes it is tough to say what might be worn and need replacement. Or upon disassembly it might be obvious.

[MoonlightVFR]

Bruce,

Thank you for your reply. I will post a TW picture. While at Bardstown I took a considerable number of tailwheel pictures of all member aircraft on field. Surprised at the variety; slight differences but same model aircraft.

What part has direct contact with the compression spring (ABI 22 or 3222 IPC) ? Could that part experience wear?

Immediately I am going to raise the tail and do the swivel test you prescribed. Am I to understand that a little resistance should be encountered L or R then PN 22 compression is probably OK? If no resistance on one side then the spring is likely broken on that side.

Regards

[bagarre]

The U shaped spring (More like a V shape with a wide base. \___/ ) is mounted to the lower half of the swivel (Casting with the wheel). The ends of the "U" strikes a notch in the steering collar, which is how the collar (what your cables attach to) apply force to the lower half and turn the tailwheel. If it turns too far, There is a bump in the casting that compresses the spring out of the notch and the lower half 'unlocks'.

When you say 'doesn't steer'. Do you mean:

A. Pushing on the rudder pedal does nothing but stretch the tailwheel spring on that side. Nothing turns.

B. Pushing on the rudder pedal causes the steering collar to turn but the bottom of the tailwheel (where the wheel is) does not turn.

C. Pushing on the rudder pedal turns both the steering collar and the tailwheel but the airplane doesn't turn.

Places to look are:
A - The king pin may be too tight, causing too much friction.
or - The friction plate was not greased at and have since seized up to some degree.

B - The U spring is broken as mentioned above.
or The unit has been pumped with so much grease, it's not letting the spring engage the notch
or the steering collar is worn beyond imagination

C - It's a Cessna 170

Hope this helps some.

[GAHorn]

The most common failure I hear about is the pawl and the spring (the one being discussed).
Compression spring is not the term used most often for that spring. More commonly it's known
as the pawl spring.

[Bruce Fenstermacher]

I used the name compression spring for the U shaped spring because that is what it is called in the AB list. It is simply called a spring in the Scott part list. It should not be confused with AB item 7 or Scott item 3 which are also called compression springs. These are the 3 or 5 springs which compress against the friction plates called thrust washers. It should not be called a pawl, though I know it has, because there is actually another part with that name in both the AB and Scott part list. The pawl is the part that compresses the U spring we are talking about allowing the lower fork to unlock.

With the wheel in the air you should be able to move the wheel so far one way and then the other and the rudder should follow. In other words a mechanical link between the bottom wheel fork, the tail wheel control horn and then the rudder through the chains.

If there is no stop on one side or the other and the fork continues 360 degrees the U compression spring is like broke on that side.

With the rudder positioned to it's extreme on either side the tail wheel fork should be able to be force to continue in that direction with a slight snap and then continue 360 until it locks back in. You should be able to do this both directions.

It the tail wheel with non unlock either your rudder chain is not tight enough to move the pawl far enough to compress the pring releasing the fork or the U compression spring is broken off and the piece is jamming the lower fork and control arm together.

If with the wheel in the air the lower fork does not move side to side with slight drag then your king pin nut is to tight or the machined surfaces of the fork and bracket are worn holding the bracket, fork and kingpin together crooked causing the drag.

There may be a grease fitting found on the top of the king pin. Some people think this is to grease the daylight out of the tail wheel. It is not. It is to grease the bearing at the bottom. But if you do grease the daylight out of it the grease will also get between the thrust plates. The thrust plates are there for friction. They function as a dampener to eliminate slight shimmy. They are adjusted with the king pin nut. They should not be dry but filled with grease makes no sense to me either.