I have done everything I know to stop my tail wheel from vibirating. The bearing are tight the springs are good and the wheel is balanced. Just put a new spring on because the old one broke. Funny thing, it does not do it every time. If I make a three point it is automatic to vibirate on asphalt, not grass, If I make a wheel landing it will not vibirate every time I lower it to the tail wheel. Help!!
Scott 3200
- Thread starter hsdexo
- Start date
We've chased the tailwheel shimmy for how many years? 
Only thing consistant we have found, is that the tendancy incrases with higher tail weights. (Get a back passenger and its worse than solo...etc.) This tends to lend itself to the leaf springs bending or twisting. Obviously the springs are not doing the violent shake, but they could be causing the tailwheel geometry to allow the 3200 to break free of its lock.
Since we have no control over the 3200, we are in the process of ordering a new batch of leaf springs (you have to order about one million of them to get a decent price), and we are trying to get a few details closely monitored when they make them. Hopefully, the design intent will come through and the top two spring will contribute more. Its not so much a change in design, but a 'getting the most out of it' thing. 8GCBC springs will be changed; the bottom spring will jump one guage size in thickness.
This increased bending stiffness should help against shimmy. I doubt it will make the problem go away. I think there are too many things plaguing the assembly back there. Slowly we will work out this bug with the line though....trying one thing at a time.
Only thing consistant we have found, is that the tendancy incrases with higher tail weights. (Get a back passenger and its worse than solo...etc.) This tends to lend itself to the leaf springs bending or twisting. Obviously the springs are not doing the violent shake, but they could be causing the tailwheel geometry to allow the 3200 to break free of its lock.
Since we have no control over the 3200, we are in the process of ordering a new batch of leaf springs (you have to order about one million of them to get a decent price), and we are trying to get a few details closely monitored when they make them. Hopefully, the design intent will come through and the top two spring will contribute more. Its not so much a change in design, but a 'getting the most out of it' thing. 8GCBC springs will be changed; the bottom spring will jump one guage size in thickness.
This increased bending stiffness should help against shimmy. I doubt it will make the problem go away. I think there are too many things plaguing the assembly back there. Slowly we will work out this bug with the line though....trying one thing at a time.
I called ACA last week wanting a new main spring for my 99 Scout. They told me they went to a new style and I needed all three. When they arrived they looked the same as my old one. Once installed though the tail sits up about 2" higher. It hasnt shook once yet. Smooth as can be. My Texas Tail Dragger always shook when it would get dry. I wonder how many people are actually running the nut to tight, thinking they are helping the problem. A friend of mine has a Swift that drove him nuts. We loosened the nut up repaced the 5 tension springs moly grease and his is happy as a clam.
Well, I would not call, the 'new style' that you got to be truely new. Basically, until the new ones come in (read: slow vendor!), We have sets that are tweaked and matched. Thus, you need all three. In cases where the airplane is used heavily, the tweaked ones don't last as long, but we are talking pretty extreme use (like 45-50 hours a week off pavement!!). New style are due in soon....I hope.
Note, there is not a serious change in that design either, just some tighter tolerance and a thicker bottom spring.
Note, there is not a serious change in that design either, just some tighter tolerance and a thicker bottom spring.
I fly a 7GCBC. After rebuilding the tailwheel (3200) and putting on a new set of spring I was sometimes still experiencing an intermittent shimmy on 3pt landings. However, 90% of the time this can be traced to landing too fast. When flaring out at an IAS of 60mph or less (with 2 notches flaps) and letting the tail touch slightly before the mains I never get a shimmy. If you drive it on 3pt before the airplane has stalled you'll get one almost every time.
mikeberg/wi
New member
I think the main fix is to keep the tail wheel vertical shaft as straight (up and down) as possible, maybe even slightly forward on the bottom (to allow for back seat weight). If it trails backward, it almost always vibrates (like a shopping cart with a bad wheel). It seems that's why they vibrate worse with someone in the back seat (more weight=bends spring=more trailing angle=nasty vibration). I've fought this problem off and on and the cure seems to be to either shim the spring or bend the spring. Of course, a stronger tailwheel spring would probably help too (as Jerry says).
