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Thread: Sundowner nose shimmy

  1. #1

    Sundowner nose shimmy

    I have a customer with a Sundowner and we've been trying to resolve a shimmy problem for about 2 years. Beechtalk said I needed to be here, so, since the customer's paying for it, I hope we can find a cure here. I've worked on a few Musketeers and Sundowners over the last 20 or so years and the only other two shimmy problems I've dealt with were cured with new shims in the nose strut. One was this aircraft years ago. When my current one started, I found the nose tire was a bit out of round and out of balance, so we got a new tire. When I broke it down, the tube had some wrinkles and had gotten a wad to one section, so we figured the new tire and tube would cure it. Nope. The nose strut is still nice and tight, with no slack in the shims. It has the newer style shimmy dampener that can be serviced. At last year's Annual Inspection, I replaced its O rings and serviced it. He flew it after the Annual with no shimmy. One landing only. Now, sometimes it does, sometimes it doesn't, so at the current Annual I'm doing, I removed the dampener and felt no slack when I worked it by hand and it's still full, well, the best I can tell it is. The only other a/c I've run into with this sort of recurring shimmy are Cessnas. The nose strut centering cable is centered, so does the centering spring contribute to shimmy dampening? I would have thought that a trailing link nose gear, by design, would be resistant to shimmying. I've had some Cessnas that even a freshly serviced dampener wouldn't work, but the new Lord dampener was a cure, but alas, AFAIK, Lord doesn't have one for the Sundowner. Any suggestions on where else to look or what else to try? Maybe a heavier spring on the centering cable? Any ideas are appreciated! thanks.

  2. #2
    I will be interested in seeing the responses as well. I have about 45 landings in my new to me Sundowner and 3 times I've had a shimmy. I experienced this in the 172 I used to fly so it isn't new to me, just curious if there is a fix. I'm sure my wife and passengers won't be too happy if it happens when they are with me.

    Peace,
    Don

  3. #3
    I have to tell you, I was thinking that it was the cable, and some jumping on the tire. I would double-check that, mine was out of center after donut replacement, and it did not shimmy. My IA and I loosed the screw that holds it in place and the gear literally snap back into the straight forward position. It took 24/1000th shim to solve the movement in the gear bearings, but even when it was loose, there was no shimmy.

    Keep in mind Frank, I am not an IA, A&P, and I did not stay at a Holiday Inn last night.
    Mike
    ps: Okay, you got me... I am a Super Hero!

  4. #4
    Frank,

    The first thing I'd check was for something amiss with the gear/engine mount. Maybe a bent or cracked support tube. Loose axle to gear fork installation? Bad wheel bearings? I'd jack the nose (weigh the stab or tie down the tail and jack the airplane) and check for side to side play of the NLG fork. It could be the knee bushings/pin. There could be a bad Heim joint in the NLG steering link, though unlikely to cause shimmy. It is remote, but the upper gear housing tube could be cracked at the bolt holes. If all else looks good, I'd pull the NLG and look it over real good.
    Last edited by Manhein; 04-19-2013 at 11:07 PM.
    Marty Vanover
    Tucson, Az.

    Where "Sandy Beech" has landed.

  5. #5
    Before you get too far into it, check the tire pressure. A few pounds less could be all the dampening you need.

  6. #6
    After you check all the easy stuff, take a mirror and look into the tunnel (there are cut outs under the side upholstery) and find the bell crank. Put Equal pressure on the pilots Side rudder pedals and see if you have any flex or movement in the steering bell crank. If there is flexing it could indicate loose rivets in the structure holding the bell crank. If this is lose, you need to either reset the rivets or R&R them. This requires removal of the firewall. I have seen this only once.

    One other thing to try, lube the nose strut until you can't get any more grease into it. If this reduces or eliminates the shimmy, you need to work more shims into the nose strut.

    It could also be pilot error. If the pilot is three pointing the landings, this could cause the shimmy. Is the pilot flying with two big guys in front? He may be way forward on the CG.

  7. #7
    Yeah - mine will shimmy once in a while as well - I've had my A&P look at it before and we never really fixed anything - although he did improve things... What I found works best - is make sure you keep the yoke full back in your lap on roll out... Honestly - before this plane - once the nose gear was down - I was pretty lazy and would just relax the pressure... Keep us posted...

    Regards,
    Scott
    Last edited by Clinderman; 04-20-2013 at 08:23 AM. Reason: c.t.

  8. #8
    Even tho the owner averages about 10 - 15 hours a year, he's owned it for about 12 years so I don't think it's his landing technique. He normally flies alone and he's not a big guy at all. Sometimes his wife goes and she's not too big either. When I did the tire earlier, I checked the strut for being tight everywhere as well as the bearings when I did the tire. Nothing loose anywhere. Unless the centering cable is just a tad off, it isn't off. I haven't seen any damage to the mount or anything else. I'm glad there are others with a similar problem, maybe it's just their nature after a while. I'm going to check that tire pressure, I usually do at the Annual anyway. An old trick on Cessnas is to put about 40 - 50 psi in nose tires and that rounds out the tire and helps eliminate shimmy, but maybe this bigger tire on the Sundowner goes the other way. That just might be it. Thanks.
    Last edited by frank; 04-20-2013 at 09:37 AM.

  9. #9
    I wonder if the gear being out of alignement could cause something like this? Maybe the gear is tracking such that it is loading the nosewheel to the side? Sounds like he doesn't fly enough to notice unusual wear on the main gear tires. Is he running 17.5 600x6 tires all the way around?

    Seems you've eleminated the really obvious stuff.
    My airplane isn't slow... it's stately.

  10. #10
    Take the front wheel off and take it to a tire shop. Have them check the dynamic balance. Has your customer done anything with the tire? Perhaps the heavy mark on the tire isn't lined up with the stem.

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