tail wheel pivot angle

[Ken]

I also find the physics of the referenced article lacking. Unless the
pivot is vertical, the rear fuselage (or spring) moves up and down
anytime the wheel turns side to side. It is impossible to always keep
the pivot vertical for most installations and that wouldn't guarantee a
solution anyway. You have a lightly damped spring system that will
always have some liklihood of shimmying and that will be sensitive to
any minor change. The author identifies lots of ways of affecting the
tune of the system and has a solution that works for him but he has gone
way too far with his conclusion. I've put my bets and theories with
Wayne's position. I'd also add that multi leaf springs usually have more
damping from the friction as the leaves slide relative to one another
and that's probably a good thing for us. That was one of many reasons
that I opted for a two leaf spring - hope it works...
Ken

Wayne G. O'Shea wrote:

I've seen this tailwheel (positive camber) argument before Ted from a couple
sources....and unfortunately disagree with it totally ! Running the
tailwheel positive camber makes for one twitchy ride, as as soon as you turn
that tailwheel slightly it will try to pass itself and turn around. Just try
hand pushing (forward) an airplane with a positively cambered tailwheel
using a tail wheel pull bar...turn the wheel about 45 degrees and see how
quickly it tries to ripe the pull bar out of your hands.

I shoot for a vertical pivot shaft/level pivot plate when lightly weighted
and of course this becomes a trailing (negative camber by attached link
drawing) tailwheel when loaded up. I still have the original tire on my
tailwheel going into 11 years now with virtually all it's tread....so there
is definitely no shimmy'ng going on back there.

Same deal with the MAM 1800 amphib nose wheels. Some owners have had shimmy
problems with them......pivot being dead nuts level.... and no way they will
trail on their own. Milling off a slight angle on the attach block so they
bolt on angled back with a "negative" caster stops all shimmy. If you went
positive camber like they are saying you would be worse off than a shopping
carts front wheels !

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[Bob Patterson]

I'll throw my vote with Wayne too ! :-)

Last year, I had a shimmy problem with the Scott 3200, for the
first time ever, on any airplane. I discovered that there is an
internal friction shimmy damper - just tighten the nut on the
bottom by 2 or 3 flats, and the problem is gone forever !! ;-)

......bobp

-------------------------------orig.-------------------------
On Saturday 03 December 2005 10:28 am, Ken wrote:

I also find the physics of the referenced article lacking. Unless the
pivot is vertical, the rear fuselage (or spring) moves up and down
anytime the wheel turns side to side. It is impossible to always keep
the pivot vertical for most installations and that wouldn't guarantee a
solution anyway. You have a lightly damped spring system that will
always have some liklihood of shimmying and that will be sensitive to
any minor change. The author identifies lots of ways of affecting the
tune of the system and has a solution that works for him but he has gone
way too far with his conclusion. I've put my bets and theories with
Wayne's position. I'd also add that multi leaf springs usually have more
damping from the friction as the leaves slide relative to one another
and that's probably a good thing for us. That was one of many reasons
that I opted for a two leaf spring - hope it works...
Ken

Wayne G. O'Shea wrote:

I've seen this tailwheel (positive camber) argument before Ted from a

couple

sources....and unfortunately disagree with it totally ! Running the
tailwheel positive camber makes for one twitchy ride, as as soon as you

turn

that tailwheel slightly it will try to pass itself and turn around. Just

try

hand pushing (forward) an airplane with a positively cambered tailwheel
using a tail wheel pull bar...turn the wheel about 45 degrees and see how
quickly it tries to ripe the pull bar out of your hands.

I shoot for a vertical pivot shaft/level pivot plate when lightly

weighted

and of course this becomes a trailing (negative camber by attached link
drawing) tailwheel when loaded up. I still have the original tire on my
tailwheel going into 11 years now with virtually all it's tread....so

there

is definitely no shimmy'ng going on back there.

Same deal with the MAM 1800 amphib nose wheels. Some owners have had

shimmy

problems with them......pivot being dead nuts level.... and no way they

will

trail on their own. Milling off a slight angle on the attach block so

they

bolt on angled back with a "negative" caster stops all shimmy. If you

went

positive camber like they are saying you would be worse off than a

shopping

carts front wheels !

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