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Tailwheel Shimmy

sj

Staff member
Northwest Arkansas
My tailwheel sounds like it is about to fly off. It was loose, I tightened it, it still shimmys and directs the nose all over the place in a fast 3 pointer. It is a Scott 3200, is there something specific that needs to be rebulit in these periodically? If a guy replaced it, what would be the best choice? :???:

Thanks!

sj

_________________


<font size=-1>[ This Message was edited by: steve on 2002-03-18 07:51 ]</font>
 
Steve, Well, let's see...There's a few things you can try:

1) Tire properly inflated?

2) Are the springs/chains loose?

3) Is there a crack in the unit? (look underneath, just above the tire. I had one shimmy horribly due to this and replacing the unit solved the problem)

4) Are your leaf springs tight and attached correctly to the airframe and the Scott. (saw a PA-12 a couple days ago that was not. I flew it and directional stablity was an issue.)

A few things to try before replacing the unit as I'm afraid you may have too in the end:

Make the spring/chain tension unequal (one side shorter). This tends to stablize the tailwheel and keep a shimmy from developing (mucho discussion in the shop about this whilst standing around looking at one knowingly)

Grease the thing properly / new tire correctly inflated. / Tighten the whole thing up while it's on a sawhorse so you can check for any continued wobble in the unit. Make sure everything is correctly installed.

Go buy a new one :smile:
 
I had the same problem and I finally fixed the problem by the unscientific "shotgun" approach.

I tore it apart and replaced the worn parts (really only the brass colored rings that allow it to caster) then greased it up and tightened the pivot bolt up a bit. It's a bit stiffer than before!

It doesn't break free to caster easily (like when you push sideways on the lift handle). That fixed it. It now will shimmey only when the plane is over gross (over 1900lbs) and on pavement.

I'll take less easy to caster over shimmey any day!

<font size=-1>[ This Message was edited by: Rick Sanson on 2002-03-18 19:50 ]</font>
 
Thanks Everybody!

Rick, when did you get your A&P??? :smile:

sj
 
Well... I'm not an A&P to be honest but...

FAR Part 43.3 Paragraph D says:

"A person working under the supervision of a holder of a mechanic or repairman certificate may perform the maintenance, preventitive maintenance and alterations that his supervisor is authorized to perform, if the supervisor personally observes the work done to the extent necessary to ensure that the work is being done properly and if the supervisor is readily available, in person, for consultation. However, this paragraph does not authorize the performance of any inspection required by Part 91 or Part 125 of this chapter or any inspection performed after a major repair or alteration."

In which case, the IA who does my annual inspections personally supervises what I (read "we") do to my airplane.

By definition, the tailwheel repair and adjustments defined in FAR Part 43 Appendix A Paragraph B titled "Minor Repairs" is subject to Part 43.3 Paragraph D (what I quoted above).

Additionally, the holder of a pilot certificate under Part 61 (that would be most all of us) who is not an A&P and owns (or operates) an aircraft operating under PART 91 only can leagally perform "Preventitve Maintenance" as defined under FAR Part 43.3 Paragraph G.

Sorry for the legal BS but I intend to know as much about my Super Cub as possible and will use the FARs to my advantage as much as possible!

:wink:


<font size=-1>[ This Message was edited by: Rick Sanson on 2002-03-18 21:53 ]</font>

<font size=-1>[ This Message was edited by: Rick Sanson on 2002-03-18 21:55 ]</font>
 
Rick, sorry, it was bad humor on my part.

But a good reminder to everyone!

sj
 
Steve make sure the pivot bolt is 90 degrees to the ground (bent tail wheel spring) as this will also cause shimmy.
 
