Cessna 152 Shimmy Dampner

[ShyFlyGuy]

Does ANYONE know how to fix a Cessna 152 shimmy dampner once and for all? Can I buy one new? My mechanic is rebuilding and/or servicing the P.O.S. every 10-20 hours. Please help.

Shy

 

[avbug]

Shy,

If the unit is rebuilt properly and is up to spec, then it should not be failing every 10-20 hours, unless you have other problems.

If you have firewall, engine mount, or nosegear problems, or are using improper landing technique, then your shimmy damper may keep on failing even if you replace it new, every time.

In a nutshell, what you have is a shaft that's attached to part of your nosegear, and a shell of the damper that's attached to another part. Inside the damper, the shaft is also attached to a disc, or fixed "piston," with a calibrated hole in it. Oil in the damper must pass through that hole to allow the piston to move in the shell. The ability of the unit to resist passage is what contributes to the damping action.

The shaft and shell are sealed with several different packings and rings. Unless your damper is being operated in a very dusty environment, these packings and rings should last quite a while. Internally, if there is corrosion or wear, the unit may be bypassing or failing to properly dampen any shimmying tendencies. If the mounting bolts are loose, or wear exists where the bolts go through the dampener at the attach points, then shimmy action may also take place, because the unit can't be held solidly enough to do it's job.

A good place to start is a close inspection of the nosegear itself. Is it damaged in any way? Are the wheel bearings adequately lubricated and in good shape? Check them carefully, both the bearings and the races, and ensure that the packings, felt, and cups that go adjacent to them are installed correctly.

Does the axle align through the forks properly? Is the axle properly secured and torqued? an improperly secured nosewheel will shimmy.

Is the wheel true and in spec? A damaged wheel can shimmy and can also easily fail...is it properly torqued together? Is the tire in good repair, and the entire wheel assembly properly balanced? Balance is important, and it needs to be checked and done each time the wheel is removed...at least every 100 hours if not more frequently. I

s the nosetire air pressure proper? Low pressure can cause these problems as well as causing binding in the nosewheel forks, excessive shoulder wear, and excessive temps...and even an explosion. Check the mains, too. If they're off, and the aircraft is pulling, this can exacerbate other problems and assist in a shimmy.

Is there play when the wheel is off, between the nosegear forks and the upper nosegear? Is there any play in the scizzor linkage? This is a very common point for shimmying, as well as an aircraft that won't track straight on the ground. Is the nosegear assembly properly lubricated? Is the nosegear strut properly inflated, and serviced properly with H-5606 fluid to the proper level? If that strut is too high or too low, it can shimmy.

How about landing technique? Is that nosewheel contacting first at all? Pushing forward after landing? Pilot technique is probably the #1 cause of nosewheel problems, including most of the wear that the entire assembly, or any part of it, experiences.

Check the firewall for buckling. The inside of the nosewheel forks for wear where a tire may have been compressed on a hard nosewheel landing...it sometimes splays out and leaves black or grey marks inside the forks, as evidence. Check the engine mounts for damage, bending, cracking paint...expecially the lower supports. You may be able to see it by checking the space between the prop spinner and the cowling for evenness above and below the spinner, or right and left, too.

Check the steering rod ends for play; another common wear point, and a source for shimmy.

Your shimmy damper may be fine...you may have other problems. If it keeps leaking like a sieve, then you have a bad damper or a bad rebuild. If that isn't the issue, if it's still shimmying and you can't seem to stop it, you may have other problems than your damper, or you may have a worn out damper internally. Without seeing the damper, it's very hard to say what you have, and not enough information has been provided to lend much comment.

Are you running a nosegear fairing? What condition, if so? When does the shimmy occur? Why is the unit being rebuilt? Is leakage occuring? Can the shimmy be stopped by the pilot? How often is your nosegear lubricated, and with what? Are you running retreads? Do you balance the nosewheel assembly? A lot more information as needed, as you've by now guessed. Check on it, and post back.

