Scott 3200 Tailwheel Shimmy Adjustment

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Re: Scott 3200 Tailwheel Shimmy Adjustment

Postby DaveF » Tue Dec 17, 2019 9:40 pm

Thanks, Gene. I've never had a shimmy problem. Maybe I should loosen up the king pin nut and see if I can create shimmy, then just tighten a bit. I'll try a different grease, too.

Frank, I'm not saying the tailwheel should be taken apart. I'm a habitual disassembler and always want to know how things work. When I first bought my airplane the pivot lock didn't work, so I had to open it up, and found one of the "wing" locking springs broken. Since then I've kept at it and always seem to find some part that needs replacement. Unfortunately, it's usually those pins. But, to your point about long times between overhaul, I don't notice any difference in performance whether the pins are there or not. The thing is very tolerant of dirt and neglect.

When I put it back together after annual, I'm going to see if, with weight on the tailwheel, the three coil springs fully compress so the bronze washers take load, or whether all the weight carried on the friction plate. I suspect the latter, except for transient high loads (like my landings).
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Re: Scott 3200 Tailwheel Shimmy Adjustment

Postby DaveF » Tue Dec 24, 2019 6:20 am

I measured the friction plate coil springs: 240 lb per inch. So three springs have a rate of 720 lb per inch. With the king pin nut loose, there's about 1/4 - 3/8" gap in the assembly, which would take 200 - 270 lb to compress completely. That's approximately the weight on the tailwheel with the airplane empty -- my bathroom scale said 250lb. So the tailwheel assembly is just about fully compressed when just sitting there empty. Any more weight or dynamic load puts the weight on the two circular bronze thrust washers, and the king pin nut is then unloaded. Its function is to pull the halves of the assembly together and set the minimum pressure on the friction plate. The looser the nut, the more the friction varies with load, and the more likely would be shimmy with a lightly-loaded tailwheel during takeoff or landing.

With the airplane empty weight on the tailwheel, it takes about two flats of the king pin nut to snug the assembly together. That means the Scott procedure of "tighten to binding, then back off a flat or two" puts it in the nominal unloaded state. But it also means that tailwheel steering has to overcome the friction plate friction and the thrust washer friction.

Did Scott intend for it to ride on the thrust washers, or on the friction plate, with the thrust washers as backup only? That's what would happen with five springs on the plate or an airplane with a lightly-loaded tail and three springs, like a 140 or Cub.

With three springs on the plate, loosening the king pin nut won't improve ground steering because the springs are compressed by the airplane weight. Loosening the nut just makes it loose but doesn't reduce taxiing steering friction.

Sorry for not having pictures to illustrate what I'm trying to describe. If you read all of this, I thank and pity you. :)
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Re: Scott 3200 Tailwheel Shimmy Adjustment

Postby GAHorn » Tue Dec 24, 2019 12:13 pm

DaveF wrote:I measured the friction plate coil springs: 240 lb per inch. So three springs have a rate of 720 lb per inch. With the king pin nut loose, there's about 1/4 - 3/8" gap in the assembly, which would take 200 - 270 lb to compress completely. That's approximately the weight on the tailwheel with the airplane empty -- my bathroom scale said 250lb. So the tailwheel assembly is just about fully compressed when just sitting there empty. Any more weight or dynamic load puts the weight on the two circular bronze thrust washers, and the king pin nut is then unloaded. Its function is to pull the halves of the assembly together and set the minimum pressure on the friction plate. The looser the nut, the more the friction varies with load, and the more likely would be shimmy with a lightly-loaded tailwheel during takeoff or landing.

With the airplane empty weight on the tailwheel, it takes about two flats of the king pin nut to snug the assembly together. That means the Scott procedure of "tighten to binding, then back off a flat or two" puts it in the nominal unloaded state. But it also means that tailwheel steering has to overcome the friction plate friction and the thrust washer friction.

Did Scott intend for it to ride on the thrust washers, or on the friction plate, with the thrust washers as backup only? That's what would happen with five springs on the plate or an airplane with a lightly-loaded tail and three springs, like a 140 or Cub.

