Shimmy issue

[jspilot]

I am admittedly pretty lacking in knowledge when it comes to airplane repair or maintanence issues so forgive me if this question is absurd.

I rent from a place where pretty much all of the 172p's have shimmy issues. I've had issues with the nose wheel shimmying on takeoff and just this past weekend on a landing. From the pilot flying perspective, the simple fix is just to pull back on the yoke and get the weight more on the main wheels but from a mechanics standpoint, is this a simple problem to fix? I know the planes are well maintained aside from this issue so I'm just wondering why they don't fix the obvious shimmy issue?

 

[poadeleted21]

Uneven nose wheel wear or possibly the shimmy damper needs overhauled.

 

[Dr. O]

Raise the nose wheel off the ground and grab the wheel yoke and shake it. If you can feel even the tiniest trace of slop in the bushings nothing you do will cure the shimmy until you replace them.

 

[frfly172]

Most likely the shimmy damper. Constant landings in the training realm , cause them to wear .

 

[Checkout_my_Six]

Raise the nose wheel off the ground and grab the wheel yoke and shake it. If you can feel even the tiniest trace of slop in the bushings nothing you do will cure the shimmy until you replace them.

this.....overhauling the damper will not cure the problem. Removing the slop in the linkage fixes the problem to where a damper is not even needed.

Most often just replacing the bolts in the linkage is a quick cheap fix. If the bushings are really bad....new bolts won't do it.

 

[James331]

What the others said, also simply having a low level of fluid in the damper can be a simple problem.

The nose wheel is really something that shouldn't be on the ground at high speed, or have much force put on it, one of the major benifits of a tailwheel plane.

Flying my amphib I always treat my nose wheels like they are made of glass.

 

[RotorAndWing]

this.....overhauling the damper will not cure the problem. Removing the slop in the linkage fixes the problem to where a damper is not even needed.

Most often just replacing the bolts in the linkage is a quick cheap fix. If the bushings are really bad....new bolts won't do it.

Also balancing the nose wheel. Lots of mechanics install a new tire and never check the balance.

 

[Checkout_my_Six]

Also balancing the nose wheel. Lots of mechanics install a new tire and never check the balance.

that....or too much air pressure is another problem.

 

[RotorDude]

And if all else fails, trust the Lord.

 

[Norman]

I've also found that tire type can affect shimmy. A round profile tire (Think motorcycle) is more prone to shimmy than a flat tread profile.

 

[bnt83]

I fought a shimmy on a 1977 Cessna 177B till I was blue in the face. It turns out that tightening the axle nut a 1/2 turn more solved it. I was afraid of cracking the nose wheel fairing with puny AN960 washers under each nut so I replaced those with AN970 washers. The bolt holes in the fairing weren't horrible but still a bit chewed up.

 

[Henning]

I am admittedly pretty lacking in knowledge when it comes to airplane repair or maintanence issues so forgive me if this question is absurd.

I rent from a place where pretty much all of the 172p's have shimmy issues. I've had issues with the nose wheel shimmying on takeoff and just this past weekend on a landing. From the pilot flying perspective, the simple fix is just to pull back on the yoke and get the weight more on the main wheels but from a mechanics standpoint, is this a simple problem to fix? I know the planes are well maintained aside from this issue so I'm just wondering why they don't fix the obvious shimmy issue?

Shimmy is caused by slack in the bits and pieces of the gear. This ends up wearing out the shimmy damper. When this happens, people fix the shimmy damper, but they don't go through the nose gear and replace the worn out bushings and bolts, so a couple months later, they have to rebuild the shimmy damper again.

Another major cause I have found for shimmy is improper preload on the nose wheel bearing. First thing to do when a plane shimmies is have someone pick up the nose wheel off the ground, grab the wheel and give it a shake, it should not move at all, there would be no sound coming from it. If that is good, grab the wheel and give it a firm flick with your wrist. It should rotate approximately 1 time before coming to a stop. If it turns more than that, it needs to be tightened.

 

[Silvaire]

All of the above but if there has been shimmy for any length of time then it is definitely necessary to replace torque link bolts, bushings and shims - common wear items that, for some reason, never get replaced. Some mechanics would rather rebuild the shimmy dampener half a dozen times which doesn't solve the problem.

Here's a good article on the subject written by Dave McFarlane

Can you stop nose gear shimmy?

 

[Dan Thomas]

Here's a good article on the subject written by Dave McFarlane

Can you stop nose gear shimmy?

He, like so many writers of articles on nosewheel shimmy, does not address the root cause: dynamic imbalance. If that nosewheel isn't dynamically balanced (not just statically balanced like most are, if they get any balance at all) the nosewheel will wobble and wear out all the linkages between the fork and shimmy damper mounting. Just replacing those parts doesn't help much.

