REFLECTOR: wheel shimmy

Stockman, Bill bill.stockman at daytonaero.com
Mon Feb 16 10:10:28 CST 2009


Brian

I have had my fair share of shimmy problems and have spent a bit of time
looking at Tigers, cirrus, columbia's etc to see what works and what
doesn't
--The spam cans and composite spam cans have few shimmy problems but all
have one big thing in common--a 300-500 lb engine and accessories
sitting on the wheel.    This tends to dampen out or avoid much of their
shimmy 
--I must say that the new rubber shock mount for the gear on the front
bulkhead has made a huge difference.    Since my last shimmy disaster
(which destroyed the OEM rubber bumper and tore up my wheel pant along
with the nose gear lock mechanism), I installed the new rubber mount and
have had no problems what so ever.   Even better, it has reduced the
horrible ride during taxi on rough taxiways.   

MTCW after 600+ hours on my 173FGE with the oversize tires---install the
new rubber shock mount before trying all these other heroic solutions.  

Bill Stockman, Senior Associate
Dayton Aerospace
 
(937) 369-4799 cell
(937) 426-4300 work
bill.stockman at daytonaero.com
-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Brian Michalk
Sent: Monday, February 16, 2009 10:04 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: wheel shimmy

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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