All good info here so far, just a few things to add. When tightening the forward leaf spring attach bolt, make sure not to crush the fuselage tube if you don't have the bushing welded in. I like to put a big washer under the bolt head in the belly to distribute the load. Make sure the pivot is 90 degrees from the ground when the plane is loaded, not sitting in the hanger empty. Otherwise when you load the plane the leaf springs bend, and the wheel casters back. Same thing happens on the high speed 3 pt. landing. Holding the stick full back puts aerodynamic weight on the tail, driving it into the ground, bending the springs out, castering the wheel back, and...shimmy. I know this is the way to teach begining students, but a little forward stick, or better yet always wheel land off the pavement and the shimmy disappears.

Mark Drath
 
2 more cents worth! (heck if you added it up you could buy a new one!) No really, the most likely cause is(straightened out spring) If you are using a stock spring it is too light! We always have changed out to a "Pawnee" spring so we could depend on the geometry staying the same. Also don't overtighten the front attach bolt!!!. If you have not had the bushing mod done or you (WILL) crush the cross tube and then drastically weaken the fuselage at that point! Also most of us have removed (every other caster tension springs (or I have known some) to cut off a turn! Also besure that the detent locking pin is properly seated (thru) the trust washers or they will turn with the tail wheel and can cause the tail wheel to lock side ways (off center) like a big rudder trim tab! (mine does this) cause the tail wheel is really worn and abused (landing in the sand,mud, water, etc). Please guys don't revert to doing "all your off pavement landings as "wheel landings" lest you endup on your nose in the soft stuff!". (that technique is for spam can drivers! Ok sorry that was about ten cents worth of opinion,

Good flying,

Tim
PS: Owners usually do equal or better work than the shop that doesn't own it! (don't hesitate though to get help to learn how to do the maintenance and repairs (right) Unfortunately there are still many mechanics that (don't fly)?
 
12Driver, you know the wheel vs 3pt debate will go on forever. Do you have any experience with the 3" extended gear/large tires on sloped beaches? I have standard lenght gear with 29" gar areo and never had a problem but I was thinking that a higher center of gravity may make things a tad tricky, any thoughts?pak
 
The debate will always rage on. My two cents worth is this, each landing will be different in the fact that conditions will always be different. You probably wouldn't use a wheel landing in soft goo, like wise if you were landing on a rock bar you would not use a three pt. It is rather interesting to watch the Supercub videos to see how the pros do it, but sit and think why they chose to land the way the did.(All of the time they were protecting the aircraft from damage and letting the aircraft mods do what they were supposed to.) Bottom line learn to do Both equally well. (Practice,Practice,Practice like we needed an excuse to fly.)
Can I get an amen brothers and sisters!!!!
 
Amen, Amen, Amen!! That was for the need for an excuse to fly, Heck just yesterday, after working on my bills I had a horrible headache, jumped in the cub, (used the excuse I needed to return a pie plate) and flew to my neighbors strip just cus! (headache gone)

Pak, I have most of my cub experience on sloped beaches on the Alaska peninsula and the tide flats of South central Alaska. Lots of times in quartering Xwinds 15-30 miles per hour! That is where the fun begins and and the practice pays off! I have had the 29/10's on a cub (for a short while) We used to chuck them in a vise, and grind the hide off of the 8ply's and try to make them work like a tundra tire? (they aren't near as bouyant period! They work great on a 185! I put several thousand hours on them in my (other plane(. AS for Landings, "Supercubc37" you are right on! you got to know how and when to use both. Generally you wheel land in big wind, so you can maintain control till the tail has to come down. (Also good point on protecting the bird (The shale and rocks fly all over hell if you wheel land a cub at any speed.
I have had std. HD gear and 3" HD gear with 26-29-30/31 and 36 inch tires-- My favorit combination (with a light cub is 3" extended gear, (the old light 26" Airstreaks and an 82/42 prop. (each to their own) As I said I now fly a PA12 with 29" tires and6" extended PA18 gear, stiff bungees and 3-4lbs of air for the soft stuff.

