REFLECTOR: wheel shimmy

[Brian Michalk]

David,

I agree with what you said, however I have another related question;
For us fixed gear guys, if we can make the wheel pant stiff enough, 
would a counterweight at the front of the fairing to get the CG in front 
of the rotation point fix the shimmy problem?

Alternately, it seems like a special damper that puts a special grease 
in shear would also work well.  There are some greases that when placed 
under shear resist shear in a linear manner.

David Ullman wrote:
> 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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