Scott 3200 high speed shimmy

[8451]

If rearward is proper do you think trying out the double bend spring is worth a shot despite the serial number mismatch?

I would suggest against putting double bend springs on an early serial number frame, that was an issue when I got my plane. Poor previous maintenance. I dropped the tail a full 3" by installing the proper single bend spring. I couldn't make a decent 3-point landing either (well, still can't but my odds have improved immensely).

Other than tail height, what issues did you experience with that geometry? Any shimmy issues that were remedied (or introduced)?

[6597]

Other than tail height, what issues did you experience with that geometry? Any shimmy issues that were remedied (or introduced)?

I do not remember any shimmy issues but that was 25yrs ago. I do remember always hitting the tailwheel on landing 3 points, just couldn't get the stall right. Although my 3 points are better I do miss the better 'over the cowl' view while taxiing from the higher spring set. I am more at peace having the correct springs and tension springs now.

I do have an old Scott document that says having the kingpin perpendicular to the ground is the most correct. It is too big to post here. Will try to find the link.

[V529]

It seems like having the tail wheel canted forward just a little bit seems to work best. (referencing the king pin angle)

Like Dave I tried a set of double bend rear springs and it made the tail wheel sit up and lean backwards, I took the spring off and went back to the single bend.............it worked much better.

[6643]

It seems like having the tail wheel canted forward just a little bit seems to work best. (referencing the king pin angle)

I think a lot of the confusion comes from ambiguous terminology. What does "canted forward" mean?

When the king pin axis is canted forward at the bottom the tail wheel has to lift the plane slightly to deviate from straight ahead. This gives it a centering tendency that tends to keep the wheel aligned with the direction of travel.

If you tip it forward at the top then the weight of the plane tends to force the tail wheel to move off center. (Or. steering off center lowers the tail and returning to center requires lifting the tail.) The steering springs then try to pull it back and you get the oscillation that causes the shimmy.

We've all seen that shopping cart with the bent front wheel that shimmies. Think about it; the wheel gets bent back from crashing into things, tilting the kingpin axis aft at the bottom.

Keep in mind, if you drop the tail on the ground the tail spring flexes in such a way as to tilt the kingpin axis back at the bottom and this will induce the shimmy. The cure is to get some of the weight off the tail wheel with the elevator.

Photo #2 above shows the (IMNSHO) ideal configuration: nearly plumb but tilted ever so slightly aft at the bottom.

[a64pilot]

Tail wheel shimmy is of course similar to flutter in that it’s a self exciting condition, think of the galloping Getie bridge as an example.
Anyway I believe the Scott or now I think AK Bushwheel 3200 has a shimmy dampener. Now I’m more familiar with the Maule tail wheel and Maule’s are infamous for shimmy, but on the Maule when the dampener gets loose or something grease on it, it goes to town.
Maybe the dampener is loose?

But as a side note, why do you guys want such a big and heavy tailwheel on such a small airplane?

[6643]

I believe the Scott or now I think AK Bushwheel 3200 has a shimmy dampener.

Yes, there are two plates pressed together by small coil springs. The tension is adjusted by tightening or loosening the king pin nut.

But as a side note, why do you guys want such a big and heavy tailwheel on such a small airplane?

Comfort!

[8451]

I didn't see it mentioned in Neal's article on the springs; was there a design change of either the tailspring bracket or the rear empennage at some point during production that made the geometry of the double bend spring "more correct" than the straight or single bend springs that shipped with the earlier serial numbers?

[6597]

I didn't see it mentioned in Neal's article on the springs; was there a design change of either the tailspring bracket or the rear empennage at some point during production that made the geometry of the double bend spring "more correct" than the straight or single bend springs that shipped with the earlier serial numbers?

That's a million dollar question, reviewing the various gear leg configurations, tailwheel springs, etc. there does not seem to be any direct correlations regarding serial numbers. I can find nothing in the parts book that makes any sense.

[phil123]

But as a side note, why do you guys want such a big and heavy tailwheel on such a small airplane?

Natural progression from a stick.

apes to man.jpg (12.12 KiB) Viewed 3765 times

[8451]

Neal's article mentions that sometimes you need to use what works, even if by serial number it may not be correct.

Here's a picture of my tailwheel geometry with a double bend spring. It has a good caster angle and I am not getting any more shimmy!

20201023_123041.jpg (378.84 KiB) Viewed 3618 times