Violent Shimmy on Landing

[Skip Miller]

You're mistaken. On the Cessna's I've flown, the nose wheel locks straight ahead when the strut extends once airborne. Moving the rudder airborne does not move the nosewheel left and right.

Not true for PA-28 fixed gear...

 

[brien23]

Cessna death wobble, the N on 172N stands for no good the H on the O-320-H stands for the engine from H*LL. A lot of old Cessna 172 fly around with nose wheel fairing off death wobble. Rather than fix it people just remove the fairing. Not sure what data they use to fly around on the main wheels with fairings on and the nose wheel one off.

 

[Tom-D]

I own a Cessna and I've noticed that if I land fast and lower the nose without slowing down it will often induce a shimmy or violent oscillation that sometimes doesn't respond to elevator input (short of lifting the nose again). If I am taking off with a forward CG it will sometimes shimmy until I use elevator to lighten the load.

You need to get that fixed.

 

[Dan Thomas]

For Pete's sake. I used to try all the suggestions made here. Spent money on new torque link bushings. Spent time and money--lots of it--on shimmy dampeners. Fooled with bearing preload. Tried all sorts of techniques when landing to try to prevent it. Did static balances endlessly.

All of it was a waste of time in most cases, since it was, as I said before, treating symptoms instead of the root cause, which was a dynamically imbalanced wheel. Aircraft tires are notoriously poorly balanced when manufactured (Goodyears are better than most) and they still use tubes, which introduce another factor and which are often not installed well and can be bunched up some in the tire and throw everything off.

It wasn't until I balanced a wheel dynamically that I finally had spectacular success stopping shimmy. I did it by cleaning the grease off the bearings and putting them in the wheel and holding the axle and spacers so that I could use a bench grinder with a wire wheel to spin the thing, and found that it wobbled something fierce. I fooled with stick-on weights in various places in the wheel, held in temporarily by cubes of foam rubber, and was able to get the wheel spinning as smoothly as silk. Regreased the bearings, put it all back together, and the shimmy was absolutely gone. Did nothing at all to the rest of the nosegear machinery. Whenever I replaced a nosewheel tire I redid the balance, or if anyone complained of the least bit of shimmy, which can happen as the tire wears.

I modified an old dynamic balancer to fit the nosewheel and was able to do it much faster. Now I work elsewhere and am back to the manual method and can't find an old balancer. Might have to build one. Some motorcycle shops have dynamic balancers that could fit a nosewheel, since bikes have the same steering geometry and shimmy issues.

Dynamic balancing makes all those expensive nosegear components last way, way longer, and stops shaking the daylights out of the radios and instruments and lights and airframe. Saves big bucks.

Goodyear sells stick-on weights for aviation. Parts number 9900 or something like that. Aviall. Don't use them on nosewheels that retract up under the engine where the sticky stuff gets hot and lets go.

Dan

 

[Tom-D]

simple cure, put it on floats. no more nose wheel.

 

[Dan Thomas]

simple cure, put it on floats. no more nose wheel.

Yup. Or convert it to taildragger. Of course, taildraggers sometimes have shimmy issues, too.

Dan

 

[Henning]

Getting all the slack out of the nose gear is cheaper than either though.

 

[Tom-D]

Getting all the slack out of the nose gear is cheaper than either though.

What if there is none, and it still shimmies?

 

[RotorDude]

What if there is none, and it still shimmies?

My mechanics tried everything over the years, and each time the cure was only temporary. It was only when they installed the Lord dampener that the nose shimmy was fixed for good. They said that's been their experience with everyone else.

 

[Henning]

What if there is none, and it still shimmies?

Haven't seen it happen. Maybe a bad tire, but usually that doesn't suddenly cause a post annual shimmy (unless they changed the tire).