Every time you get new tires for your car, they get dynamically balanced. Airplanes, which are often worth far more than a good car, don't. It's no wonder their wheels shimmy and hop.

Dan

 

[Henning]

He, like so many writers of articles on nosewheel shimmy, does not address the root cause: dynamic imbalance. If that nosewheel isn't dynamically balanced (not just statically balanced like most are, if they get any balance at all) the nosewheel will wobble and wear out all the linkages between the fork and shimmy damper mounting. Just replacing those parts doesn't help much.

Every time you get new tires for your car, they get dynamically balanced. Airplanes, which are often worth far more than a good car, don't. It's no wonder their wheels shimmy and hop.

Dan

Yeah, you just can't sell people something that only effects 15 seconds of any flight and has an operational work around, pilots are just too cheap. I brought this up years ago, and the consensus was, 'nope', line up the dot is good enough.

 

[bnt83]

They are a strange animal for sure.

I did everything to my Cessna 150 when I first got. New bolts, New bushings, even pulled the whole strut and re-shimmed the steering collar. The shimy damper bolt bosses seemed a little worn but those parts are made of unobtanium so they got reused. I never had a shimy after that.



I'm not sure how it can work so well but I replaced the PLASTIC TORQUE LINK BUSHINGS in the 1968 Cessna 177 and a 1977 177B over 10 years and have yet to do it again. The 1968 has flown roughly 400 with tons of landings and the 1977 probably 250 hours.

http://mcfarlaneaviation.com/Products/?ID=31143040&Make=Cessna&Model=177&PartNumber=TL-KT-10&

 

[rbridges]

my mooney had this problem, too. the steering horn was worn and needed to be replaced. Basically, bushings were worn and slop was evident. In the mooney, you can get an idea by moving the rudder back and forth. Before it was replaced, I had nearly 2" of play. Now it's 3/4".

 

[nrpetersen]

The Cessna nose wheels will center themselves and lock on extension such that you can't observe the lost motion in the anti-shimmy linkage. The Cessna service manual doesn't show how to work around this (at least it didn't).

I suggest collapsing the nose wheel strut, then pull the tail down, then vent in just enough air to allow the strut to only partially extend. Then wiggle & analyze. It is probably the cross yoke (title?) that needs shimming. The fuselage side of the shimmy damper may need a slightly oversize bushing too. Lathe time.....

 

[Dan Thomas]

The Cessna nose wheels will center themselves and lock on extension such that you can't observe the lost motion in the anti-shimmy linkage. The Cessna service manual doesn't show how to work around this (at least it didn't).

I suggest collapsing the nose wheel strut, then pull the tail down, then vent in just enough air to allow the strut to only partially extend. Then wiggle & analyze. It is probably the cross yoke (title?) that needs shimming. The fuselage side of the shimmy damper may need a slightly oversize bushing too. Lathe time.....

By dynamically balancing the nosewheel, I have eliminated shimmy even on airplanes with worn steering collars and torque link bushings and shimmy damper mounts.

All of that hardware is rather light and there are too many places for slop to build between the damper and fork. An often overlooked place is that "cross yoke" you mentioned--that's the steering collar--and that bit, after thousands of hours, develops a looseness that shimming can't fix because the oleo housing gets worn on the left and right edges of the top of its bottom collar where that steering collar rests. The collar rocks, allowing some undamped movement of the wheel fork. And the rocking wrecks the small joint between the torque links.

Dynamic balancing early on prevents all the wear in the first place.

http://www.gsp9700.com/technical/4202T/6GLOS003.htm

Dan

 

[Henning]

The Cessna nose wheels will center themselves and lock on extension such that you can't observe the lost motion in the anti-shimmy linkage. The Cessna service manual doesn't show how to work around this (at least it didn't).

I suggest collapsing the nose wheel strut, then pull the tail down, then vent in just enough air to allow the strut to only partially extend. Then wiggle & analyze. It is probably the cross yoke (title?) that needs shimming. The fuselage side of the shimmy damper may need a slightly oversize bushing too. Lathe time.....

Fill the hole with Devcon Aluminum Putty, re drill properly sized.

 

[Henning]

By dynamically balancing the nosewheel, I have eliminated shimmy even on airplanes with worn steering collars and torque link bushings and shimmy damper mounts.