 

[ShyFlyGuy]

Avbug,

Thank you for your in-depth reply. Since the aircraft is used in a flying club, there is much training going on. Durring stage checks, I've noticed that students are not relieving the pressure on the nosewheel by keeping backpressure on the yolk durrinng the beginning of the takeoff roll. The shimmy starts at about 20-30 knots. I shout and scream and slap them around, as do the instructors, but they just don't usually get it. So that's one problem, but I don't care how you take off, that shouldn't happen. I doubt it happened with the new aircraft, and I doubt it should happen now.

As for the landings, I can not vouch for any of the landing techniques, since the first stage check I give them is post-solo, and I only see 3 landings out of perhaps 50 that they've had to date. Generally speaking, the landings aren't nosewheel landings. But they give up on the landing as soon as the mains hit. So the nosewheel hits hard at 55 knots on the rollout. It shakes like an epeleptic elephant and they just jam on the brakes. I lay into them like a drill sergant after I grab the yolk and provide backpressure until about 20 knots, when the nose gently falls to the runway. (One problem, regardless of student technique is short-field landings, wherein it is necessary to apply full breaks and backpressure, often setting off the elephant.) So I can tell you that it's getting its workout through the takeoffs and landings.

That being said, I had the mechanic replace the steering bungees, shim the strut, and overhaul and service the dampner. This solved the problem for about 100 hours, but also cost me nearly $800. I can't afford to do that every hundred hours.

The dampner looks like a relic from WWII and is dripping fluid continiously. It is loose on the mounts, and seems generally useless. We're not using a nosewheel fairing, although I'd guess that adding mass would help reduce the shimmy.

I'm going to send out a club newsletter lecturing on the landing technique, but that's all I can do about the landings. Doesn't Lord make NEW shimmy dampners? Thanks again for your response. I'm thinking about installing a squat switch that will send an electrical jolt to both seats if the nosewheel is compressed too much. That way, the pilot has incentive not to let the nosewheel drop and the passenger/instructor will also keep a very keen eye on the stupid jackhole next to him.

Yours,

Shy

P.S. I might add that this is the angriest post I've ever made on this site. Touchy subject.

 

[a&p cfiguy]

it's not the shimmy dampner

It never is. Those things are useless! They couldn't stop a shopping cart wheel from shimming! It's the steering collar. After years of use, it gets worn. Every time the nose shimmys it wears even more. Only way to fix it once and for all is to remove the whole strut assy, take the collar off, and install shims (as per the parts manual), until it's once again stiff. But not too stiff that you cant steer it. There are three different size thickness shims to get the proper fit. If you have a good fit, it'll never shimmy, no matter what technique landing you use. It'll even not shimmy if the shimmy dampner is not installed! As long as the steering is tight. Mechanics like to put this problem off to bad landing techniques, etc cause they don't want to disassemble the whole gear. But actually, once you do it, it's not that hard. 2 hours tops.

 

[UGAflyer]

I had a problem with my 172 nose wheel shimmying..several guys looked at the damper and everything and couldn't find a problem...well last annual I had to get a new set of tires and problem solved. I think the nosewheel was out of balance. I really don't know what that means exactly but that's what I have heard.

 

[TrafficInSight]

"loose on the mounts" is normal. Like Avbug and the rest said I'd look for play in the nose gear itself.

 

[GravityHater]

I think Lord makes a new PMAd SD that is a lot better.
Usually its someone who doesn't know to hold aft yoke when taxiing/taking off as you say. Seems like repeated abuse from this then leads to worn parts and then the shimmy is always a problem.

 

[TrafficInSight]

If everything is normal then you really shouldn't have to hold the yoke back to keep it from shimmying.