My humble opinion.......... :roll:
My humble opinion.......... :roll:
Craig has a great point, actually.
No matter how bad the geometry, in order to start the dynamics of shimmy, you have to excite the system first. If you don't excite it enough to begin with, it won't happen. The more the geometry is out of shape, the more sensitive the system is to excitation, in this case.
One!! of the 6 springs came in yesterday. So, the judgement is still out on whether these are going to help.
No matter how bad the geometry, in order to start the dynamics of shimmy, you have to excite the system first. If you don't excite it enough to begin with, it won't happen. The more the geometry is out of shape, the more sensitive the system is to excitation, in this case.
One!! of the 6 springs came in yesterday. So, the judgement is still out on whether these are going to help.
THIS MAY BE JUST TOO SIMPLE, BUT COMING FROM A SIMPLE MIND, IT WORKS FOR ME. I THINK YOU FOLKS ARE CORRECT WHEN YOU POINT TO TAIL WHEEL SPRING DEFLECTION AS THE ROOT CAUSE OF TAIL WHEEL SHIMMY, BUT NO ONE MENTIONS JUST SIMPLY NOT SUCKING THE STICK ALL THE WAY BACK ON ROLLOUT. THIS ALSO REDUCES WEIGHT/ SPRING DEFLECTION/ SHIMMY TENDENCY. IT MAY SEEM OBVIOUS, BUT I HAVE FLOWN WITH SEVERAL MODERATELY HIGH TIME PILOTS WHO BLINDLY FOLLOW MINDLESS TAILWHEEL TECHNIQUE AND GRIT THEIR TEETH AS THE TAILWHEEL HAMMERS AWAY. BACK OFF ON THE BACK PRESSURE AND SEE IF THE SHIMMY PROBLEM DOESN'T GET BETTER.
I operate a 1977 Decthlon CS for tailwheel conversions and aerobatic intruction.
I had a lot of problems with shimmy - my previous aircraft, a super decathlon, had to be welded for that reason.
I found there was a gap/clearance between the tailwheel assembly and the spring, which allowed the complete tailwheel assembly to be able to move sideways, pivoting around the bolt.
I put a sheet of cobber in between, which filled up the space sideways, and tightened it hard.
After this I have had practically no shimmy. I recal only once where a student made a poor crosswind landing and pulled the stick fully back - other than this one incident, no shimmy so far.
I have experimented with less air pressure - no help, only result was a blown tire.
I teach the students, when we do 3 point landings, to leave the stick where it is when touching down and only mooving is aft slowly
Regards Lennart Wahl, Aerobatic instructor, Denmark
I had a lot of problems with shimmy - my previous aircraft, a super decathlon, had to be welded for that reason.
I found there was a gap/clearance between the tailwheel assembly and the spring, which allowed the complete tailwheel assembly to be able to move sideways, pivoting around the bolt.
I put a sheet of cobber in between, which filled up the space sideways, and tightened it hard.
After this I have had practically no shimmy. I recal only once where a student made a poor crosswind landing and pulled the stick fully back - other than this one incident, no shimmy so far.
I have experimented with less air pressure - no help, only result was a blown tire.
I teach the students, when we do 3 point landings, to leave the stick where it is when touching down and only mooving is aft slowly
Regards Lennart Wahl, Aerobatic instructor, Denmark
Since this topic has surfaced again I would like to add my comments on rebuilding Scott 3200 tail wheels.
I have rebuilt a number of tail wheels, to do this I have made a number of tools to remove bushings, bearing races and pins.
One thing everyone should have before starting is an overhaul / assembly manual which was emailed to me as a PDF file from Avox.
The one item I have found on every tail wheel that I have worked on is the Spring #18 pn. 3200-00 needs adjusting. Extra clearance here results in backlash in the fork. This free play allows a certain amount of steering slop. This is corrected by first removing #27 pn. 3205-00 bronze bushing, then pushing #21 pn. 3257-00 pin through into the bore. (On one case we had to pull the pin out because it was too damaged to push through).
The spring and shims #19 & 20 can be removed. You take the shims from the inside and installed one or both behind the spring to remove the spring free play. While you have the bronze bushing out slide it over the pivot stud and check for wear. If it slops around - replace it, do not re-use it.