 

[yakdriver]

You need to find the cause as it will damage the gear after it happens a few times. My friend's 182 did it half a dozen times then it broke both the upper and lower nosegear mounts on the last landing. The gear leg jammed up between the carb and engine mount at about a 20 degree angle. prop was about an inch off the ground. I was giving a guy a checkout in a T-28A and it developed a severe shimmy on landing roll out. Both cowl doors flew open and it ended up it broke both lower longerons at the engine mount. It ended up a log expensive rebuild. Don't mess around with shimmy as it can lead to bad things. Don

 

[Tom-D]

Haven't seen it happen. Maybe a bad tire, but usually that doesn't suddenly cause a post annual shimmy (unless they changed the tire).

OMG there is some thing you haven't seen/done

It is rather easy to not get the axel tube in correctly. When this happens, it takes a little taxi with load to allow the tube to find its proper location, when it does, the load on the wheel bearing is entirely lost.

Look at it as the tube is being pinched between the forks not allowed to pass into the fork's hole that is its proper position. When the tube is pinched between the forks, the long bolt that holds the axel assembly will take the proper torque, and the wheel will rotate freely.

It will not be forced into its proper position until load is applied. then the long bolt is 1/4 to 3/8ths inch loose.

 

[weirdjim]

Oh, come ON people.

The thing didn't shimmy BEFORE the annual.

It shimmied AFTER the annual.

It ain't worn scissor joints
It ain't out of round tires
It ain't worn nose steering parts.

Some mechanic took something off and didn't tighten it back down afterwards.

The comment was that the nose wheel pant was replaced. For that the nose tire had to come off. It had to go back on. That particular process has always been a problem in that the nose axle (NOT AXEL) tends to settle down after a few minutes of taxi and loosen up. That is a well known phenomenon and if you can't simply remove the cotter pin from the castellated nut, tighten the sucker up, replace the cotter pin, then somebody doesn't understand maintenance very well.

Thanks,

Jim

 

[Henning]

OMG there is some thing you haven't seen/done

It is rather easy to not get the axel tube in correctly. When this happens, it takes a little taxi with load to allow the tube to find its proper location, when it does, the load on the wheel bearing is entirely lost.

Look at it as the tube is being pinched between the forks not allowed to pass into the fork's hole that is its proper position. When the tube is pinched between the forks, the long bolt that holds the axel assembly will take the proper torque, and the wheel will rotate freely.

It will not be forced into its proper position until load is applied. then the long bolt is 1/4 to 3/8ths inch loose.

Sounds like the bearings end up loose then and provides slack (improper preload) in nosewheel, which is what I said. Much cheaper to reassemble properly than convert to a float plane or conventional gear.

 

[brien23]

You need to fix this if the people working on your plane cant fix it go somewhere else. Removing the nose wheel fairing is not a fix, since it's a N model 172N you might just want to get rid of it .

 

[Tom-D]

Sounds like the bearings end up loose then and provides slack (improper preload) in nosewheel, which is what I said. Much cheaper to reassemble properly than convert to a float plane or conventional gear.

Is this a who said it first??

read post 21.

 

[bnt83]

Is this a who said it first??

read post 21.

You mean post 14?

 

[Tom-D]

You mean post 14?

All means the same thing, 1 thing causes the other. It must be assembled wrong before it can come loose.

But what we don't know is, was it tight in the first place?

 

[weirdjim]

You mean post 14?

NO, 14 said it could be tire unbalance OR loose hardware. That's like saying your fever could be the flu or appendicitis. 21 nailed it head on without any intermediate malarkey.

Jim

 

[Clark1961]

But what we don't know is, was it tight in the first place?

Well it was tight the first time but she loosened up over the years...looked into a trade-in but that seemed expensive...

 

[Stingray Don]

Just got the plane back from the shop today. They said the damper needed to be serviced. I did 5 quick trips around the pattern and no shimmy. A good sign but time will tell. Thanks everyone for the input.

 

[Henning]

You'll wear out another damper if the underlying cause isn't addressed.

