Re-Arching a Tail Wheel Spring

My tail wheel shimmied recently for the first time ( after 265 hrs) so I reread Gilbert Pierce's Essay on the subject. Upon disassembling the tail wheel, I discovered that the two 1/4 inch bolts holding the spring in alignment were bent and the rubber pad distorted ( on reassembly, I eliminated the rubber pad and installed new 1/4 inch bolts). Also the washers under the 7/16 bolt at the front were yielded some -- I know Steve Pierce says to use hard washers; I will get some for the next time I have it apart. Also the tail wheel tire was way out of balance so I installed a new one.
The main problem though was that the tail wheel pivot was at about a 4 degree negative caster angle.
This picture was taken before disassembly right after landing with a shimmy. No one in the plane & about 36 gallons of fuel.

Doctor Randy recently had his tail wheel shimmy & bought a new Bush Wheel spring set which he is very happy with. His airplane with an unknown quantity of fuel (full?, 36 gal) & no one in the plane sets with a 6 to 6.5 degree positive caster angle. Since he is happy with this set up, I aimed to duplicate the angle his plane sets at.
Here is Randy's tail wheel.

So, happily, I have a friend, Don Rosacker, who has a 50 ton press in his hangar. So with another friend, Tim Loth, Don & Tim Re-Arched the springs using a rough drawing I had made as a guide.
Don is the handsome octogenarian on the right. Tim is the much younger dashing guy on the left.


I also have another set of Pawnee springs that Grant recently sold me at Wipaire (they got them from Univair). So we can compare the originals with the re-arched springs.


At the rear edge of the spring, the re-arched set is 1 3/8 lower than the originals. Gilbert said that his ended up 1 1/2 inches lower.
The reassembled tail wheel on N18SY now has a little more arch than I was aiming for. Randy's is 6 to 6.5 degrees positive. Mine now is 7 to 7.5 degrees positive. Gilbert warns that too great an angle will make it hard to steer. I haven't flown it yet.
Our Super Cub, N18SY, after re-arching the Pawnee spring set. It now sets at a 7 to 7.5 degree caster angle; no one in plane and 36 gal fuel.

With Vivian in the back seat (120 lbs) and me in the front seat (210 lbs) and with 36 gals of fuel, the caster angle comes down to about a positive 6 degrees.
I measured these angles by taking a side picture of the assembly as shown then either using the metal panels on the wall of the hangar for reference or a plumb bob to establish a vertical line. Then on the 8x10 photo, I drew the angles, measured the legs, calculated the tangent then looked up the angle for that tangent. Repeatability seemed to be plus or minus 0.5 degrees.

I did much the same thing Darrel. But, I rearched too far. Direct from Univair too flat. Re-arched too far equals no shimmy, but harder or "less willing" to steer. As long as there is a positive caster at all weights and CG, it won't shimmy.

As I'm certain you know, Steve and Gill Pierce have done a bunch of work on this too and wrote it up here and on Steve's website. Glad yours is working for you.

Calvin, did you measure the angle you ended up with or could you post a picture? I'm curious to home in on what positive caster angle becomes too much. I might be there. Can anyone else give some advice on this subject?

Darrel,

My advice is to fly it awhile with the re-acrched spring. Chances are that some of the re-arching will relax back out and you might end up nearer your target angle. These springs are spring steel (AISI 5150 or similar) and are hardened to a reasonably high strength, most likely in the 40 to 45 Rockwell C hardness range (approx. 125-150 ksi yield strength). When you bend these springs in the hardened condition, they end up with quite a bit of residual stess. This causes them to bend back in the direction of their original shape under lower stresses than the yield strength of the material. Just fly it for awhile and see where is ends up.

Jeff

Thanks Jeff for that insight. I will fly it and after a while measure the angle again as you suggest.

I have re-arched mine a number of times and experimented with it a various angles, never really measured it. I've had mine with more arch than you have in the picture and it was fine, steering was not a problem. The biggest problem with too much arch is ground handling with a toe bar, the wheel is just laid over too much at 90* to turn well with the tow bar.

I got tired of needing to re-arch the spring when I was away from the shop on a trip or something so just now replaced it with a new BushWheel spring. Hopefully, this will be a better solution, it is a bit thicker steel than the one I took off. That one was a Calumet Cub tail spring.

Thanks Gerald, comforting to know you have experienced higher angles with nothing too weird happening.