All of that hardware is rather light and there are too many places for slop to build between the damper and fork. An often overlooked place is that "cross yoke" you mentioned--that's the steering collar--and that bit, after thousands of hours, develops a looseness that shimming can't fix because the oleo housing gets worn on the left and right edges of the top of its bottom collar where that steering collar rests. The collar rocks, allowing some undamped movement of the wheel fork. And the rocking wrecks the small joint between the torque links.

Dynamic balancing early on prevents all the wear in the first place.

http://www.gsp9700.com/technical/4202T/6GLOS003.htm

Dan

Devcon Aluminum Putty can fix that worn strut as well.

 

[Dan Thomas]

Devcon Aluminum Putty can fix that worn strut as well.

Legally? I used to use some of Devcon's great putty products in the non-aviation machine shop industry, but I have never seen them used on anything airworthy.

Dan

 

[Hiperbiper]

Something else to check it the tire itself. Those of us old enough to remember car tires with bias plys (not radials) can remember what happened when you ran a tire through a pothole or drove over a curb at speed...a cord or two would break and the tire would go out of round and vibrate like hell!
Checking is free; have the tail tied down and place some kind of pointer close to the tire. Now spin the tire and watch to see if the tire is has gone egg shaped.
Cheap flight school tires and students slamming the nose wheel down repeatedly will cause a out of round tire shimmy.

Just something cheap to check...

Chris

 

[nrpetersen]

Devcon Aluminum Putty can fix that worn strut as well.

That must be some impressively strong stuff - if it can react one end of the shimmy damper with only a #10 screw in bearing X about 1/4 inch long!

Wait - I think there was a 1/4 inch dia bushing loose in the strut forging. Maybe it came from Cessna that way? I made one that was a few thou over size & that took care of the shimmy problem.

Otherwise, how do you do a dynamic balance on a nose wheel? Take it to a tire shop?

The nose wheel tire tread would develop an obvious " inverse crease" in it after a couple 100 hrs of service. I tried grinding that part tread down on a sander but that only worked for a while.

 

[Checkout_my_Six]

That must be some impressively strong stuff - if it can react one end of the shimmy damper with only a #10 screw in bearing X about 1/4 inch long!

kinda sounds like a JB weld fix....

 

[Henning]

Legally? I used to use some of Devcon's great putty products in the non-aviation machine shop industry, but I have never seen them used on anything airworthy.

Dan

It's MilSpec for like duty, I can't see why not.

 

[Dan Thomas]

Otherwise, how do you do a dynamic balance on a nose wheel? Take it to a tire shop?

The tire shops can't mount the nosewheel on their balancers. I used to do the dynamic balance by hand, then modified an ancient mechanical dynamic balancer that worked well. Then I went to work for a different outfit and am back to doing it by hand.

The bearings get all the grease cleaned off them so the wheel rotates easily on its axle. You hold the axle and bearing spacers in both hands and spin the wheel using a bench grinder with a wire wheel. Once it's spinning a bit, take it away from the wire wheel and you can feel the static imbalance (a vertical shake in line with the wheel's rotational plane) and the dynamic imbalance (a wobble).

Now the fun begins. You start by using lead stick-on weights (Goodyear makes an approved version for aircraft) but use small cubes of foam rubber to hold them in the wheel against the outer rim, on both sides of the wheel. By experimenting with various weights and locations, you'll find the sweet spots that remove all static and dynamic imbalance. It takes a while but once you've done a few it gets quicker. Clean off the wheel, stick the weights in place, regrease the bearings, and put it all back to work.

An electronic balancer, like those used in the tire shops, would be nice, but it would cost a bundle. One of these days when I get motivated I will build a basic machine that will do it quickly for a lot less and see if I can get anyone interested. I used to design and build some automated production machines. That was the most fun anyone could have at work.

Nosewheel shimmy is one of those universal headaches, a cure for which owners would gladly pay for once they saw the difference and understood that unaddressed shimmy eats radios and instruments and loosens up a lot of expensive stuff on an airplane.

I dynamically balance tailwheels, too, but in the tailwheel's case the problem is more likely a distorted tailwheel spring that tips the steering axis forward at the top, like a bent shopping cart caster that drives one crazy.

Dan

 

[Henning]

That must be some impressively strong stuff - if it can react one end of the shimmy damper with only a #10 screw in bearing X about 1/4 inch long!

Wait - I think there was a 1/4 inch dia bushing loose in the strut forging. Maybe it came from Cessna that way? I made one that was a few thou over size & that took care of the shimmy problem.

Otherwise, how do you do a dynamic balance on a nose wheel? Take it to a tire shop?

The nose wheel tire tread would develop an obvious " inverse crease" in it after a couple 100 hrs of service. I tried grinding that part tread down on a sander but that only worked for a while.

