The Murphy Builders Site

Here are two more articles on tail wheel caster angle. If you can decipher
these you are doing pretty good.



Caster angle definition - http://vaa15.org/tips/2004-7.html

Tire wear and shimmy - http://vaa15.org/tips/2004-10.html





The tail wheel manufactures are not much help either. Matco did respond and
they say it should be set up so the pivot plane is horizontal when loaded.



The reason the concern on the pivot angle started with the design of the new
aftermarket stingers Dale is working on (since MAM can't seem to make it
happen). If I understand Dale's new forward bracket correctly it is not
adjustable up and down so there is now way to change the angle of the
stinger so he wants to get it right from the get go. If the stinger has the
9.5 bend at the tail wheel attach end like the only prototype stinger that
is on Ted's you will be starting with a pivot angle already fairly negative
(as defined in this diagram
http://merlin.alleg.edu/group/eaa1194/graphics/tws.jpg ) which is not ideal
(from most peoples perspective, but not all).



I see some guys prefer a slightly negative angle but I suspect it is just a
degree or so and not a noticeable visual angle like on Ted's current set up
http://www.i1ci.com/Flying/Moose/Tail/I ... pTailW.jpg



So far no one that I have personally talked to (all guys with thousands of
TW time) likes to set it up with the negative angle and I couldn't find any
at the airport set obviously negative but a lot obviously positive. A
search of the Super Cub forum found most (but not all) agreeing that when
the tail leaf spring gets old or flattened out and the tail wheel goes
negative shimmy gets worse.



So what is the ideal pivot angle? - I guess what ever works.



FWIW.



Scott

Moose 174











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Hi Scott

So far I haven't seen an analysis that convinces me. The most extreme
negative angle that I've seen was on a Scott equipped Rebel with that
stiff aluminum spring and he said it never shimmied but was somewhat
difficult to turn. No surprise there. The pivot angle may be the easiest
thing to change but I don't believe it is the most important factor.

Did you notice in the two pics that you referenced how much more aft the
tailwheel axle is on the Scott? A yoke that keeps the wheel forward like
on the Moose pic is not likely to be very tolerant of positive castor
IMO. (ie. with a vertical pivot axis, the extended vertical pivot axis
passes much closer to the axle on the Moose) For a Scott or similar I
suspect Matco's advice to target zero is as good as you are likely to
get and some positive is probably also fine. However personally FWIW I'd
favour vertical or probably a bit of negative with that Moose tailwheel,
especially if it worked well for Ted.

Lots of ideas out there but I don't see much effort to quantify the
effect and there is lots of mud in the water. A nugget in the article
may be the reference to tailspring torsion. Something like a thin single
leaf steel spring that twists easily has got to be far more prone to
shimmy. When I measured them, even the old fibreglass Rebel spring was
stiffer in torsion than a one leaf steel spring. A multi leaf was
better. A thick aluminum spring should also be better than a single
thinner steel one. A round cross section stinger would probably be best
but of course there are other considerations.

Ken

Scott Aldrich wrote:

Here are two more articles on tail wheel caster angle. If you can decipher
these you are doing pretty good.



Caster angle definition - http://vaa15.org/tips/2004-7.html

Tire wear and shimmy - http://vaa15.org/tips/2004-10.html



The tail wheel manufactures are not much help either. Matco did respond and
they say it should be set up so the pivot plane is horizontal when loaded.

snip




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Thanks Ken,

Scott
-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Ken
Sent: Thursday, December 08, 2005 10:57 AM
To: rebel-builders@dcsol.com
Subject: Re: Moose tail wheel pivot angle

Hi Scott

So far I haven't seen an analysis that convinces me. The most extreme
negative angle that I've seen was on a Scott equipped Rebel with that
stiff aluminum spring and he said it never shimmied but was somewhat
difficult to turn. No surprise there. The pivot angle may be the easiest
thing to change but I don't believe it is the most important factor.

Did you notice in the two pics that you referenced how much more aft the
tailwheel axle is on the Scott? A yoke that keeps the wheel forward like
on the Moose pic is not likely to be very tolerant of positive castor
IMO. (ie. with a vertical pivot axis, the extended vertical pivot axis
passes much closer to the axle on the Moose) For a Scott or similar I
suspect Matco's advice to target zero is as good as you are likely to
get and some positive is probably also fine. However personally FWIW I'd
favour vertical or probably a bit of negative with that Moose tailwheel,
especially if it worked well for Ted.

Lots of ideas out there but I don't see much effort to quantify the
effect and there is lots of mud in the water. A nugget in the article
may be the reference to tailspring torsion. Something like a thin single
leaf steel spring that twists easily has got to be far more prone to
shimmy. When I measured them, even the old fibreglass Rebel spring was
stiffer in torsion than a one leaf steel spring. A multi leaf was
better. A thick aluminum spring should also be better than a single
thinner steel one. A round cross section stinger would probably be best
but of course there are other considerations.

Ken

Scott Aldrich wrote:

Here are two more articles on tail wheel caster angle. If you can decipher
these you are doing pretty good.



Caster angle definition - http://vaa15.org/tips/2004-7.html

Tire wear and shimmy - http://vaa15.org/tips/2004-10.html



The tail wheel manufactures are not much help either. Matco did respond

and

they say it should be set up so the pivot plane is horizontal when loaded.

snip




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