Great documentation and illustrative photos, Darrel..

MTV

Thanks Mike. Always appreciate your comments.

Darrel, if you need help "de-arching" your spring let me know and I can perform one of my patented hard landings and take the arch right outta that spring...for a nominal charge, of course.

Thanks Randy, but I have expertise in that area myself.

WindOnHisNose said:

Darrel, if you need help "de-arching" your spring let me know and I can perform one of my patented hard landings and take the arch right outta that spring...for a nominal charge, of course.

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WOW... what kinda rate do you get for droppin a friends plane on it's tailwheel ? This may be a business venture we need to inspect

Finally flew the SC today with the spring re-arched. Steering was not noticeably harder with the 7 degree positive caster angle so all is well. No shimmy.
Also, I changed the steering links to add the slack recommended by Bushwheels. Don't think I will leave them that slack. I prefer more positive steering.

This tailwheel ( a Maule - I think) shook so violently last week on landing @ Anoka that the steering springs & chains separated from the plane- later found on the runway!

Darrel, My first experiment with re-arching the springs I went too far. You couldn't steer it because when you tried to steer it you were actually lifting the tail up because way too much angle. I think if you had too much you would know by now. The arch on the AK Bushwheel tail spring came from a New Old Stock Piper PA18 tail spring that was hanging on a hanger wall in Birchwood. The metallurgy is part of the issue from my conversations with Bill Duncan on the subject. The starting angles are the other. I have several letters from different sources to Light Plane Components (former owner of the Univair Tri-Pacer to Pacer STC) and Univair complaining of tail wheel shimmy on converted PA22/20s. Spring angle was the key. The Pacer is pretty heavy on the tail and is more prone to losing it's arch. Have replaced several Pawnee springs on heavy Super Cubs that lost their arch. Haven't had any issues with the AK Bushwheel spring yet on SCs or Pacers.

Steve, after the shimmy a few weeks ago, I bought a new Pawnee spring set from Grant at Wipaire. Got a pretty good deal as they had had it in inventory for 8 years, & Grant is a good guy! The spring originally came from Univair. But when I took off my original Pawnee spring, also from Univair, (the one on the plane when the tail wheel shimmied), I couldn't tell the difference in their angles. One laid right on top of the other -- although my top spring, the short one, might have been sprung a little. So I don't think my spring had de-arched much if any. I rather think that I finally found the "right" conditions for it to shimmy. And I think ALL Pawnee springs supplied by Univair are angled wrong. I am no spring expert but Stewart made a good point about material & hardness. These springs should never de-arch if designed & manufactured correctly for this application. I am stuck with a Pawnee spring because my plane is certified and has the 2000 lb. gross weight STC from Wipaire which clearly lists the Pawnee spring as a requirement to satisfy the STC.
Anyway, kind of handy to have a second "baseline set laying around.
I am happy with 7 degrees after today's ride. It steered just fine.
By the way, I made two landing at 1650 lbs. and afterward the caster angle still measures 7.3 degrees. So if I decide I need 5 degrees, I might have to hire Doctor Randy to straighten it out some.
I still think the steering chain & springs are too long following Bush Wheel recommendations -- wish I could add half a chain link back in.
After First Flight -- Still 7.3, approx. 7 degrees caster angle.

Darrel,

While all your threads are of great interest, this thread has taken on a new significance.


I guess I'll be calling the Bushwheel crew tomorrow to see if they have any Pawnee springs in stock.

Jeff

Jeff. I bet that rattled your teeth. Mine shimmied wildly just once -- once was enough! However, I have talked to a lot of friends with tail wheel planes that shimmy now and then. For some reason, they just shrug and don't plan to make any changes. I don't want a shimmy -- EVER. Easy enough to buy a stronger correctly arched spring or re-arch one.
Thanks to Gilbert Pierce, there is no mystery about how to fix the problem.
If you look at the Bush Wheel video of how to set up tail wheel chains, you will be struck by the seemingly steep angle of the leaf spring. I would wager that one will not shimmy. http://www.youtube.com/watch?v=QtokU8mIDQk

It is no wonder that the tail post sometimes cracks off of the lower longerons. Did you notice how much the fuselage was twisting (belly fabric wrinkling) several feet ahead of the tail?

Yes, the tail really gets a workout.

