Nose wheel shimmy c182H

Discussion in 'Maintenance Bay' started by benyflyguy, Mar 23, 2020.

  1. benyflyguy

    benyflyguy Pattern Altitude

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    I have a nosewheel shimmy mid speed. On take off and landing. It a a specific speed range. I keep back pressure elevator to minimize and it still does it. We have had the tire replaced-balanced-shimmy damper refurb you name it. It shook apart a wheel pant on us. Finally got a vid or it
    Fast forward to 10 minute mark for takeoff.
    any thoughts???


    I started to think about rudder trim. If the rudder is trimmed differently will that trim the nose gear as well? We’ve spent thousands trying to get this right.
     
  2. NordicDave

    NordicDave Cleared for Takeoff

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    This is popcorn topic.

    Usual suspects in order of likelihood:
    • Loose scissors joint links, need McFarlane link rebuild it
    • Shimmy dampener rebuilt
    • Worn locking collar
    • Sometimes Tire issues
    Personal experience with a few planes, it's commonly all the above replaced or overhauled one part at a time hoping not to have to dive into the whole set-up. Tire is least suspect unless there is uneven wear pattern. It's more expensive to take a piecemeal approach. Though sounds like to may have done all this already.

    I'd remove the wheel pant and see if the shimmy goes away. Sometimes the pant will catch, shake, and release air in a cycle.

    I'm sure the next 3 posts will disagree widely.
     
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  3. wheaties

    wheaties Pre-takeoff checklist

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    This post agrees with @NordicDave. Nose wheel shimmy is, unfortunately, common and can be aggravated by a hard landing (or a series of rough landings.)
     
  4. Dan Thomas

    Dan Thomas Final Approach

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    I have stuck my oar in on this plenty of times. I have cured numerous cases of nosewheel shimmy by simply dynamically balancing that nosewheel, without touching anything else. Dynamically, not statically, which is about all any airplane wheel gets. If it isn't dynamically balanced, all those expensive parts can only mask the problem for awhile until they're worn out again in short order.

    Finding someone to dynamically balance it is the problem. Motorcycle shops often have a dynamic balancer that will fit a nosewheel. The seals and bearings must be out of the wheel. I built my own balancing machine when I went to work in another shop, based on an old machine that I had converted to do nosewheels in the flight school.

    You have to think about it. When you buy new tires for your car, what do they do to them before they install the wheels back on the car? They dynamically balance them. That wheel has to be spun to determine the amount and location of the imbalance, and simple static balancers cannot do it, despite what some vendors of static balancers tell you. I, and a lot of other old guys here, remember the days when tire shops had only static balancers, and the steering wheel would shake constantly in cruise, with some speeds being very bad. ANd that was in cars, where the suspension and steering stuff is far heavier and rigid than that in an airplane. The shimmy damper in the airplane is just an attempt to stop shimmy caused by dynamic imbalance and dates from the old days when static balancing was standard practice. Cars NEVER used any shimmy dampers at any time, and dynamic balancing eliminates the need for them. It's funny how we complain about the archaic technology still being used in GA aircraft, and then go and use archaic methods to try to cure shimmy.

    Do a search on POA for nosewheel shimmy. I'm not going to redefine it here anymore.

    If there are guys posting here giving advice on what to fix, and they have never done a dynamic balance and experienced the result, their advice is badly lacking.

    See this: http://www.aircraftmaintenancespecialties.com/tech.php?id=3

    TLDR? An excerpt:

    Nose Wheel Tire & Wheel Assembly Balance
    Balancing the nose wheel/tire assembly is the most important point to check in trying to solve a shimmy problem. Aircraft tire and tube manufacturers paint a red dot on the tire for the "light" spot. A yellow stripe on the tube, or the valve stem should be aligned with the red dot for coarse balance during tire buildup. Then the tire/wheel assembly is balanced with a static type (bubble) balancer and generally does an acceptable job.

    However, the preferred method, and sometimes the only method that can solve persistent shimmy problems, is dynamic balancing. (Dynamic balancing is when the wheel/tire/bearing assembly is spun and the proper weights and locations determined electronically). Very few light aircraft maintenance shops have the capability to do spin balancing, so AMS suggests that local motorcycle shops be contacted until one with a spin balancer is found. Usually these shops have the necessary mandrels to mount an aircraft tire/wheel assembly. Balance is achieved by affixing lead weights to the wheels.

    Bold emphasis mine.
     
