REFLECTOR: Nosewheel shimmy_3

Scott Derrick scott at tnstaafl.net
Thu Jun 29 09:35:56 CDT 2006


Sid

how far can you rotate the nose wheel each way in degrees with a 172 
shimmer damper attached? 

Cessna's have a mechanism to stop rotation so you aren't using the 
shimmer damper as a stop which I presume can cause premature failure of 
the damper.  Must think about that.

Scott



Sid Knox wrote:
> 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.
>
> ************ note **********
> This is the fourth attempt to post this _3 message. The first had too 
> many
> photos so I tried again with two photos (one 109 KB the other 113 KB) and
> still reject. I tried a third time with the same two and another reject
> (why would 222 KB be rejected??) so here it is again with just one photo.
> I'll follow-up with a separate message with another photo and yet another
> message with another photo, etc., until I get them all sent. And this is
> susposed to conserve bandwidth??
>
> Sid Knox
> Oklahoma
> Velocity N199RS
> W7QJQ
>
>
>
>
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