Let fly,

Tim
 
pak, You will only gain a little over 2" in height with the 3" gear. About the same difference you have now between running your tires soft or fully inflated. I have 3" gear with 29's and really like the attitude. Does not seem top heavy at all. Tail comes up a little slower taking off down slope, but not much. The big difference is in gear width, which is about 4.5 inches. Lots more stable when operating cross slope. But harder to get up on one wheel when on curved strips or with crosswinds. If you have never flown with extended gear, try to fly a Cub with it on before you buy to make sure you really want it.

LANDINGS
Here comes the Sermon, and it aint even Sunday.

All the good high time Cub drivers I know hardly ever make a honest wheel or 3 pt landing. It is usually some combination of the 2, a tail low wheel landing, keeping that fragil little wheel in the air and maintaining control till nice and slow, then setting it down. How high the tail is depends on the wind conditions and terain. Guess I have to much time in the tail draggin spam cans, but a 3 pt is a graceless, uncontroled "plop" back to mother earth, hard on the equipement and only good for very soft terain, (if it's soft enough to need to do this, you should reconsider landing there). A true high tail wheel landing is only good for long landing areas and extreme crosswinds, (again, if the crosswind is strong enough to need to do this, you should reconsider landing there). Some may say that making wheel landings will hasten your joining the famous "High Tail" Squadron. But if you take little planes into tight places, it doesn't matter if the little wheel is up or down if you screw up. It's like the go-fast guys landing with their wheels still tucked in the wings, "There or those that have, and those that will". The only way to prevent this is to keep your skills sharp, and always keep your sh.. together when landing. Stay safe.
Mark Drath
 
What is your take on the potential for helping my nose heavy condition in my 12 with the additional weight of the "big fork and tire" versus the loss of "angle of attack" during take-off

Remember I have already got a significant gain in leading edge height due to the addition of "6" inch extended PA-18 gear and 29" tires? I am currently swinging a 82/41 stick on my 0320/160?

Thanks

Tim
 
PA-12 CG

You know what's weird Tim? I've flown a friend's newly rebuilt PA-12 a few times in the last month and it's very tail heavy. He has stock length 18 HD gear, 8:50's, the 82/44 and a 160 but I have to run near the forward limits of the trim with him (290lbs) in the back seat and a 3/4 load of fuel. (Big tanks) Is this normal for a 12? I wouldn't dare throw anything in the baggage compartment much less the extended baggage in this condition. :eek:
 
Final Chapter

Folks,

I installed (under the watchful A&P eye) a new tail spring and brand spanking new tailwheel and they work GREAT. Also, the new VG's (see my post on VGs) are AWESOME!

:p sj
 
Nose heavy Fat Cub

There you go Tim, just make good friends with a 290 pounder and keep him in the back. I'd still avoid the "big fork and tire" due to stress on the fuselage, even though your AOA should still be OK. I had a long talk with Jim Drometer the other night. He has the STC for the 80" wood prop on 12's and 18's. I need to go fly his 12 soon, he says it really performs, (light 150 hp with the wood). He also wants to bolt the wood onto my 18 to see how it performs. I don't think he has tried it on a 18 as light as mine yet. Should be interesting. May help the nose heavy Fat Cubs. Loses 25 lbs on the nose compared to a Borer. I won't be able to go see him till the shop slows down and the weather straightens out, it could be a while. I'll post my findings after I do. The only solution I have come up with is to trade the Fat Cub for a nice light stripped down 18.
 
Tail Wheel Shimmy

Most likely it was your detents and plates were worn out. The wheel detents in the straight forward and 90 degree positions. If it free wheels back and forth too easy the tail will track all over the place. I ocassionally take mine apart and clean and inspect the detents, etc for ware, dirt and lube. Yes it has a grease zerk, but nothing is better then a hand applied layer of grease. Also your rudder horn and tail wheel ears need to be parallel. A lot of guys get the horn in the rudder crooked. In this case you will need to shorten on chain to compensate. Also the heavy set of Scott springs will help. My two cents. The other comments about the tail spring are also contributing factors. Crash
 
