REFLECTOR: Shimmy Damper

[Chuck Harbert]

Sid, nice job. I think you've solved the shimmy problem with the least 
expensive alternative using a proven a/c part. Thanks.

Chuck H

>
> Shimmy damper_3
>
> One of the local knowledgeable mechanics reminded me that Cessna for years
> had a hydraulic shimmy damper on their nosegear.  I examined and took 
> photos
> of an older 150 and 172 to see how they did it.  The same guy gave me a
> discarded damper that someone had tried to get apart and essentially 
> ruined
> (by FAA standards).  I took it apart, honed the bore, filed off the rough
> edges on the piston, put in new o-rings and snap rings, and filled it with
> automatic transmission fluid (ATF).  Total cost less than $2.
>
> I made up the mounting hardware initially from cereal box cardboard to get 
> a
> general idea of fit and then from thin Al (quick and easy to modify) only
> strong enough so that I could check geometry.  Finally, I made up what you
> see in the photos from 1/8 inch mild steel ( I do not have any chrome-moly
> plate).
>
> I took (angular) drag data for both the original Belview setup and for the
> Cessna damper.  It became immediately apparent that the ATF was too thin
> so I replaced it with Shell 100 (50 wt) aircraft oil.  This stiffened it 
> up
> considerably and that what I am currently using.
>
> It was during the initial fit-up process that I discovered that the fork 
> had
> been bent bad.  I noticed it when I was pulling (by hand (dump-valve 
> open))
> the whole gear up into the nose of the plane to check for fit.  One side 
> of
> the wheel lightly dragged on one of the two alignment "posts" (I don't 
> know
> the official name for these).  I considered trying to straighten it but I
> don't have a large enough press to do that sort of work so I ordered a new
> fork assy.  It has since arrived but I am still experimenting with the
> original as it works. just looks a little strange.  The new fork is
> considerably stronger than my old original.
>
> Ok, time for taxi testing.  Big nut backed-off as before.  Two high-speed
> runs (fast enough to fly the canard and let it plop) and no shimmy.  Time 
> to
> go around the pattern.  Take-off, no shimmy.  Retracted the gear. lights
> out.  Extend the gear. both lights ok.  Landed and let it roll for awhile
> and still no shimmy.  Taxied back, easily made both sharp turns into the
> hanger.
> This may be a keeper.
>
> More photos to follow... I am pushing the bandwidth limit now.
>
> Sid Knox
> Oklahoma
> Velocity    N199RS
> W7QJQ
>