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IAC Discussion Summary


IAC Discussion Summary

Tail-Wheel Shimmy

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Disclaimer: These IAC pages are developed by individual IAC members and do not represent official IAC policy or opinion.


Collected by: Stu Pressey SP7732@aol.com


Question

We have a tail shimmy problem on a SkyBolt (io540) with a Scott 3450 tailwheel. We get a shimmy on roll-out.

From: Stu Pressey SP7732@aol.com


Summary

How to correct the shimmy.

Minor points:

1. Pilot Skills: If you are unsure of your ability to fly a tail dragger, schedule a check ride with your FSDO.

2. Are there any worn or movable internal parts in the castor assembly? Replace worn and/or missing parts and secure any play.

3. Leaf springs: Are they secure and true to the airframe. The leaf springs may have a yaw and/or roll angle alignments to the airframe. Correct (with shims) to the true aircraft centerline.

4. Coil steering springs: These springs do not have anything to do with the shimmy. We have found the tail will shimmy just as well with tight springs or lose springs and even unequal tensions springs. When you want to find out for your self be sure to have a good supply of spare tires available. We suggest the loose or asymmetrical configuration. Remember these are called steering springs not shimmy springs.

Major point:

90% of the time the shimmy is here: The castor axle centerline.

This is the axle the tail wheel pivots around. This axle must not be vertical; it must be past vertical. It's hard to see to judge the alignment. So when you are looking at a side view, the top of the axle must be more aft than the bottom of the axle. It tilts back. This axle must not tilt forward. We used a level to confirm this. Make sure the ground the aircraft is on is level, then with the aircraft in a static 3-point attitude our final setting was one bubble past vertical. (This should be an easy reference for most pilots since they fly one ball out of trim.)

Also the nut on the bottom of the axle need to be snug so the wheel castors with a slight resistance. This will keep the wobble out of the bearing/axle.

There are some other suggestions and corrective actions, which in essence alter the angle of the castor axle centerline.

From: Stu Pressey SP7732@aol.com


Answers

On a 3200 if you snug the through bolt so that there is a little friction when the wheel swings side to side much of your mentioned problem will go away. If you go too far on the friction thing your wheel will not center in flight and will cause the plane to veer. Easy on the snug thing...

If that fails you will probably need to check the friction plates and the, what is it called, plate with the three pins sticking up. These will have a tendency to break off and can cause your symptoms. Try the through bolt first. Let me know what you think!

From: Carl Petersen champ@citabria.com


I have a Scott 3200 on my 1977 Great Lakes. Every year or so, I have to tighten the nut on the axle pin by one "flat" on the nut. At one point, when I tightened it, it became so tight the wheel did not pivot. So then I (instead of tightening the nut) changed the size of one of the washers under the nut and set the nut back to the original position. That way the play went away, but the nut position wasn't changed and the wheel pivoted freely. I noticed the first sign of the shimmy came when I had a big passenger in the front cockpit. After a few weeks of regular use, it shimmied with all passengers, then I got out the wrench. Good luck. A shot of my plane BTW is at the URL below. Keep it low and slow!

From: John Marshall


I know it sounds incredibly simple, and you have probably tried it, but here's an idea:

when I bought my Decathlon, I had the same problem. I ran it by my mechanic, and he reduced the air pressure in my tailwheel by 8 - 10 pounds or so. the shimmy stopped and has never been heard from again. good luck.

From: Charles Clegg, CClegg6251@aol.com


I assume that it did not always shimmy. If this is true, have you changed the tailspring lately? Perhaps it is to soft, letting the pivot tilt forward when the tail is loaded beyond static?

From: Paul Tollini paul@tollini.com


When was the last time you took the tailwheel off the aircraft? When was the last time the bushings were replaced? When was the last time the wheel was greased? When you lift the tail of the aircraft, are the leaf springs securely attachede to the airframe? Any one or a combination of the above MAY be the source of your problem.

From: Eric Rood ericrood@freenet.columbus.oh.us


I have a C-180 that had that same damn problem for 14 yrs. before it was finally cured..... and everyone but God tried over that 14 yrs. There were two factors which, fixed at the same time, affected the problem. The first was alignment. The NEAR vertical axis upon which the wheel rotates side to side is just that. If it is perfectly vertical there is no stable point. The second was the internal damping within the top portion about which the rotation takes place. Although I had disassembled and cleaned.... etc this stuff several times, it was worn & I could not detect it by visual inspection. Call the tailwheel manufacturer's engineering dept. It is the shortest route to a fix.

From: Bert H. Berrong vulcan11@flash.net


I have had a similar problem on a Cessna 170A and never have resolved it completely. It has a Scott 3200. With weight in the back seats and baggage it is the worse and with max up elevator. I have had it whip so bad that it picks up the back of the aircraft off of the ground.

I found it is worse after you grease the tail wheel. Someone told me to loosen the tension on the springs (that connect to the rudder cable) and this did help. I think the springs being tight can make it worse. I think there is also a spring kit that has asymetrical spring tension (one of the springs is stronger than the other). The type of springs that compress (have the wire inside) as apposed to the 'tension' springs also help.