 

[NW_Pilot]

It never is. Those things are useless! They couldn't stop a shopping cart wheel from shimming! It's the steering collar. After years of use, it gets worn. Every time the nose shimmys it wears even more. Only way to fix it once and for all is to remove the whole strut assy, take the collar off, and install shims (as per the parts manual), until it's once again stiff. But not too stiff that you cant steer it. There are three different size thickness shims to get the proper fit. If you have a good fit, it'll never shimmy, no matter what technique landing you use. It'll even not shimmy if the shimmy dampner is not installed! As long as the steering is tight. Mechanics like to put this problem off to bad landing techniques, etc cause they don't want to disassemble the whole gear. But actually, once you do it, it's not that hard. 2 hours tops.

I agree! Something like that was done when I had my front strut rebuilt last winter had a bad o-ring has not had a shimmy since. Was worth the extra hour .

 

[avbug]

This was my point. You have several problems here.

That people aren't holding the stick back for taxi isn't the problem...you already have slop in the gear assy, and it's just being manifest by taxiing too fast, failure to keep pressure off the gear, etc...those are symptoms, but not the problem.

Your shimmy damper is shot. If it's dripping fluid, it's shot. It may just require a rebuild again; that may fix it, but so long as the nosegear assy has this much slop, from all the sources previously named, it will shimmy again even if you replace the damper every few hours with a new one...you'll ruin the damper over and over because the damper isn't really the problem. You have a bigger problem and the damper can't fix or control it, so the problem damages the damper.

When the nosegear is continuously allowed to shimmy, it's not just the steering collar. You'll get wear occuring in a number of other places, previously discussed. Replacing steering bungies is great, but you have a lot of wear places. Often mechanics like to inject grease everywhere, and the problem seems to go away for a few hours. Then it comes back. The nose strut and nosegear assy takes a beating, especially on a training airplane. The design of the nosegear on that airplane is conducive to shimmy; it's inherant in the design. For that reason, constant, frequent maintenance and strong technique are necessary if the shimmy is to be avoided.

It's a catch 22, really. When the shimmy starts, it causes excessive wear, that causes more shimmy. And it can be attributable to a number of sources. You can band-aid some of the sources and make it go away for a little while, but that doesn't fix the problem, and it comes back. When it does, it only makes things worse.

A nosewheel shimmy can be violent, with a lot of force and pressure on a number of components. Let it go on long enough, and it can even break an engine mount, or a longeron. The steering collar probably needs attention. The steering rod ends probably do, too. You probably need to look at the scizzors for shimmying again, and the mounting bolts and flanges for the shimmy damper likely have play in them as well.

Yes, student technique does this to a training airplane. You can't really blame the students so much; that's part of the learning process. If you're going to put an airplane in a training arena, then you can expect increased maintenance costs due to accelerated wear and damage. If the problem is only partially fixed, you can expect it to come back as the same things keep happening to the airplane. But even without the students abuse (and that of the instructors) you're still going to see the problem return, because the problem is never really fixed.

Take the shimmy damper itself...it's dripping fluid, and obviously not going to work. It shouldn't be leaking a drop. Does this mean it's bent or bad or excessivly worn? Not necessarily, but it's taken a beating from the shimmy...the shimmy hasn't been fixed, so you can't expect the damper to last...the problem has never really been taken care of. Fix the problem, then overhaul the damper, and you should see it last a long time. Put a new damper or another damper on there, and the problem will re-occur again and again, just like it is now. Remove all the play from the system (nosegear system), and then overhaul the damper again.

Another help is constant pressure on the rudder pedals. Tailwheel pilots have long been taught this, and most instructors in nosegear aircraft don't teach or understand it. Out of habit and long years of flying conventional gear aircraft, I keep substantial pressure on the pedals all the time (when it's really rough and violent in the summer, often my feet go numb). This tends to help eliminate shimmy, keeps tension on rudder cables to avoid slop, and eliminates accelerated wear from flopping around, as well as ensures more positive rudder control. Part of the reason that it's not usually taught in nosegear equipped aircraft is that they're conducive to laziness, and generally the instructors have never been properly taught, themselves. The rigging is usually a little different (leverage and T-bar, vs. direct cables), but it's still no excuse. Avoid pressing on the brakes if you're going to press on the pedals.