 

[Tom-D]

Well it was tight the first time but she loosened up over the years...looked into a trade-in but that seemed expensive...

Which first time?

Factory? You can bet it didn't shimmy when new.

 

[Tom-D]

You'll wear out another damper if the underlying cause isn't addressed.

The dampener was the underling cause.

this fix is temporary. It needed fluid because it leaks.

 

[Henning]

Pick the nosewheel off the ground, give it a solid flick. If it spins more than one to 1.25 revolutions, you still have a problem.

 

[weirdjim]

Which first time?

Factory? You can bet it didn't shimmy when new.

Um, Tom, you need to read a little bit of human anatomy into this answer.

Jim

 

[weirdjim]

Well it was tight the first time but she loosened up over the years...looked into a trade-in but that seemed expensive...

Depends on how old you are and what the trade-in conditions are. For me, at 71 and looking at my last flight square in the face, the expense wasn't the consideration. Enjoying my last few flights in peace and harmony were worth any expense I had to endure.

I consider the "expense" prolonging that last flight by at LEAST ten years by getting rid of the shimmy, chatter, and squealing with the trade-in.

Jim & Cyndi

 

[Dan Thomas]

The dampener was the underling cause.

I'd still say he has a dynamic imbalance. Motorcycles don't use shimmy dampeners and their steering geometry is exactly the same as a light airplane's nosewheel. They get their wheels dynamically balanced, same as cars, and shimmy is not a problem. If a bike has an imbalance it will show up if the rider takes his hands off the bars.

If there is a continuing dynamic imbalance, it will sooner or later loosen all the linkages between the fork and shimmy dampener and he will be back to where he started and in need of a lot of expensive repairs.

Dan

 

[Tom-D]

I'd still say he has a dynamic imbalance. Motorcycles don't use shimmy dampeners and their steering geometry is exactly the same as a light airplane's nosewheel. They get their wheels dynamically balanced, same as cars, and shimmy is not a problem. If a bike has an imbalance it will show up if the rider takes his hands off the bars.

If there is a continuing dynamic imbalance, it will sooner or later loosen all the linkages between the fork and shimmy dampener and he will be back to where he started and in need of a lot of expensive repairs.

Dan

You can have a brand new strut, balanced tire, and every thing perfect, but if that shimmy dampener isn't serviced correctly. it will shimmy. They didn't put it on there because it isn't needed.

 

[Tom-D]

And, OBTW, the only way to get all the air out of the dampener is to assemble it totally submersed in fluid.

 

[Dan Thomas]

And, OBTW, the only way to get all the air out of the dampener is to assemble it totally submersed in fluid.

Clamp the dampener's fork carfully in a soft-jaw vise. Pull the body up until the top end of the shaft is inside the cylinder. Wrap duct tape around the top of the cylinder to form a dam, and fill that with fluid. Let the bubbles out. Work the dampener slowly to get any air from beneath the piston, and let more air out. Take it out of the vise and rock it a little to get any trapped air out of the top corners. Push the shaft back in and take the tape off.

The dampeners on some Cessnas must have some air left in them. Classic case is the Cardinal, where the dampener is inside the cowling and gets really hot and will explode if filled right up. There's a decal on it--or should be--that refers the mechanic to the service manual for instructions on filling it.

And of course, a dampener that has some air in it isn't a terribly efficient dampener, so that nosewheel needs dynamic balancing.

Dan

 

[bnt83]

The dampeners on some Cessnas must have some air left in them. Classic case is the Cardinal, where the dampener is inside the cowling and gets really hot and will explode if filled right up. There's a decal on it--or should be--that refers the mechanic to the service manual for instructions on filling it.

Those darn placards tend to be missing!!!! Setting up ignorant mechanics for failure.

The FG Cardinal dampers have a fill plug too. No submersion in hydraulic fluid necessary.

 

[Tom-D]

Which aircraft were we talking about again..? Is it too difficult to tell the difference between a 172 and a 177 retract?


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