nose- or tail-wheel shimmy

[RadioSaigon]

A search here has revaealed countless threads in which shimmy is mentioned, but apparently none in which this event -and how best to handle it is discussed.

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?

[coke drinker]

Always use the reduce pressure on the tailwheel-wait for the shimmy to stop-and then reapply pressure method.

[Ovation]

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.

Can't help much on the cure except unloading the nose wheel as you are doing now, 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 :ouch: (even with a forward CG).

[Chimbu chuckles]

I remember experiencing shimmy on the tailwheel of the 185 in my first days in PNG and my then CP explaining that the tw was actually doing 360s (rather than a high frequency wiggle) as we rolled down the runway...looking at the marks on the tyre this seemed a reasonable explanation...it certainly felt like that...my instant reaction was to push the stick forward an unload the wheel which worked.

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 remember having an ongoing battle of will with the other pilot (there were only ever two of us at Simbu Aviation) when I later became CP over how tight the tailwheel bolt should be on the 185 - He always loosened off the bolt to give easier ground manouvering - it came out of the steering range and into castor easier - this meant it was also more prone to shimmy. I always wanted it tighter which meant it took more effort to get the tw to castor but it never shimmied. We also had an ongoing good natured argument over who got to fly the 185 and who got to fly the Islander on any given day - we both loved to fly the 185.:D

Maybe he was prescient - he died in that Islander the following year.

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 ****fight 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.

[kingRB]

one particular Cessna (RG) I did plenty of hours in during my commercial training had the most violent nosewheel shimmy.

First time I encountered it by myself, I was late in a take off roll, I just about sh@t myself the noise and vibration was so severe. I hadnt experienced anything like it before and my first assumption was I had blown a tyre.

I dont know how many times it had been written up on the MR on that aircraft, then cleared by the engineers - it still did it.

Interesting though, it would never shimmy in the landing roll - only on take off.

I found with practice that just enough back pressure on the elevator to relieve the weight on the nose stopped the shimmy until you are airborne... Similar to a soft field take off technique.
With that aircraft it only required very subtle movements to stop it... But by god what a difference it makes if you dont.

I know a bloke who ran off a runway due to shimmy on take off - he did not have any idea what it was, panicked, aborted the take off and ended up going through a fence. Can be serious business!

[185skywagon]

I agree with all Chimbu has said above.
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.

185.

[Mr. Hat]

Have had the shimmy get right out of control. Its not nice. Its worth getting your engineer to look at it because based on what i experienced i think structural failure is possible.

[PyroTek]

I was doing training in a 152 once seemingly without dampers. or ****ty shimmy dampers.
it seemed that when at a fast taxi speed it came on.. twice it happened to me from memory.. the only two times i flew the aircraft.
So, what we did was, first time.. pull the nosewheel off the ground.
Otherwise we had to slow to a very slow taxi speed.. walking pace even for it not to shimmy like that.
My instructor seemed to be oblivious to the fact (and never attempted to explain what was happening), and I had to ask Chuckles about it.

Chuckles was great help. :ok:

[Exaviator]

Am I stating the obvious here. The cure is simple. Write it up in the Tech Log and get the engineers to fix it! :hmm:

[tinpis]

What Chuckles said.:ok:

[Cilba]

It happened to me a few times in a Citabria, until an engineer advised me to get the tyre pressure spot on. This can be a fiddly job with such a small inner tube. There was no further shimmy with the correct tyre pressure.
I never had shimmy in a 185, but I did have the nut come undone, resulting in the tail wheel falling off. There wasn't much damage as I wheeled it on at Lae, and vacated very slowly on the "skid".
Cilba

[werbil]

Amphibious seaplanes can have shimmy problems with the nose gear - and like the tail draggers when they do the nose wheels do 360s around the castering pins.

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.

[sockedunnecessarily]

Exaviator is the one with the correct answer. It is not supposed to do that and needs to be written up and fixed - end of story.

Could tell you a million things it is not good for. Top three would be engine mounts, landing gear firewall mounts and damage to tubes in CRT glass displays.

[AerocatS2A]

Have had shimmy problems in a Shrike and the engineer's solution was to remove some pressure from the oleo. This seemed to fix the problem.

[ZEEBEE]

Am I stating the obvious here. The cure is simple. Write it up in the Tech Log and get the engineers to fix it!

What is not so obvious is that some aircraft (Cresco, Pac750 et al) are just prone to nwheel shimmy regardless.

An operator in WeeWaa told me that the best way to prevent the shimmy is to pump the scissor link full of grease often and it is less likely to suffer, but as others have stated, stopping it immediately it occurs is paramount.

On the 210 the shimmy can be prevented entirely by using judicious back pressure to keep the weight on the main wheels at any speed above 40 knots.

[NZScion]

On the 210 the shimmy can be prevented entirely by using judicious back pressure to keep the weight on the main wheels at any speed above 40 knots.

Which has a secondary effect of acting as a decent speedbrake reducing brake wear :ok:

[ZEEBEE]

Which has a secondary effect of acting as a decent speedbrake reducing brake wear

Well, yes and no.
It doesn't do anything for brake wear during take-off :} but does help on the landing roll.
The other benefit of course is that it puts most of the weight of the aircraft on the main wheels allowing the brakes to work more effectively (if you want to use them, that is) thus reducing tyre wear.. ie less chance of flat-spotting.
The reduction of shimmy commencement is an added bonus.

The reverse is true for the tail wheel machines of course, where judicious FORWARD pressure on the stick after landing can actually improve braking and in turn reduce the tendency to shimmy.

Too much will result in even better braking by tipping the aircraft on it's nose. Except in 180/185's where I've seen Ag pilots stand on the brakes with the tail up AND forward stick to keep it there. Definately no chance of shimmy there:E

[james ozzie]

King RB - I had EXACTLY the same experience as you on the 172RG (in South Africa) and the shimmy was constantly being fixed and then recurring. It got to the point when I would include the shimmy in my pre-take off pax briefing(!) Always on T/O and cured with some back pressure, as you say.

(as an aside, I never figured out that spring loaded steering - on one occasion I returned to the ramp as I was sure the thing was not steering properly. Engineering them proceeded to drive it round in tight circles but were also unsure if it was OK. In the end I just flew the thing...)

The shimmy presumably has its origins in the spring loading on the steering.

[rutan around]

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.

[Chimbu chuckles]

Was shown some pretty big dings in the top of a C210 elevator horn balance recently from prop-wash flinging rocks at it while the stick was held hard back...the owner was less than happy with 'some young pilots'.

I would suggest there is a happy medium?

[Connaught]

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

[Centaurus]

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..

[rutan around]

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

[tio540]

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.:bored:

[gassed budgie]

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.

[Connaught]

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????)