Nose or Tail-wheel Shimmy

[RadioSaigon]

On a recent flight with another pilot, we both experienced shimmy of varying intensities during our (respective) landing rolls. The techniques to deal with it varied too... the other pilots response seemed to be to ignore it, with the panel disappearing in a blur of lateral movement (intensifying the longer it was left unresolved), a rapid tightening of my sphincter as I visualised the stresses being placed on various airframe systems and components and a generally exceptionally uncomfortable ride for all aboard -until I hopped on the brakes myself due the PF's lack of action.

On my landing, similar scenario, vastly lower frequency of vibration. At 1st sign of shimmy, I came full-back on the elevator which (naturally enough) raised the nosewheel off the ground, stopping the shimmy, after which I re-landed the nosewheel with no further problems.

I'm interested in people's opinions of:

• what causes the shimmy -other than piss-weak shim dampers of course!
• what methods/techniques do others use in response to shimmy?
• what method/technique is best?
• what are the potential downstream issues likely to be faced on an airframe subjected to repeated, unresolved shimmy?

[theIronPaw]

Always use the reduce pressure on the tailwheel-wait for the shimmy to stop-and then reapply pressure method -but a word of caution: when rolling after landing some light aircraft have enough elevator authority at full aft travel to scrape the tail skid (even with a forward CG).

[TestPilot]

What causes it?

Unbalanced wheel or tyre/uneven wear/not fitted 'square'/loose/not greased regularly etc...and/or a worn out shimmy damper for nosewheel aircraft.

I never experienced tw shimmy on grass that I can remember - seemed to only happen on sealed runways - one of the reasons I eschew 3 pointers on tarmac.

I think, for tw aircraft, keep the system greased with fresh grease regularly, keep it slightly on the tight side of perfect with respect to how easy it castors (if there is a clutch that gives a steering/castoring range) and keep the weight off it as long as possible on tarmac. Unloading the wheel if it shimmies is just basic airmanship. Certainly unrestrained shimmy does enormous damage over time and WILL lead to failed components - over the years in PNG I had several occasions where a brief shimmy caused the center bolt to fail completely and the whole shitfight departed the aircraft.

On a nosewheel aircraft you risk damage to the firewall where it mounts in fixed gear aircraft and, in retractable gear aircraft, you are hammering the extension/retraction system components which could easily lead to not being able to extend the nose gear one day...with all that entails.

Nut torque on the king post of the scott T/W unit is critical to minimising shimmy. The later McCauley units have a series of compression spring around the T/W head, that apply friction to the fork to stop it from shimmying.

To stop it, if it starts, requires you to remove load from the T/W by polling forwards(185 etc). It is worse on the tar.

The 210 is also a bit prone to shimmy. Keep dampers and any associated links in good order. If it occurs, apply back elevator and remove the load.

If left unchecked, potential damage would be substantial.

On the 185, you cannot keep your feet on the pedals if the shimmy gets going.

Write it up in the Tech Log and get the engineers to fix it!

[jfp]

Non engineering factors that seem to encourage shimmy include:
Higher groundspeeds (light or or tail wind, heavy loads)
Hard surface (never had a noticeable shimmy on grass or loose gravel)
Dropping the nosewheels onto the runway (rather than easing them down)
Rough runways
Forward Centre of Gravity

If you can't lift the nose gear back into the air, a significant reduction in speed is often the only thing that will stop a bad shimmy. Sometimes unloading the nose gear can even make the shimmy worse. Heavy braking is often the only option - and all it does is reduce the number of oscillations by reducing the time that the wheel shimmys.

I've seen a bent trolley axle and destroyed trolley blocks after only four landings after new trolley blocks had been fitted. Of those four landings it was only the last that experienced shimmy - and apparently it was so violent it nearly pulled the aircraft off the runway.

[DeSarge]

Nose wheel shimmy is largely a design fault - the nose wheel leg should slope forward from the vertical by at least 23 degrees. When designed that way a shimmy damper really isn't needed as the wheel tracks properly by itself. Look at how a bicycle's steering is arranged. Riding no hands is easy but try riding one with the handlebars reversed 180 degrees thus making the steering leg effectively vertical or even sloping back.

Knowing the above won't help much in changing an existing design fault but it does partly explain why hauling back on the stick stops shimmying - you are increasing the forward slope nearer to 23 degrees. Another reason pulling the nose up helps is that all three scissor hinges then ride in less worn areas thus taking out some of the play. There are quite a few linkages between the nose-gear leg and the shimmy damper on Cessnas. All these linkages wear, and the sum total of play can be quite large. Some people jack their aircraft, grab the nose-wheel and wriggle it, and conclude that they haven't got much play. However, the nose-wheel is not in this position when it shimmies. It has the weight of the plane on it, and this is why checking for play should not be done while jacked. The best way is to roll the aircraft forward on to two flat sheets of plastic or metal with plenty of grease between the two sheets. This allows you to wriggle the nose wheel leg laterally to determine the amount of play and where it is. It is amazing how much more play there is in the normal running position than when hanging off the jacks. Personally I get my 2 flat sheets by cutting the sides out of a rectangular-shaped 1-litre oil container, which are usually already lubed, and readily available.

Finally, holding the nose high on a Cessna 210 should be mandatory as it helps prevent propellor damage from sucking in rocks. Since I have to pay for my props I would be most unhappy seeing someone startup, taxi, take-off or land my aircraft without the stick being well back.

[wrx185]

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 has been 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.