REFLECTOR: wheel shimmy

Tue Feb 17 14:30:11 CST 2009

David, I virtually prototyped a counterweight ahead of the fork and still got shimmy, except at a lower frequency.   It was then that I virtually prototyped the dynamic absorber I mentioned.
Gordon
  ----- Original Message ----- 
  From: Al Gietzen 
  To: 'Velocity Aircraft Owners and Builders list' 
  Sent: Tuesday, February 17, 2009 12:01 PM
  Subject: Re: REFLECTOR: wheel shimmy

  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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