Nose Wheel Shimmy Dampener
Here's the shimmy dampener system that Sid Knox devised.
"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 Belville 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)."
After two years, no problems... the only thing I would do different next time would be to make it look a little better.
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.
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).
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....
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 (rotary) 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 is 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 one. It has since arrived but I am still experimenting with the original as
it works ok... 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, made both sharp turns into the hanger
easily. This may be a keeper.
Q: Whats the turning radius with the damper connected?
A: Don't know the aircraft turning radius but the measured swing of the wheel
is about 45 deg left, 30 deg right. Obviously, I don't have the geometry
correct but it does what I need. Between my hanger and the taxiway is a 90
deg turn followed immediately by a 120 deg turn in the opposite direction.
Before (using the Belview washers), it was a major pain-in-the-butt
negoiating these turns... often going off into the grass. Now, with the
free-turning nosewheel, it is simple. And, no shimmy, no worrying about
adjustments.
Q: When turning in tight quarters do you bottom out or top out on the damper often? No worries the damper would break from over extending?
A: For the above mentioned turns, I feel no indication that I have hit the
ends.
Q: How big is that damper? It looks big in the picture, got a part number?
A: It is a damper off of some older model Cessna that a friend had in the scrap
pile because someone had "ruined" it (by certified standards) trying to get
it apart and damaged the piston and bore. I cleaned it up, got some o-rings
from AutoZone and it feels good as new. I am using Shell 100 for a working
fluid. I tried hudraulic oil (John Deere) but because of the scoring in the
bore, it did not damp as much as I wanted...it had essentially two
orfices... the proper one and the score. So, the heavier oil compensates
for the higher internal bypass. There is *no* leakage to the outside.
This is probably not a good mod if one must buy a good damper as they are
- very* expensive! (typical Certified B.S. pricing).
Sid Knox
Oklahoma
Velocity 173 RG N199RS
Starduster N666SK
KR2 N24TC
W7QJQ
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