Or you walk into the hanger to the sight of a pool of teltale red fluid spreading from your nosewheel.
Either way, it's time to rebuild that shimmy dampener. Here are some pictures and information to help make that job easier.
I'm going to assume that removing the shimmy dampener from the airplane
does not pose a significant challenge. In any case you'll be doing this
under the watchful eye of your A&P, so any little details there can
be easily resolved. The tricky parts start right after the dampener is removed from the airplane. Many people who try to dissassemble this device, even certified mechanics, get a little lost right after removing the C clip from the end of the dampener.
The trick is to use that port that shows in the first picture to blow air into the body of the dampener. The pressure will help separate the parts.
The process looks like this. Just an air hose with a non-vented nozzle
that fits into the little fill hole we noticed in the top picture.But there is a trick. Most people will start by removing the end of the chamber, starting with the C clip. But a person who has done this before will start with the internal pressure compensation piston.
To do that remove the plug from the end of the rod that passes through the dampener. A spring will pop out. Hold the end of the shaft over a bucket and apply air pressure to the fill hole. A cute little piston with an O ring will pop out of the inside of the shaft, or at least it should. This piston is shown in the next picture next to the plug that holds the spring in.
If your dampener was leaking from the end of the shaft, this is the O-ring
you care most about. It can be seen in the picture to the right. In this
picture the spring is inserted into the shaft only to show that it's
hollow.. the piston goes in before the spring on reassembly.Once the inner piston is out (along with a lot of fluid) remove the C ring from inside the end of the dampener body and apply air pressure again, holding your hand over the end to catch the parts.
It is likely that you will not be able to get enough pressure in the
body to remove the end plate unless you block the now-hollow shaft. You
might be tempted to use the plug to do that, but if you note in the
picture to the right, that plug has a vent hole in it. Use a suitable bolt, or insert the piston and put the plug in to hold it. You should be able to coax it out with a longnose pliers after this is over.
With the hollow rod plugged, make sure the C clip is removed, pressurize the housing again and catch the end plate as it pop off.
Once the end plate is out, pull out the inner C clip and slide the
rod assembly out. Take careful note of where the parts all line up
as you do so. Just about now your bench should have all these parts on it. If you have planned ahead it's a good time to verify that you have O-rings to replace all the ones that you will be taking out.
Note that one of the parts is a little like an O ring, but square and
hard, with a break in it. Most of us have seen O rings turn into something
that looks a lot like this, but this is not a dead O ring, it's a packing.
It's suppose to be hard, and needs that split to be lifted over the lip
to fit into the slot. The rest of the parts are O rings, pistons and snapring critters, nothing too surprising here.
Things get a little more interesting when you look at the piston and start to figure out how it works.
Of course the end plate and it's O rings are there to keep the fluid in.
And the O ring on the piston seals it as it moves back and forth through
the housing. But what are these little orfices we see, as pointed to by
the arrow? Those are carefully metered ports that let just the right amount of fluid through, setting the proper dampening rate.
It is these ports that some of the FG folks have looped a piece of safety wire through to increase the dampening of their dampeners. As you can see, the RG has much smaller orfices.. at least they must be since you could never get a piece of safety wire through those holes. Depending on your screen size and settings, this picture is about twice life size.
In the piston itself we see a roll pin across the shaft. Not surprising,
since something needs to hold the piston. But this pin also keeps the
inner piston from moving past that point as it travels down inside the
hollow rod. OK, all that's simple, but how about the orfice just under the O ring? That one leads down into the hollow center of the rod, where it acts on the little piston that we first removed.
It too is a tiny hole, because it has only one purpose: to releave the pressure in the housing as heat builds up.
Also note that the roll pin will keep the inner piston from moving past that point as it travels down inside the hollow rod. So the pressure will stay on the side of the inner piston opposite the spring.
You see, the Cardinal is the only Cessna who's shimmy dampener lives up inside the cowl instead of out in the airstream. As a result it tends to get hot during flight.
As far too many FG operators have found, this becomes a problem in the FG when the dampener is serviced incorrectly. When a mechnic services the FG dampener just like all other Cessnas, the dampener will heat up on the next hot or extended flight and the pressure will blow the end off the dampener body. That's why the FG must be serviced with a specific volume of fluid, as marked on the placard that should be on the dampener
Back to the RG.. in these dampeners this excess pressure will simply push down into the hollow core and push the inner piston up against the spring that we first removed.
As a result, the RG dampener may be completely filled.
That's the end of the theory. All that's left now is to replace all the O-rings and work with your A&P to put the assembly back together. It's a lot easier than taking it apart!
One more little trick.. it can be very difficult to get all of the air out of that little fill hole. Start by filling the body about half way, then fill the hollow inner rod and insert the piston, spring and plug.
Now fill to the top. A spray-cleaner tube can be attached to the end of a pump-type oil can to push the fluid down inside the port while the air and bubbles can freely exit around the tiny tube.
Reinstall, have your A&P sign off the project and taxi at will!
Keith Peterson, CFO Webmaster