Fellows, one of the main reasons for shimmy in the 3200 scott is over greasing. What happens is the anti-shimmy plate gets grease on it and don't anti anymore. Take the unit apart and and clean it up good. At reassembly keep grease off the plate. Now, the part I am refering to is in the top portion and is a fiber type plate . there is another fiber plate in the very top that is stuck in there. these parts need to be greaseless. After installing the fiber plates comes the pressure plate, the part with the 3 pins sticking out of it,then the unit is assembled with just a smearing of grease. Be sure all 5 pressure springs are still there on assembly. after assembly pump in no more than 2 pumps of the grease gun. At any other greasing , no more than 2 pumps.
Steve
 
Here is an article my Dad wrote after a lot of research on tailwheel shimmy in his airplane. He is a mechanical engineer and has to have a logical explanation for everything. In other words he doesn't believe in PFM (pure #@%&in magic). It solved his problem.

Steve

I noticed when flying my Piper Clipper heavily loaded I experience tail wheel shimmy on my Scott 3200 when landing on a hard surface such as concrete or asphalt. Several years ago I was parked at the landing end of runway 36L at Oshkosh. I always get tail wheel shimmy when landing there. This afforded me the opportunity to observe many landings as I lounged under my wing. It was here that I made the discovery that about 50% of the tail wheel airplanes landing on 36L experienced tail wheel shimmy. I believe the grooved runway exacerbates the problem. Anyway those tail wheels were not just shaking side to side, they were rotating around their pivot axis 360 degrees and doing so violently. On my recent trip to Alaska I had tail wheel shimmy on almost every landing unless I really greased it on. On my return I vowed to solve the problem.

I started the quest for a solution on the Internet. I was told that if you raised your tail wheel tire air pressure it would assure the tail wheel would shimmy no more. I was told to reduce the tail wheel air pressure. I was told I had too much grease in the tail wheel. I was told that if the tail wheel didn?t spit grease at you when walked by it, it did not have enough grease hence the shimmy. I was told to loosen my steering springs, I was told to tighten my springs. I was told that the pivot axis must be absolutely vertical so that the surface the tail wheel swivels on is parallel with the ground hence the pivot bolt would be vertical. Mine was. I was told the pivot bolt must face forward at the top, I was told the pivot bolt must face aft at the top. So what did I do? I took the tail wheel apart and made sure it was mechanically in top-notch condition and adjusted to the manufactures specifications-again. It was. Then I tried each and every remedy listed above except changing the angle of the pivot bolt, no help. The Scott 2000 tail wheel does require some tension on the steering springs to control the unlock tension and hence shimmy. The Scott 3200 installation instructions say that chain tension is not required or recommended.

Next I got out an old 1950?s auto repair manual that explained king pin front wheel suspension systems and steering castor angle. If you have ever pushed a grocery cart through the supermarket with one of the front wheels shaking side to side you have experienced wheel shimmy and improper castor angle. What I learned from the chapter on steering alignment was basic steering geometry. To measure your steering geometry, drop a line drawn parallel to and through the pivot axis and extend it to the floor and make a mark on the floor where this line hits or use a straight edge parallel to the steering axis shaft. Next drop a line vertically from your tail wheel axle to the floor or again use a straight edge and make a mark on the floor. This will also be where your tail wheel contacts the floor. Now move this line or straight edge that passed from the axle to the wheel/floor contact point horizontally until intersects the pivot axis line at the pivot axis midpoint The line that is parallel to the steering axis must hit the floor ahead of the line dropped vertically from the wheel axle. The angle formed by these two lines is your castor angle. The larger the castor angle the better as far as tail wheel shimmy is concerned. In other words, the farther ahead of the tail wheel that the steering axis line hits the floor the greater the castor angle and the less likely that will have shimmy. To put it another way, the steering axis pin or bolt must be vertical or tilted with the top pointing behind or to the rear of the airplane when the airplane is fully loaded. Emphasis on fully loaded.