I must be misreading what you wrote, I though you had a hogged bolt hole in aluminum that you needed to resize back down to the correct size? This is a completely valid repair product developed for that purpose and used at all levels of industry to make economical repairs just like that.

I would think a tire shop can, if not, check with a local go cart racing shop, they'll be able to tell you who has the equipment.

 

[petrolero]

I had a shimmy problem and had my shimmy dampener repaired and it fixed the problem unless I do something dumb like push the nose down while carrying too much speed on landing.

 

[Henning]

I had a shimmy problem and had my shimmy dampener repaired and it fixed the problem unless I do something dumb like push the nose down while carrying too much speed on landing.

You didn't fix the shimmy, you masked it. The shimmy is there working to break the bandaid/damper again. It's like arm wrestling, just because there is no motion, does not mean great forces are not being exerted.

The only way to fix a shimmy problem is to eliminate slack in the system. If you get out all the slack from the wheel bearings to the control arms, you wouldn't need a shimmy damper. If you have enough slack, you just eat more shimmy dampers, and allow the underlying cause to deteriorate until it turns from an afternoon with the tail tied down and making some adjustments and maybe replacing a few moderate price parts, you end up replacing a whole torn up assembly with one from the salvage yard.

 

[petrolero]

You didn't fix the shimmy, you masked it. The shimmy is there working to break the bandaid/damper again. It's like arm wrestling, just because there is no motion, does not mean great forces are not being exerted.

Since there is no shimmy, by definition, the shimmy is fixed.

They may have tightened up the linkage and replaced bushings too, I don't recall off hand.

 

[RotorAndWing]

It's MilSpec for like duty, I can't see why not.

Can you provide documentation that shows it can be used for a repair on a nose gear? Does the manufacturer have it in the MM that allows it?

Is there an AMOC that allows it to be used in a nose gear repair?

 

[Henning]

Since there is no shimmy, by definition, the shimmy is fixed.

Read the edit, sorry. No, it really isn't, it is damped, that's all a shimmy damper does is damps the physical result of the oscillation in force, but it does nothing to eliminate the underlying cause of the oscillation in force. The force is still there, just being controlled, for now. But it will eventually win again and you will have to reseal the shimmy damper again.

 

[Henning]

Normally it does not take me long or cost much to elimate shimmy from a plane. Seriously the majority of the time it's the nosewheel bearing preload that is too slack, that is the root cause, and from that everything else gets worn out. There are some planes that never see a shimmy problem, I bet dollars to donuts you pick up the nose wheel and give it a solid flick of the wrist it'll spin once. Most of the other parts are already built with common, standard, cheap, easily replaced bushings. If the yoke head has slack you can't get out, there is Devcon, but usually if you have the wheel right and everything else right, you end up well within operating tolerances.

 

[Norman]

Can you provide documentation that shows it can be used for a repair on a nose gear? Does the manufacturer have it in the MM that allows it?

Is there an AMOC that allows it to be used in a nose gear repair?

But, but, but, Henning said it so it must be true. I read it on the internet.

 

[RotorAndWing]

If the yoke head has slack you can't get out, there is Devcon, but usually if you have the wheel right and everything else right, you end up well within operating tolerances.

Can you show any manufacturer's MM that approves Devcon in repairing a nose gear yoke?

 

[Henning]

Can you show any manufacturer's MM that approves Devcon in repairing a nose gear yoke?

No, I am sure it does not exist in the MM, but as you know, that is not the only qualifying source of repair data. Devcon is a MilSpec material designed for exactly such repairs. It is commonly used, and the application is consistent with the labeling. There is also the Owner Produced Part allowance. For me to do such a repair meets all the criteria. I do not see where this would be considered a major repair either.

 

[RotorAndWing]

No, I am sure it does not exist in the MM, but as you know, that is not the only qualifying source of repair data. Devcon is a MilSpec material designed for exactly such repairs. It is commonly used, and the application is consistent with the labeling. There is also the Owner Produced Part allowance. For me to do such a repair meets all the criteria. I do not see where this would be considered a major repair either.

So how would you sign this repair off (providing you had an A&P)? What data would you reference?

 

[petrolero]

Read the edit, sorry. No, it really isn't, it is damped, that's all a shimmy damper does is damps the physical result of the oscillation in force, but it does nothing to eliminate the underlying cause of the oscillation in force. The force is still there, just being controlled, for now. But it will eventually win again and you will have to reseal the shimmy damper again.

I edited my reply as well. They may have tightened up the play in the system as well - in fact they very likely did.

 

[Henning]

So how would you sign this repair off (providing you had an A&P)? What data would you reference?

The A&P typically references "Replaced with owner repaired part"