The tension on the bolt is also important as this preloads the friction on the tailwheel. I could not find any specs for this. Too tight and the tailwheel will not move with the rudder and can preload trim into the rudder if the tailwheel is not exactly straight on takeoff.

I think the problem is a geometry problem where the castor is changed with up elevator (max down force on the tail). I have also installed a new tail spring to no avail.

From: Herman Dierks dierks@austin.ibm.com


I was recently involved in the flight testing of the Grand 51, a full size composite mustang with a T53 turbine. It had a similar problem, It did not shimmy initially, then it did, once. After that it shimmied severely all the time. To cure it we did 3 things:

1. new tire, the shimmy may have damaged the tire and it was slightly out of round, and also out of balance

2. reduce the pressure in the gas strut, this increase the castor angle that the tire pivoted on and put the tire farther behind the pivot point.

3. switched from a donut tire (12.50 SC to a flat tread tire) I dont think a donut tire is available for the maule so it would not apply in your case.

One or more of these solved the problem and we haven't had a hint of a shimmy in over 40 hr.. I our rush to get to OSH we never went back and reverse engineer the problem to find out what the cure was. My guess is a combination of 1 and 2. On a maule type system you would want to make sure that the pivot was level. If the front of the pivot is higher than the back of the pivot adding weight will push the tire to the left or right. This could induce a shimmy. So could an out of round or balance tire.

From: Doug Rozendaal dougr@petroblend.com


I have owned 4 citabrias with a version of your tailwheel.I found a fix that worked for me.It would take an hour of typing to relate it to you.Lets do it by phone.I would call but I dont have a name or phone number. Im at 602-962-1373 after 7 PM MST

From: Bill McIntyre acromac@worldnet.att.net


I suggest you check for any play between the leaf sping attach point and the Scott tail wheel housing . I know there it a large bolt that can be tighten very tight but will not do the trick. I had the same problem on a Decathilon and solved the problem by making a "U" shaped stainless shim and placed it between the leaf and the Scott tail wheel housing . In your case it may take two shims. It solved the shimmy problem for us........I am a A&P with 34 years . Hope to hear from you.

From: Bill


I have heard that this kind of problem could be fixed by having different springs on the left and right hand side. My Christen Eagle has a Maule (smaller than your Scott) with clearly different springs. A brochure I've got states: "Quote, Maule's latest on tailwheel connector springs stops shimmying. A high and low frequency spring is used to upset the frequency of the tail wheel. The lighter spring is used on the left side. Unquote. Both springs have same length and diameter but the wire uses is conciderably heavier gauge on one than the other. Maybe it's a lead??

From: Lars Svensson


Tailwheel shimmy is a congenital Skybolt disease. It is caused by the far forward gear axle center of the Skybolt gear which results in horrendous weights being carried by the tailwheel. This results in high loading of the spring, tire and wheel pivot, all contributors to the excitation of a shimmy.

Generally there are three culprits in the shimmy gang:

1. An improperly arched or too soft tailwheel spring which allows the tailwheel pivot to trail under load. 2. An improperly inflated or damaged tire which provides excitation for the shimmy. 3. A worn tailwheel mechanism which has free play and will support the excitation.

Many try to eliminate shimmy by tightening the steering springs but this rarely has any beneficial result. A good, healthy and shimmy free tailwheel will not shimmy with NO steering springs attached. Usually the most traceable cause is the spring which allows the pivot to incline to a trailed geometry under load. A Skybolt with, say 85 pounds on the tailwheel when weighed level will have well over a hundred in the three-point attitude and almost three hundred with you and a pax in the seats. It is a formula for shimmy. Try to assure that your spring is tough enough to resist flexing to the point that the pivot achieves a trailed geometry and either change to a stronger, three leaf one or have yours re-arched. It is all alchemy I'm afraid, good luck,

From: Rob Dorsey Rob@skydanceraviation.com


My Cessna 180 has a Scott 3400 tailwheel, and early in my ownership of the airplane, it developed a severe tailwheel shimmy. On dismantling the Scott, I found that there's a pressure plate and clutch disc arrangement that is used as a damper. This damper is pre-loaded by small, stiff springs which fit into vertical holes in the upper housing. Mine had only one of these pre-load springs. There are perhaps eight holes. I added two springs, and it hasn't shimmied in 19 years.By the way, I believe that the nut on the main vertical bolt is supposed to be tightened all the way, and the springs are relied upon for the pressure on the damper. I wouldn't swear to this though; it's been a while since I had it apart. The springs are available from Cessna dealers, and they're shown for the Scott 3200 (I don't know if these are the same as for the 3400) in the illustration in the 1997-98 Aircraft Spruce catalog. Ref. item #5.

From: Doug Sowder DSowder@aol.com


There was an article in Sport Aerobatics several years ago on this subject. I believe it suggested that the angle of the tailwheel pivot needed to be slightly forward of vertical. In any event the article is in one of the Tech Tips manuals. If you do not have access to them, let me know and I will mail of fax it to you.

From: Bruce Green brucegreen@juno.com



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Last Update: Sat Aug 29 16:11:25 2009


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