nose- or tail-wheel shimmy
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95% of the time the tire is out of balance and should be either changed or balanced, or occasionally if there are 2 nose wheels; a pressure difference
like a car with a shaky steering wheel
if you are using grease in the scissor links to take away shimmy it is past time to buy a new set of scissors, or shim/rebush them
rutan, you have never apparently dealt with an aircraft with a trailing link style landing nose gear, if the wheels are behind the leg then they have no option to go straight and follow it, admittedly the ones i work with have hydraulic NWS, but they are quite manageable with differential power and brakes to steer, and the other ones i work with have a vertical legs and work well when all the wheels balance weights are attached (they do fall off now and then) (i believe all boeings, emb 120, cessna 441, and metro have vertical legs, so the 'bicycle' theory needs work more to convince me)
my experience only
like a car with a shaky steering wheel
if you are using grease in the scissor links to take away shimmy it is past time to buy a new set of scissors, or shim/rebush them
rutan, you have never apparently dealt with an aircraft with a trailing link style landing nose gear, if the wheels are behind the leg then they have no option to go straight and follow it, admittedly the ones i work with have hydraulic NWS, but they are quite manageable with differential power and brakes to steer, and the other ones i work with have a vertical legs and work well when all the wheels balance weights are attached (they do fall off now and then) (i believe all boeings, emb 120, cessna 441, and metro have vertical legs, so the 'bicycle' theory needs work more to convince me)
my experience only
Last edited by Connaught; 16th May 2009 at 12:46.
My flying school cross-hired a Cessna 150 at Essendon. The nosewheel tyre pressure was very low and we had to pump it up to correct pressure before we flew it. On take off and landing there was severe nosewheel shimmy and we wrote up the defect in the maintenance release and returned the aircraft to the owner (he owned his own flying school).
The owner went absolutely ape when he discovered the defect was written up and demanded to know why we had pumped up the nose wheel tyre to the correct pressure.
Turned out he knew about the nosewheel shimmy but decided he didn't want to incur maintenance costs to get it fixed so he simply dropped the tyre pressure to less than half of the correct pressure and that minimised the nose wheel shimmy. The other reason why he spat the dummy was because his maintenance release sheet had been squeaky clean and our defect entry spoiled the look of it..
The owner went absolutely ape when he discovered the defect was written up and demanded to know why we had pumped up the nose wheel tyre to the correct pressure.
Turned out he knew about the nosewheel shimmy but decided he didn't want to incur maintenance costs to get it fixed so he simply dropped the tyre pressure to less than half of the correct pressure and that minimised the nose wheel shimmy. The other reason why he spat the dummy was because his maintenance release sheet had been squeaky clean and our defect entry spoiled the look of it..
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Connaught
On reflection I was unfair to use the term "design fault" ."Design compromise" would have been more accurate.Designers have to find a balance between the superior strength,lighter weight and compact size of a vertical NGL and the need to have a lot of complex antiflutter (that's what shimmy is) devices. On an airliner it's probably a no brainer but on smaller stuff there's a fair bit of head scratching needed.
I haven't any experience with trailing arm nose gear in bigger aircraft but I know in smaller ones eg small Grummans,Vari-ezes ,Long-ezes Bede 4s etc the trailing wheel will flutter often breaking the NW fork. These aircraft must be checked often for correct shimmy damper tension. If you are looking for an example of trailing wheel shimmy at very low speed just observe the wheels of a Woolworths shopping trolly next time you go shopping.
Chimbu
I would rather pretty up now and again a pair of lead filled elevator bell horns than buy a new set of C210 prop blades. That being said I've certainly seen pilots going instantly to full power on loose gravel strips-it's just a shame they don't have to pay for the damage. I'm sure they'd learn quicker that way.
Cheers
On reflection I was unfair to use the term "design fault" ."Design compromise" would have been more accurate.Designers have to find a balance between the superior strength,lighter weight and compact size of a vertical NGL and the need to have a lot of complex antiflutter (that's what shimmy is) devices. On an airliner it's probably a no brainer but on smaller stuff there's a fair bit of head scratching needed.
I haven't any experience with trailing arm nose gear in bigger aircraft but I know in smaller ones eg small Grummans,Vari-ezes ,Long-ezes Bede 4s etc the trailing wheel will flutter often breaking the NW fork. These aircraft must be checked often for correct shimmy damper tension. If you are looking for an example of trailing wheel shimmy at very low speed just observe the wheels of a Woolworths shopping trolly next time you go shopping.
Chimbu
I would rather pretty up now and again a pair of lead filled elevator bell horns than buy a new set of C210 prop blades. That being said I've certainly seen pilots going instantly to full power on loose gravel strips-it's just a shame they don't have to pay for the damage. I'm sure they'd learn quicker that way.
Cheers
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Every C150, 172, 172 and 182 I see taxi has the control column fully forward, with or without instructor, and in nil wind. The poor poor nose leg, no wonder they shimmy.
I've found over the years the thing most responsible for nosewheel shimmy is simply lack of proper maintenence. None of the Cessnas that I regularly fly (172 182 182RG & 210) ever have a problem with shimmy at the front end. As rootin around has suggested, if all the hardware is torqued up to where it should be, the slop is taken out of the mechanism and the shimmy dampener is serviced when required, there shouldn't be a problem.
On the 210 that was purchased recently, it was obvious that the shimmy dampner had never ever been serviced since it had rolled out the factory door back in '82 and in 1700 hours of flying. Go figure!
I might add it definitely pays to keep these parts serviced and in good repair. When the pre-purchase was done on the 210, it was noticed (strange how noone else noticed though) that there was quite a siginificant bend or curve if you like, in the top nosewheel torque link. I'm assuming that at some point the front oleo was over inflated by some siginificant margin. We priced a new one from Cessna and had our breath taken away when told "yep, got one on the shelf and it's gonna cost you $3,300.00"!!! That's $3,300 for just the top torque link and nothing else. Ouch! We made alternate arrangements.
On the 210 that was purchased recently, it was obvious that the shimmy dampner had never ever been serviced since it had rolled out the factory door back in '82 and in 1700 hours of flying. Go figure!
I might add it definitely pays to keep these parts serviced and in good repair. When the pre-purchase was done on the 210, it was noticed (strange how noone else noticed though) that there was quite a siginificant bend or curve if you like, in the top nosewheel torque link. I'm assuming that at some point the front oleo was over inflated by some siginificant margin. We priced a new one from Cessna and had our breath taken away when told "yep, got one on the shelf and it's gonna cost you $3,300.00"!!! That's $3,300 for just the top torque link and nothing else. Ouch! We made alternate arrangements.
Last edited by gassed budgie; 17th May 2009 at 13:16.
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hahaha
forgot about the shoppin trolly,
and yes the major portion of my experience is a very big beefy airplane that fly lotsa people around, coincidently they still have scissor links that need inspecting, and serviceing, replaceing and shimming and have the 'shimmy damper' incorporated into the nose wheel steering actuators
thanks for that the shoppin trolly made me laugh (4 castoring wheels, who's idea was that????)
forgot about the shoppin trolly,
and yes the major portion of my experience is a very big beefy airplane that fly lotsa people around, coincidently they still have scissor links that need inspecting, and serviceing, replaceing and shimming and have the 'shimmy damper' incorporated into the nose wheel steering actuators
thanks for that the shoppin trolly made me laugh (4 castoring wheels, who's idea was that????)