    Last edited: Mar 24, 2020
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  5. benyflyguy

    benyflyguy Pattern Altitude

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    You are right. I have heard this before and I just checked with our shop and is static balancer. We aren’t flying too much right now so I’ll have the wheel pulled and take to a motorcycle
    Shop. This is the only thing we really haven’t tried- really should be the first thing I guess. I will keep you posted.
     
  6. Dan Thomas

    Dan Thomas Final Approach

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    A static balance can actually make things worse. If the imbalance is on one side of the wheel, as in left or right (it's seldom in the center), and the tech installs the weight opposite the heavy area but on the wrong side of the wheel, it's statically balanced but now the dynamic imbalance is even worse. Static balancers think in two dimensions, while the problem is in three dimensions.
     
  7. ktup-flyer

    ktup-flyer En-Route

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  8. Larry Korona

    Larry Korona Pre-Flight

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    Good idea, but would the weights that a non-aviation repair shop put on the wheel be acceptable?
     
  9. Dan Thomas

    Dan Thomas Final Approach

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    It's the standard practice. Aviall sells Goodyear stick-on weights for this purpose. The wheel has to be really clean and dry, but they stay put. The only time I had any fall off was on our R182, and that nosewheel retracts into the well immediately below the engine and gets pretty warm. The weight stickum can let go. The hubcaps kept them from escaping altogether.

    As the nose tire wears it might be necessary to rebalance. Getting those weights off can be fun; the tape usually stays stuck to the wheel and weight and delaminates through its center. Then you have to carefully scrape the tape off the wheel and clean it.


    The weights go inside the wheel's cavities and centrifugal forces help keep them there, too.

    Cessna published a Service Information Letter long ago on this subject. CE-84-21, IIRC, Can't find it now. An older one, NL-81-5, describes the Goodyear static balancer and the self-sticking wheel weights. Straight from Cessna, still listed in their service document listing, so it carries more authority than I do. https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=37619

    Remember, that balancer will not solve nosewheel shimmy problems. It's not a dynamic balancer, yet shops keep using it on nosewheels and have no success dealing with shimmy. We used it to balance main wheels to stop common wheel hop. With the brake discs on the inside of the main wheel you can't dynamically balance it anyway. Can't put weights in that side.
     
  10. nrpetersen

    nrpetersen Pre-takeoff checklist

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    Assuming the strut assembly is similar to a 172, there is a design feature that locks it to center when it is fully extended. This has to be defeated when analyzing the lost motion in the linkage connecting strut yaw to the to the steering/shimmy damper. This is easiest done by:

    1) Vent all the charge from the nose strut until the strut fully collapses.
    2) Raise the nosewheel by tying the tail down.
    3) Bleed in enough atmospheric air to allow the strut to extend about half ways. This allows the nose gear to be manually wiggled similar to shimmy, and the linkage can be more correctly evaluated for lost motion between the steering damper and the wheel shimmy.
    4) Chances are there is excess roll motion in one of the elements of the linkage that is not obvious unless you do this half way extension test. Shims spacers etc are available. Spend $$ on same.
    5) Fill the shimmy damper, reassemble, and recharge.

    This is based on a MechEngr w 50 years of 172 experience. YMMV.
     
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  11. Larry Korona

    Larry Korona Pre-Flight

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    Good information! Thanks!
     
  12. gdwinc

    gdwinc Filing Flight Plan

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    I'm curious to know if anyone has tried using Dynabeads to balance a nosewheel? Might be easier than tracking down a motorcycle shop to do a dynamic balance.
     
  13. Dan Thomas

    Dan Thomas Final Approach

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    They'll statically balance a wheel, but will they move to the sides to dynamically balance it?
     
  14. gdwinc

    gdwinc Filing Flight Plan

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    I don't know about moving to the sides, but this is a quote from their website:
    The way we do this is by offering a Dynamic Balancing Solution, a high-density ceramic bead that, when easily installed, continuously balances your tires as you drive. The amount of material will distribute itself in weight and position dependent on the balance requirements of the individual tire.
    Again, I'm not advocating this as a solution, just curious if anyone has tried...
     
  15. nrpetersen

    nrpetersen Pre-takeoff checklist

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    I think theoretically Dynabeads will gradually migrate to a constant rotating radius of the interior of the tire. Yes, that may in time improve the static balance assuming constant tire tread thickness, but I don't see how it will do anything at all for the dynamic balance. It also requires time to work its way around the tire inside, something that is not available in a tire that rolls only a little bit before called on to spin like hell - and a widely varying speeds.

    I would also question the balance after a wheel has been parked for some time. It probably actually be worse, when suddenly spun up.
     
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  16. Dan Thomas

    Dan Thomas Final Approach

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    With our oh-so-modern inner-tube tires, getting the balancing beads in there would be fun. And inner tubes often develop wrinkles that would hamper bead movement.
     