With three springs on the plate, loosening the king pin nut won't improve ground steering because the springs are compressed by the airplane weight. Loosening the nut just makes it loose but doesn't reduce taxiing steering friction.

Sorry for not having pictures to illustrate what I'm trying to describe. If you read all of this, I thank and pity you. :)


I admire your logic Dave, but I believe the procedure for tailwheel assembly/adjustment and use of the compression springs was arrived at by trail-and-error when it was adapted to aircraft.

I believe you’re overthinking this. Firstly, the weight on the tail of an empty 170 is more nearly 125 (plus or minus) certainly less than 150 lbs.
Secondly, this wheel was not designed for aircraft. It was designeed for warehouse trolleys and adapted to aircraft. (See the air pressure chart provided by Scott to observe the weight-range for this wheel.)

Also, it should be obvious that this is not a truly steerable tailwheel. If that had been the intention then a solid connection from rudder-input-cables to the wheel-yoke would have been a solid connection instead of clutch-plates, springs, and locking/un-locking pawls and detents. An aircraft needs to be able to fully swivel as well as accept sudden jolts etc. from unimproved runways. Everyone who flies this airplane (or any similar one) quickly discovers it’s only “encouraged” to steer and that sharp steering commands require judicious use of brakes. (Try taxying in a cross-wind greater than 20 knots and one will quickly learn to use heavy-braking and opposite
270-degree turns rather than 90-degrees when turning downwind.) :lol: BTDT :lol:
Thirdly, it would seem to me that adding weight to the aircraft would increase the friction of the clutch plates and improve steering. But that doesn’t occur due to the simultaneous increase in momentum due to mass increase which actually increases the workload of the assembly. Pilot technique remains similar regardless of increased weight, and any shimmy also remains or worsens. (The primary reason for shimmy is improper caster of the tailwheel. Increased weight negatively affects that caster. Perhaps that increased friction helps the clutch fight that tho’.) A positive angle of caster will diminish shimmy.
Also the steering chains should be taut...not loose. The tension-springs in the steering chains are for the purpose of preventing damage to the airplane rudder-cable circuit and they cannot transmit steering OR dampening efforts to the tailwheel unless they are strong (use heavy-duty versions and never the Maule compression-types) and taut instead of dangling.
Anyway, the angle of the leaf springs should be examined if shimmy is persistent.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight.
An originality nut (mostly) for the right reasons. ;)
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Re: Scott 3200 Tailwheel Shimmy Adjustment

Postby gfeher » Tue Dec 24, 2019 2:47 pm

Dave, the only thing I can add to what I've said is that I tighten the kingpin nut per the Cessna SNL with the tailwheel removed (e.g. after re-greasing it), rather than as installed with weight on it as you describe. Maybe try tightening it per the SNL without weight on it.
Gene Feher
Argyle (1C3), NY
'52 170B N2315D s/n 20467 C-145-2
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Re: Scott 3200 Tailwheel Shimmy Adjustment

Postby DaveF » Tue Dec 24, 2019 9:59 pm

What? Me? Overthink? No WAY. :D

I set the tension on the bench, too. There’s no reason to do it on the airplane under load. I did it just to see how the adjusted position compares with the nominal loaded position. Surprisingly close, as it turns out.

I was hoping to see a way of making the steering more effective, either by changing the nut adjustment or number of springs, but no. For what it’s worth, I did learn that the thrust washers carry a lot of the load.

I’ve never had shimmy. I wish I could reduce the friction enough to cause it.
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Re: Scott 3200 Tailwheel Shimmy Adjustment

Postby canav8 » Wed Dec 25, 2019 8:33 pm

All tailwheel adjustments are correctly adjusted when the tailwheel is in an unloaded state. The 3200 is a universal tailwheel to fit many different aircraft. The reason for the springs is to correctly handle the load for break out. The 3 of the springs is all that is necessary for the 170 configuration. Shimmy is usually associated with king pin orientation relative to the ground and not the tailwheel assembly.
52' C-170B N2713D Ser #25255
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