When my airplane was empty the steering axis bolt was vertical. When I loaded the airplane, the tail wheel spring compressed and the top of the steering axis bolt was pointing to the front of the airplane. This would put the extension of a line drawn through the steering axis behind the tail wheel contact point. Bad news-it will now shimmy. You don?t want the castor angle to be too large because it will make steering on the ground more difficult. The large castor angle will tend to lift the rear of the airplane slightly as you turn the aircraft. This is called self-centering effect. Having the steering axis bolt vertical or inclined slightly with the top pointing back when fully loaded should be sufficient.

So how do you correct this angle? There are two easy solutions. If your airplane is like most, the spring is bolted at the front to the airframe with a bolt that passes through the spring leaves. The spring then rests on a pad several inches behind the point through which the through bolt passes. Usually the spring is clamped to the pad at this point. You can add a shim between the pad and the spring to increase your steering angle. Or you can take the route I took. I took the spring off and laid it on a piece of poster board and traced out it?s arch. Then I took the spring to a spring shop and had them re-bend the spring until the tail wheel end of it was about 1-1/2 inches below the original. In other words I increased the arch slightly. Walla-no more shimmy when loaded.

One other point. You should carry sufficient air pressure in your tail wheel to keep the tire firmly attached to the rim when it hits the pavement on landing. Due to the small diameter of the tail wheel, it accelerates very rapidly on contact. If you have insufficient pressure in the tire it will slip on the rim and cut the valve stem. You now have a flat tail wheel tire. I know- it?s happened to me twice. I now carry a minimum of 45 pounds of pressure in my Scott tail wheel. The same thing can happen to your main tires but with more surface contact area around the rim it is less likely to happen unless your plane lands at very high speeds.
 
GREAT POST! I am glad to see that it has been realized that it is the improper caster angle that causes most shimmy.

Hence this is why on most cubs that are worked? that a Pawnee spring is used in replacement of the original. the original is 1 1/4 inches wide and is much lighter. they work fine for a while! I have seen nearly all of them loose their arch, (noticable when loaded) over time.

Great post! (replace the spring) with a heavy duty one if you haven't already.

PS: the reason a new one works (for a while) is it is not worn, or mis adjusted either.


Tim
 
How?

So how DO you replace your spring with a Pawnee spring? It's wider, requiring a different rear mounting bracket, which would cover the bolt mounting holes.....or would it?
 
What is the Pawnee spring part Numberd? Where is the place to get them? Univar? gd
 
you can get the spring from Univair, Stoddards, they are 1 1/2" wide and they mount in the original hole with(out the centering bracket/retainer in the rear) instead the bolts are the retainer with just a flat strap across the bottom.

I don't remember the part number, however there is a 2-hole spring as well for the 2 hole HD scott 3200 tail wheel. (not entirely necessary) for most flatlander cubs.

Tim
 
The last few landings I have had the tailwheel shimmy and the spring clips will either break or come off. At last annual the spacer between the spring and wheel housing was replaced. My first few landings after the annual did not have this result. The difference in the last few landings is I have had a passenger, i.e. additional weight. Any ideas on what might be happening? I replaced the clips with new ones, but even they came loose.

John
 
had the same problem, but when the spacer was replaced it was fine. prior to the spacer being replaced, i would apply forward pressure on the stick to alleviate/eliminate the shimmy.
 
Read this article and it will give you some insight into what causes tailwheel shimmy. My Dad wrote it after researching his engineering manuals and experimenting a little. He is a mechanical engineer and it comes out in everything he does. I have used this process on Super Cubs, clippers, Maules and Huskies.

http://www.pierceaero.net/tws.php
 
Hi,
I just got lots of tail wheel shimmy when I greased it up a few days
ago. Never had it before. Landing on pavement two out of three times, 100% of the time when I was heavy, caused concern. I had to let the plane decelerate on it's own before I could do anything.
Dave.
 
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