  17. GaryV

    GaryV Pre-takeoff checklist

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    Sometimes a shimmy isn’t a shimmy.

    I’ve been fighting a ‘shimmy’. I couldn’t find anything else wrong so I took my nose wheel to a local motorcycle shop to have it dynamically balanced. The guy that does the balancing at the shop is a pilot that owns a Maule.

    He pointed out how out of round the tire was and helped me use a belt sander to get it back in round. The first time he checked it, the balance was off by more than an ounce on one side. After we removed the high spots, and got it back to round, it didn’t need weights to balance.

    The tire is new enough that I couldn’t see any obvious issues until watching the tire while it was spinning. When it was turning quickly you could see that it had a high spot. I used a great pencil to lightly touch the contact surface as it was spinning and it turned out that there were several high spots before we touched it up.

    Thinking back I did have a light ‘shimmy’ shortly after installing the tire. Obviously that should have been a red flag but I missed it. My tire guy said that as time went on the problem just got worse to the point where the tire was bouncing on each rotation when the tire was lightly loaded like shortly after touchdown or just before rotation. I interpreted the bounce as a shimmy.

    Now that the tire is round my ‘shimmy’ went away. I never checked a new tire for out of round before but will be checking them in the future. I did check my mains while I was at it an they are round.

    gary
     
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  18. brien23

    brien23 Cleared for Takeoff

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    Cessna did not balance the nose wheel with weights when new and your C-182 aircraft when new did not have that problem. You along with others have parts worn out in the nose gear that are causing the problem. Find someone to fix it right and get away from the idea that the nose tire needs weights to balance it will fix all.
     
    Last edited: Apr 1, 2020 at 10:52 AM
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  19. Dan Thomas

    Dan Thomas Final Approach

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    This overlooks something really important. When Cessna built that airplane, all the parts were new and the tolerances were all very close. The tires were US-made, not from Asia like so many are now. Those tight clearances and good-quality tires meant that you felt nothing even if there was a bit of imbalance. Now, a mechanic can replace the shimmy damper mounting bolts and bushings but he can't economically replace the oleo housing with its wallowed-out mounting hole or the oleo strut with its wallowed-out hole. He can't economically replace the shimmy damper cylinder with its wallowed-hole either. He can take the oleo apart and re-shim the steering yoke to reduce the rocking on the housing, but he can't replace the metal worn off the top side of the support flange and so he can't remove all the rocking. He can replace the torque link bushings and bolts but he can't economically replace the nosewheel fork that has the wallowed-out torque link mounting holes in it, nor the oleo housing with its wallowed-out holes. (Edit: those holes get wallowed out because the mechanic doesn't torque the bolts up properly.The bolts pass through a bushing (sometimes called a spacer) that is inside the torque link bushings, and the bushings are supposed to run on the spacer, which is supposed to be pinched between the lugs on the oleo and fork or strut collar so it can't move. They get left loose, and the bolt ends up pounding the holes out on those aluminum castings and the spacer's ends eat divots in the inboard sides of the lugs. There is nothing in Cessna's manuals or service bulletins about it. From an engineering pespective it's an adequate setup if the thing is assembled right. Most aren't.)

    A guy could spend vast sums on all this stuff. And even if he did, a dynamically-imbalanced wheel will wear it all out again in short order. I learned to fly in a six-year-old 172 in 1973 that had awesomely horrible nosewheel shimmy. Less than 2000 hours on it, likely. So bad it blurred your vision when it happened. With a US-made tire, too.

    Now we have "aviation quality" bias-ply tires that aren't round sometimes and have high spots off-center sometimes. An off-center high spot moves the center of the contact footprint around and makes the wheel want to turn one way or the other. That's one issue, but the other is that these tires are often way out of dynamic balance. Having inner tubes doesn't help. They're not uniform either.

    Still no radials for light airplanes. It's silly.

    If the tire/wheel assemblies on a brand-new 1970 Cessna had been dynamically balanced and kept that way, it would still be on its original steering stuff and it would still be tight.

    Cars always get dynamic balancing and have for a lot longer than you younger guys have been alive. You have never experienced the runaway front-end shimmy we older guys sometimes did when the wheels got a static balance only, and constant steering-wheel shake in cruise. And steering stuff that was worn out as a result. And, like I said before, if you have never fixed nosewheel shimmy by dynamically balancing that nosewheel, your experience is deficient. You can argue against it all you like but it means nothing if you haven't tried it.
     
    Last edited: Apr 1, 2020 at 7:52 PM