At the top of the tail wheel assembly is #3 pn. 3207-00 Thrust Washer which is a phenolic plate that locks into the top. Before I install a new one I clean it with alcohol and sand both sides with 400 grit sandpaper to remove the gloss. If the Thrust Plate Assy. has any wear or grooves you can sand it flat or better replace it.
It’s a good idea to replace all 5 compression springs #5 pn. 3233-00. when I assemble the tail wheel I use little to no grease between the phenolic plate or the thrust plate. I want this to have friction and not act like a bearing. Grease the bushing, tapered bearing and the bronze thrust bearings #7 pn. 3206-00.
The castle nut should not be over tightened - 15 in Lbs min then, if required continue clockwise to access cotter pin hole.
Over tightening this nut to control shimmy creates more problems.
In use, I do not believe you need to apply more than one shot of grease at a time to lube the bronze bushing. Do not pump it full till grease comes out of every where.
Alaska Bushwheels now has PMA'd Scott parts and are less expensive. I believe Chief sells them.
My $.02
Regards
Morris
I have rebuilt a number of tail wheels, to do this I have made a number of tools to remove bushings, bearing races and pins.
One thing everyone should have before starting is an overhaul / assembly manual which was emailed to me as a PDF file from Avox.
The one item I have found on every tail wheel that I have worked on is the Spring #18 pn. 3200-00 needs adjusting. Extra clearance here results in backlash in the fork. This free play allows a certain amount of steering slop. This is corrected by first removing #27 pn. 3205-00 bronze bushing, then pushing #21 pn. 3257-00 pin through into the bore. (On one case we had to pull the pin out because it was too damaged to push through).
The spring and shims #19 & 20 can be removed. You take the shims from the inside and installed one or both behind the spring to remove the spring free play. While you have the bronze bushing out slide it over the pivot stud and check for wear. If it slops around - replace it, do not re-use it.
At the top of the tail wheel assembly is #3 pn. 3207-00 Thrust Washer which is a phenolic plate that locks into the top. Before I install a new one I clean it with alcohol and sand both sides with 400 grit sandpaper to remove the gloss. If the Thrust Plate Assy. has any wear or grooves you can sand it flat or better replace it.
It’s a good idea to replace all 5 compression springs #5 pn. 3233-00. when I assemble the tail wheel I use little to no grease between the phenolic plate or the thrust plate. I want this to have friction and not act like a bearing. Grease the bushing, tapered bearing and the bronze thrust bearings #7 pn. 3206-00.
The castle nut should not be over tightened - 15 in Lbs min then, if required continue clockwise to access cotter pin hole.
Over tightening this nut to control shimmy creates more problems.
In use, I do not believe you need to apply more than one shot of grease at a time to lube the bronze bushing. Do not pump it full till grease comes out of every where.
Alaska Bushwheels now has PMA'd Scott parts and are less expensive. I believe Chief sells them.
My $.02
Regards
Morris
Good Reply Morris.. lots of details that are sure to help everyone.
Any experience would be appreciated!
Re: '72 Citabria 7GCBC
1. Are the "New" leaf springs available? I believe mine are 1 and 3/4 inches wide.
2. and if so.. how are they working out for everyone. Did the redesign help?
3. Is rebending the springs (without heating) just as good as the new ones?
4. Does anyone have a pattern to follow to bend the springs (for the spring shop)
5. My spring hole and "bump" for keeping the springs lined up are loose (the "bump" is smaller than the hole). This allows the springs to "wander" (read: not line up perfectly) a little bit, even though everything else is tight. Is this normal? Fixes?
Thank you,
David
Any experience would be appreciated!
Re: '72 Citabria 7GCBC
1. Are the "New" leaf springs available? I believe mine are 1 and 3/4 inches wide.
2. and if so.. how are they working out for everyone. Did the redesign help?
3. Is rebending the springs (without heating) just as good as the new ones?
4. Does anyone have a pattern to follow to bend the springs (for the spring shop)
5. My spring hole and "bump" for keeping the springs lined up are loose (the "bump" is smaller than the hole). This allows the springs to "wander" (read: not line up perfectly) a little bit, even though everything else is tight. Is this normal? Fixes?
Thank you,
David