REFLECTOR: wheel shimmy

[cgputney at peoplepc.com]

Dave, my dynamics book addresses shimmy and touts a solution using TWO nose 
tires locked together.   The differential scrubing damps the oscillation. 
I modeled our nose gear in a rigid body kinematic program at work using a 
single point center contact and it shimmied.   Applying a two pound, tuned, 
dynamic absorber eliminated the shimmy, but only if I ignored the strut 
flexibility.   I'm back to locking the nose gear!
Gordon Putney

 ----- Original Message ----- 
From: "David Ullman" <ullman at robustdecisions.com>
To: <reflector at tvbf.org>
Sent: Monday, February 16, 2009 8:35 AM
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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