Tailwheel cables slack

[daedaluscan]

I am having an issue with my tailwheel control cables. I have a 1956 B model with the internal pulley set up to these cables.

I installed a rebuilt tailwheel a year ago, with new chain. About a month later I took out one link each side to maintain a snug tension. Over the last couple of weeks I have had quite a bit of tailwheel shimmy (that I have fixed with bearing adjustment and grease), and today noticed that the cables were very slack during my walkaround. Very little tailwheel steering when taxiing. I needed to take out three links (two on one side, one on the other) to get the tension that I am used to (just off tight).

I am not happy with this so will get my AME to look at it before I fly. I feel that there is an issue if I need to continually retension the cables.

I have looked at the drawing in the parts book and think that either a cable is slipping at the attach point on the rudder cable, or there is an issue with one of the pulleys.

What is really frustrating is that I cannot see anything behind the bulkhead. I assume I have to take off the fin and horizontal stabilizer? Seems like a lot of work. I am thinking of finding one of these to have a look

http://www.amazon.ca/Milwaukee-2310-21- ... B001AOVBK8

Any input appreciated, Charlie

[GAHorn]

The "internal steering" system has numerous problems of it's own.

One of them is hidden damage from split/cracked bulkheads which allow the pulleys to flex out-of-position, loosing their adjustment, but not displaying looseness when static on the ground.

Shimmy is usually the result of the "kingpin" at the incorrect angle. The TOP of the pin should be AFT of the BOTTOM of the pin. This attitude is set by the mainspring (which some folks mistakenly think is OK to "re-bend" to a customized curvature.)
See: http://www.cessna170.org/forums/viewtop ... mmy#p82024

Several topics discuss this problem throughout the forums.
One such topic is:

http://www.cessna170.org/forums/viewtop ... eel+shimmy

[daedaluscan]

I have taken off that inspection plate, but did not know what to look for as I had not looked at the drawing. Nothing obvious torn out of place but it was a bright sunny day and hard to see in there even with a flashlight.

The pulleys that are visible through the little fairings where the cable exits seem to be in good shape (though there is some rubbing of the cable on the fuselage which suggests my tailwheel spring is flexing a lot. I do have the correct forward castor at rest, but not sure under load), but I would like to see the upper pulleys.

I do have the correct springs.

My AME is coming with a borescope to have a look.

thanks for all the advice.

[canav8]

Easy guys as George Stated. Tail chains should not be taught while the tail is on the ground. Put the tail on a saw horse and unload the tailwheel. adjust chain tension when unloaded to be taught. The reason is when the tail spring becomes unloaded it actually gets longer and will put tension on the rudder control horn on older 170,A,B with the tail springs. On the newer set up it can cause a sawing effect on the tail pulleys that you are talking about. Tail shimmy is generally caused by a fatigued tail wheel spring causing the tailwheel kingpin to be slanted as opposed to perpendicular to the ground. If that is ok then I would go after the tailwheel action not the pulley system. The Scott 3200 tailwheel or the bushwheel equivalent uses 3 springs to get the job done on the 170. No More and No Less. The clutching mechanism can wear quickly if you do not use enough grease and may not work correctly if you use to much grease. Lots of threads on this topic in the search. Doug

[canav8]

No Three springs. It comes with 5 springs. The Scott 3200 was designed to be used with all tailwheel aircraft but dependant on the tail load you must adjust your clutching spring tension. Most people do not know how to service the 3200.

[canav8]

Every 3200 I've seen had all 5 springs installed. We've had at least 10 in the last 20 years on multiple airplanes and never saw any of them missing springs. All of them functioned flawlessly after they were rebuilt by our mechanic, with most problems coming from the flat spring and wear on the notch where it engages.

All the exploded views from Scott and Alaska Bushwheel show it with 5 springs required. I am no expert by any means, I just go off of source documents and let my IA/A&P do the work while I watch.

I'll have to ask him about omitting springs and learn some more about when/why that would be desirable!

Aryana,
You are correct. The highest wear components of the tailwheel are the pieces you exclaimed. They are the most common problems to having a tailwheel unlock in one direction but not the other. But none of this solves the original posters problem of tailwheel shimmy or linkage chain lengths. Back on thread topic.
Charlie, is the tail spring geometry correct?

[canav8]

He stated he had looked in the tail already and nothing looked awry.

