REFLECTOR: wheel shimmy

Tue Feb 17 12:01:13 CST 2009

David;

A few years ago I also took a close look at the shimmy issue; although
perhaps with less expertise than you.  One of my first conclusions was that
the flat profile tire was indeed a worst case.  Shortly thereafter I spoke
with another Velo RG driver who used a round profile nose tire who found
that he still had to maintain pivot drag, although a bit less, in order to
avoid shimmy.  But I'm guessing it is a step in the right direction.

My further analysis later led me to believe that the issue is quite closely
related to flutter of an airfoil, and that the key would be to 'balance' the
nose wheel assembly around its pivot axis.  I even went so far as to
determine the required mass and the distance from the pivot to the 'center
of mass' required.  For my plane (RG, so no wheel pant) it would be 3.7# at
3 1/8".  I proceeded to determine the dimension and support for a lead
weight to do the job.  It's a relative small piece of lead.

That's as far as it went. I haven't taken the time to pursue it - one reason
being I'm hesitant to take the risk of doing it and testing it.  I recall
another builder had tried the concept with bad results; but I'm not sure he
chose the weight for proper 'balance'; without which it could exacerbate the
issue.

Perhaps you (or anyone) could lend your knowledge of dynamics to support or
debunk this idea.  Perhaps a combination of this and a round profile tire
could allow minimal pivot drag, no required pilot operation, and full
articulation of the wheel.

I learned (inadvertently) that when the pivot drag on my plane got less than
about 10# I got shimmy that did not cause damage - so I keep it at about
12-14#. Definitely a nuisance for low speed taxi.

Al

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of David Ullman
Sent: Monday, February 16, 2009 6:36 AM
To: reflector at tvbf.org
Subject: Re: REFLECTOR: wheel shimmy

I know the wheel shimmy topic comes up often.  I am now making the front

wheel pant for my SEFG and began to think about it (my velocity is still 2

years from flying).  In gradual school I took lots of dynamics courses and

looking closely at the tire/strut system on my plane, I appear to be

building a shimmy factory.  With the tire that came on the plane

(3.40/3.00-5 slick with a flat profile 3" wide), it cant help but shimmy. 

To have shimmy you must excite it with an off center contact point between

the tire and the surface.  Two ways to get this is to have the strut

misaligned side-to-side (I carefully aligned mine, but the first hard

landing will take care of that) or have a tire that can put the contact

point off to the side - like my tire which can have contact 1.5" off to

either side. Exasperating shimmy is the flexibility of the front strut and

the tire (rated at 50psi, but rim says only 30psi).  Like I said a shimmy

factory. 

I looked on the web and there are lots of different 3.40/3.00-5 tire

profiles.  So why did my plane come with a 3" wide foot print. I could see

this if I was going to do soft field landings and had a heavy nose load, but

I don't have either.  Seems that I am going to find a tire with contact in

the center, as peaked as possible, before I finish making my wheel pants. 

Any thoughts on front tire profiles?

David G. Ullman PhD ME

Robust Decisions

ullman at robustdecisions.com

541-754-3609

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