Tailwheel Attachment Bolt Torque PA-22-108

[weddlece]

Hello

I have a tailwheel conversion PA-22-108. I noticed yesterday after flying and pushing the airplane back that the upper tailwheel body and tailwheel shim (the wedge-shaped piece required between the tailwheel and spring with the Scott 3200 on the Univair conversion) was very loose, with visible movement of the shim and tailwheel casting relative to the spring. This was with the tailwheel fork rotated 180 degrees out from its normal trailing position after pushing the airplane backward. I placed a socket on the single attach bolt and it rotated, albeit with a fair amount of resistance. I then rotated the tailwheel to its normal position in line with forward travel of the aircraft, and there appears to be almost no slop between the tailwheel, shim, and spring. I've always noticed some side play due to flexing of the spring, but what I saw after pushing the airplane backward seemed excessive. Is there a torque spec published anywhere for the single bolt tailwheel attachment? I couldn't find anything in the Univair drawings or in the Scott 3200 literature. I noticed that one of the spring leaves has a longer hole, I suspect to allow for some play as the spring flexes. I just wanted to make sure the guidance wasn't just to "tighten it some more" before I took that route. I have the narrow tailwheel spring, by the way. I believe it is Univair P/N PA20TWS135. Another question I would have is whether the attach bolt should be torqued the tail raised or with the weight of the aircraft on it. Thanks for any help.


Regards,

Curtis

[Gilbert Pierce]

The bolt thru the spring to the fueselage is AN6-24. that is a 3/8-24 bolt in tension. Using the AC43-13-1B Table 7-1 you would use the line for fine thread bolts in tension. With an AN310 nut which is a tension nut the torque should be 390 in-lbs or 32 to 33 Ft-lbs. This is important as this bolt has an impact load on it. The bolt must be tensioned higher than the expected impact load or it will fail in tension due to fatigue.

The bolt thru the tailwheel to the spring is a AN7 bolt. The same table for a 7/16-20 bolt is 840 in-lbs or 70 ft-lbs. This bolt also must be loaded in tension also for the same reason as above.
I recommend using Grade 8 washers under the nut and the bolt head. I also retorque these bolts during the annual inspection.

Tail up or down should not make any difference in the torque.

[weddlece]

Gilbert,

Great answer - thank you. I guess I should just not overthink it and go to AC43.13.

I think that's a good idea to use the Grade 8 washers, especially under the bolt head - it looks like the points of the bolt head have gouged the tailwheel casting slightly over time.

Looking at the tailwheel and the Univair drawings has led me to a couple more questions:

The tailwheel has the Scott 3243-01 channel-shaped spacer to adapt it to the narrow spring. At the forward edge of this spacer, between the spacer and the leaf spring, is a wedge-shaped spacer that is the width of the leaf springs, the length of the spacer, about 1/4" thick at the front, and about 1/16" thick at the rear. This wedge shifts the tailwheel fork pivot axis to a more negative angle (lower portion of the axis more forward), I presume to reduce tailwheel shimmy. I can't find any reference to it in a drawing, nor any part number in my documentation, nor any similar product online or in a catalog. Has anyone seen/used a similar spacer?

I have seen discussion of the Univair PA20TWS135 being "too flat" and of some people having this spring assembly reworked to a smaller radius bow before installing. Is there a reference drawing for the proper bow, and how is it measured? Is it possible or advisable to upgrade the Colt to a PA25 spring? If so, are the brackets readily available?

Lastly, my leaf springs have thin strips of nylon between them. The nylon strips are the full width and length of the spring leaves. They have started to extrude and "squish" out between the leaves, and the leaves have staggered a bit over the six years I've owned the airplane. Is this some technique to improve spring action?

Thanks for any help. Have flown this airplane for six years just greasing the tailwheel pivot and bearings each year, but I think it needs a little attention due to the compression of those nylon strips between the springs whose purpose I don't really understand, and the consequent loosening of the mounting bolt.


Curtis

[andya]

Here is a template for having the Tail Spring re-arched.

https://299b3543-a-62cb3a1a-s-sites....attredirects=0

May need to adjust the size of the picture so when printed the rule scale matches an actual ruler for sizing.

I used this to have a brand new Univair spring re-arched to get the tail wheel pivot axis vertical when in stalled and typical payload in the airplane

Gilbert has one on this sight as well.


Here is a link to the site where I stored this which has a large assortment of helpful information for learning about our SWP.

https://sites.google.com/site/shortwinginfo/

If you get some goofy notice that indicates there is some immoral info upon selection disregard and continue to the sight
Have no idea what I did when I set this site up to get that message

[Steve Pierce]

Sounds like someone added a shim to fight tail wheel shimmy. The template can be used to get the proper arch in your stock q 1/4" wide tail spring. I prefer the 1 3/4" wide tail spring from AK Bushwheel, better arch and no twist like you noted on your stock spring. I fought the loose head on my Super Cub for a while and eventually after seeing Tony's and his experience went to the two bolt head and never another loose tailwheel head since.

[weddlece]

Steve,

There is definitely a shim in there. Seems would be more effective to have the spring re-arched like the AK Bushwheel spring. It is definitely 1 1/4" wide.

How is the 1 3/4" spring installed with the welded bracket? Is there an adapter available, or does a new bracket have to be welded to the lower aft end of the fuselage? I don't see any way the bolt holes wouldn't interfere with a 1.75" spring with the bracket I have installed right now.

I assume it would be possible to order a 2-bolt head from AK Bushwheel and retrofit my Scott with that? I guess that the 1 3/4" spring also eliminates the Scott 3243-01 spacer requirement. I suspect it would be an improvement, but I'm not quite at the level of off-field work that you and Tony do!

Any thoughts on the nylon strips between the spring leaves? I think that's contributing to some looseness and am probably just going to take those out. This is probably where I'm going to find that the brackets aren't tight enough without them. Perhaps re-arching the spring would help with that as well.

Thanks!


Curtis

Sounds like someone added a shim to fight tail wheel shimmy. The template can be used to get the proper arch in your stock q 1/4" wide tail spring. I prefer the 1 3/4" wide tail spring from AK Bushwheel, better arch and no twist like you noted on your stock spring. I fought the loose head on my Super Cub for a while and eventually after seeing Tony's and his experience went to the two bolt head and never another loose tailwheel head since.

[weddlece]

Well, 1 1/4" Univair spring off my project airplane looks way better than the spring I had on there, so I'm going to reassemble with that and new hardware. Comparing my old spring, newer known Univair PA20TWS135, and a loaner PA-25 spring, they all have the same bow, which is less than what is in the template. Think I'll leave the wedge in there for now and see how it does.


Curtis

[andya]

Unless Univair has changed their spring, I would recommend you have it re-arched as I did before installing it.

[Gilbert Pierce]

Unless Univair has changed their spring, I would recommend you have it re-arched as I did before installing it.

They won’t change it because they say it complies with their drawing ????

[weddlece]

Well, airplane is flying again with "new" old PA20TWS135 spring. Definitely fits up tighter without the nylon spacers and with the bolts torqued properly. Seems much more stable than with previous spring. New bolts next annual, and will re-arch the spring to match the template, delete the wedge, and see how it works. Tailwheel head and 3243-1 spacer show a bit of hammering at the points where they interface with the spring and at the point where the bolt head sits on top of the attach hole. May consider replacing these items, since these non-uniform surfaces will contribute to further wear. Will still consider an upgrade to a 1.75" spring and a two-bolt tailwheel head, though I'm not quite sure there is a "legal" way to do this with my aircraft.