Darrel Starr said:

Jeff. I bet that rattled your teeth. Mine shimmied wildly just once -- once was enough! However, I have talked to a lot of friends with tail wheel planes that shimmy now and then. For some reason, they just shrug and don't plan to make any changes. I don't want a shimmy -- EVER. Easy enough to buy a stronger correctly arched spring or re-arch one.
Thanks to Gilbert Pierce, there is no mystery about how to fix the problem.
If you look at the Bush Wheel video of how to set up tail wheel chains, you will be struck by the seemingly steep angle of the leaf spring. I would wager that one will not shimmy. http://www.youtube.com/watch?v=QtokU8mIDQk
View attachment 12031

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It sure did rattle my teeth. Enough that I do not want it to happen again. Thanks for the link to the Bushwheel video on how to set up tail wheel springs. I'll give it a shot when I install the new spring. I also have to thank Gilbert Pierce for an education on tail wheel shimmy. Before I took the SuperCub apart in 2007, I re-arched an old spring per his on-line advice. It solved the problem. Unfortunately, that tail spring broke and I am repeating history. I am going to get the Pawnee spring with the proper arch and solve this thing for good.

skywagon8a said:

It is no wonder that the tail post sometimes cracks off of the lower longerons. Did you notice how much the fuselage was twisting (belly fabric wrinkling) several feet ahead of the tail?

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Thanks for pointing that out. I completely missed the wrinkles earlier. Based on how violent the shimmy is, I am not surprised that the whole tail is distorting. Makes me want to get this thing fixed even sooner.

jrussl said:

Thanks for pointing that out. I completely missed the wrinkles earlier. Based on how violent the shimmy is, I am not surprised that the whole tail is distorting. Makes me want to get this thing fixed even sooner.

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My tail wheel used to burst into song at in opportune moments. Tail was pretty light so new stock spring and all was usually great but every now and then when loaded heavy or I landed just so she would sing off key.

Frustrated, I studied it and noticed that the bolts were pretty long and an old salt told me Piper used to shim em, so I put a 1/2" shim between the spring and the rear attach bracket and presto never anouther note.

OLDCROWE said:

My tail wheel used to burst into song at in opportune moments. Tail was pretty light so new stock spring and all was usually great but every now and then when loaded heavy or I landed just so she would sing off key.

Frustrated, I studied it and noticed that the bolts were pretty long and an old salt told me Piper used to shim em, so I put a 1/2" shim between the spring and the rear attach bracket and presto never anouther note.

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Thanks for the suggestion of the shim. As I only had a three leaf .25" thick x 1.25" wide tail spring, I decided to go with a new one that is three leaf but .32" thick leaves x 1.75" wide Pawnee Spring. The Bushwheel folks were helpful as usual.

I made an earlier comment that I did not think it was shimmying on grass....well the following video proved me wrong. While the shimmy is greatly reduced, it is still there. It will be interesting to see how much it wiggles with the new spring.

Jeff, for reference, if you still have the old setup on the plane, how about publishing a straight side view with the tailwheel on the ground so we can look at the caster angle.
Also, another request, when you get the new spring set, please get a side view of that too & video the action again as you did before. Wondering if the changes will totally "quiet" the action or will it wiggle some just not enough to be real noticeable.

This video, along with Darrel's input brings up the thought, does the increased size (mass) of the AK bush wheel make a difference in the shimmy/anti-shimmy qualities of the tail wheel? Should the AK bush wheel be attached at a different angle than the Scott, which has been so capably shown to us by Steve Pierce? Should it be mandatory that a wider/stiffer spring be installed with the bush wheel? Jeff's narrow spring wiggles like a wet noodle.

Darrel Starr said:

Jeff, for reference, if you still have the old setup on the plane, how about publishing a straight side view with the tailwheel on the ground so we can look at the caster angle.
Also, another request, when you get the new spring set, please get a side view of that too & video the action again as you did before. Wondering if the changes will totally "quiet" the action or will it wiggle some just not enough to be real noticeable.

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Darrel,

I'll weight the airplane down and take some pictures of the angle before and after the change. I'll also get some more video after the change.

Skywagon, I had a pretty back shimmy without the baby bushwheel and an old spring. My experience tells me that if the caster angle is bad, all tailwheels will shimmy.

Jeff

I installed the new tail spring today and got some before and after pictures.

Jeff, that is great information. Do you have the angles or would you like me to measure them. It is obvious that the 1 1/4 spring isn't up to the task while the new spring is. Nice work!