REFLECTOR: wheel shimmy

[Sid Knox]

> Sid, you still here?

> Scott

Yes, I still lurk in the shadows...

The link Brett Ferrell ("Master of Archives and
Texts") provided shows much of it but some was edited
probably for space considerations.

http://wiki.velocityxl.com/index.php?title=Nose_Wheel_Shimmy_Dampener

Follows is the whole story along with a few more photos.
The mod is completely successful for me.  I have not done anything except
fly with it.  Even the ChiCom hardware store bolts are still there.
The only change is that I installed a new fork to replace the original
that was tweaked when the first experimant (lead weight) failed.


****************** from June 2006 archives  *************************
Nose wheel shimmy.
One of the recurring irritations of an otherwise great airplane is the
nosewheel anti-shimmy scheme. Velocity forum readers may recall this subject
comes up periodically, stirs up a few people, and then dies with the
generally lame advise to keep the Belview washers clean and tightened to
some seemingly arbitrary value and stop whining.

To get into my hanger I must make a sharp 120 degree right followed
immediately by a left 90. Sometimes I make it and sometimes I end up out in
the weeds.

After the last time, I finally decided I was pissed enough to try and do
something about it (no, not get another hanger).

I recalled in a previous go-around of this subject, someone had posted a
link to an Air Force site (?) telling how the problem was solved on the
F-15. Unfortunately, the article was 100% PR on what a wonderful group
Managemant
put together and 0 % on how they did it. But there was a photo that showed
something hung on the nosewheel fork. I discussed the Velocity problem and
the F-15 problem fix with local owner/operators and it was generally agreed
that probably there was a mass, either solid or liquid (mercury?) inside.
Well, the mass idea would be easy to try so I melted some lead and cast a
rectangular weight. It ended up weighing about 25 oz. No "design" involved,
just a "nice" size. I mounted it pointing forward as seen in the photo.
Mounting was crude as this was to be a feasibility experiment. if it worked,
I could build it better. If it didn't, then no big time lost.

With the big nut backed-off to near zero drag, I made two high-speed taxi
tests. Two was all I could do together because of danger of overheating
brakes. (Figure it out. well over 100 KW for several seconds time dumped
into the tiny brakes. we're talking some serious heat here!).

No shimmy observed. I was feeling pretty smart until my teenage son
pointed-out one potentially serious flaw in the scheme. As I was digesting
that one, I thought of another probably catastrophic flaw.

This message is getting rather long so I will continue the tale in a
follow-up. Anyone want to guess what are the two flaws? (clearance up in the
nose when retracted was not one of them... there is a bunch of space up
there).

*****************

Shimmy damper_2

Various persons thought of the two flaws I had in mind. 1) gravity rotating
the wheel while up inside the nose, and 2) slipstream rotating the wheel
causing jam-up during retraction or worse, touching down ("plopping down"?)
with the wheel pointing aft!!  This one would probably rip the gear leg
loose!

So, back to the experiment. I simply turned the assembly around so that the
weight was pointing aft.

High-speed taxi test.  Got brief burst of severe shimmy.  Ok, I need more
weight (?) so I cast another similar block and mounted it alongside the
first.

High-speed taxi test.  SEVERE shimmy.  The assembly departed the airplane
and left me with no friction, no damping.  Yanking the stick back to keep
the nose wheel off was of course short-lived as the canard soon lost lift.

Found most of the pieces (see photo).  Back in the hanger, a quick look
showed no obvious damage.  This assessment was to change later. I'll get to
it in another post.

Meanwhile, I was working on another idea....

************************

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.

*********************  end of archive  (from June 2006    ******


I can send only one photo now as I am pushing the bandwidth limit.
I'll post a second message in a few minutes with more photos

Sid Knox
Oklahoma

Velocity  173 RG  N199RS
Starduster  N666SK
KR2         N24TC
W7QJQ



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