[Bruce Fenstermacher]

Though off topic when used on a 170 the 3200 had only 3 of the 5 springs (#7) installed. Makes no difference in the end if the king pin nut is adjusted to cause enough drag through the clutch plates to eliminate shimmy. Of course when eliminating shimmy king pin geometry should be correct before adjusting the king pin nut.

As for adjustment of the chains on the old system there is disagreement among the troops as I recall on two points. As I recall some among us say there is no change in chain tension whether the tailwheel is on the ground loaded or not and so one could adjust the chain link count to remove slack. The second opinion is that the geometry of the tailwheel under load changes when free hanging, does tighten the chains and so the slack should be removed with the tailwheel off the ground.

i'm of the opinion that the chains do get tighter when the tailwheel is off the ground because that is what happened with both of my 170s. However I also know I don't want any slack with the wheel on the ground when I need steering. I can not adjust the chains with no slack with the wheel on the ground and no tension when it is off. Just can't be done. So I adjust the chains by with no slack on the ground and try not to think of the added tension while in the air. At least while under the high tension while in the air the wheel should be moving as free.

BTW Scott says there is no adjustment of the control springs tension. What I think they meant is there is no adjustment other than adding or subtracting links. But two other things I found can be done. The first is to add a AN115 shackle to the chain. I like to put it in between the control horn and the spring. A shackle is a different length than the chain link and so you can adjust approximately half a link. The other trick only you can decide to use is to slightly squeeze the thicker part of several links this lengthening them. I did this and had no trouble with the chain for years but you won't find the method approved anywhere.

[canav8]

Thanks Bruce. I have been looking into offering a control horn STC to offer longer throw to fix the notorious steering problems of the Cessna Tailwheel aircraft. FAA has been giving me trouble though. Still working on it.

[canav8]

Nice video. The only way to truely tell without aerodynamic loading is put the tail on the sawhorse and see what happens. Im sure yours is no different then ours.

[GAHorn]

Easy guys as George Stated. Tail chains should not be taught while the tail is on the ground. Put the tail on a saw horse and unload the tailwheel. adjust chain tension when unloaded to be taught. The reason is when the tail spring becomes unloaded it actually gets longer and will put tension on the rudder control horn on older 170,A,B with the tail springs. ....

That's not exactly what I said...and that's also not exactly true....
It's been often said that the tailwheel descends in-flight and adds additional "pull" on the steering chains... and that is not correct. There is actually no difference in tension placed upon the steering chains when the aircraft is airborne or not...if the tailwheel steering geometry is correct and the mainspring has not been altered.

(A large segment of fellow Members disagreed with me on this until we demonstrated it at the Branson convention airport seminar, where we used Steve Grimsley's airplane which, while sitting on the ground, the steering chains were just barely taut. We then lifted the tail up into the air and sat the fuselage on a box...and the steering chain tensions did not change one iota!...they were exactly the same tension. Several were surprised.)

Think of the steering arm transcribing an ARC which is equi-distant from the bellcrank as the tailwheel is loaded/unloaded.
(And in any case...that is one reason for the tension-springs in the steering chains...to absorb any minor differences.)

The distance between the bellcrank and the steering arms do not change with the tailwheel descending ...IF.... IF... the mainspring is original and un-altered.... HOWEVER...if the mainspring is WEAK...and has either lost it's bends or has been deliberately straightened....then the tailwheel, while sitting on the ground, is actually HIGHER in relation to the fuselage than it should be. If the steering chains are adjusted to this improperly positioned tailwheel.... they will tauten even greater as the tailwheel descends to it's ordinary position.
If your tailwheel steering chains are changing tension from sitting on the ground versus airborne.....then your mainspring is suspect.

[johneeb]

To clarify, do you think that also applies to the later type steering that has the cables oriented horizontally?

Arish watch the tailwheel steering chains closely in this video!
http://s8.photobucket.com/user/aaryana/ ... s.mp4.html

[C170U2]

I would agree with George. For what it's worth, I tested the tension when I installed my new springs and found no difference on the ground and when elevated off the ground. I used my highly calibrated fingers for this test...

[canav8]

George, I should also add it depends on the condition of your tailwheel spring. a New spring has a greater arc then a gently used spring.

[daedaluscan]

Wow. Firstly thanks to everyone for the response, his is a lot of information.

I borrowed a USB borescope and took some rather poor photos, and did not find anything obvious. No tears int the bulkhead or shredded pulleys that I could see. Photo quality is pretty poor though.