I was pondering the ES nose wheel shimmy issue while walking the dog, and it occurs that we have enough information and smarts in the LML community to put this one to bed.  It is just to persistent not to warrant a full court press.

 

Disclosure: I am finishing a Lancair IV, and so can comment only as an “outsider.” 

 

Below I offer some basic insight into vibration, speculate on causes, and offer some suggestions, but they are all based on the anecdotal reports of LML, some work in shock/vibration many years ago, and a course in vibration theory in engineering school during the Paleocene.  With that in mind….

 

Some Basics

 

Shimmy is vibration, and shimmy suggests vibration at or near the natural frequency (resonance) of an oscillator which is somehow excited to do its thing: oscillate at its preferred frequency. 

 

Mechanical oscillators usually trade energy between a spring (anything that can store energy as a function of deflection) and the kinetic energy of motion in which the spring energy is traded for moving a mass.  You need a spring to store energy (it can be a beam, strut, engine mount or some other bendable structure), a mass to wiggle, and an excitation force to start the wiggle.  When the frequency of the exciting force equals the natural frequency then – bingo! - you have resonance and lots of big wiggles – IF the oscillator is not adequately dampened by friction.  

 

The excitation force can come from within the mass (a vibrating engine), from the outside world applied to the mass (a gust bending your wing) or from through the spring from the “base” moving up and down such as a speed bump thumping your car through the suspension.  The responses are all roughly the same for purposes of this discussion – when excitation force equals natural frequency, you get resonance -  so we will use the simplest model.

 

 

An example is shown below, copied from my old theory of vibrations text book.

 

 

Here we have a base anchor point (top), a spring and a mass with a damper in parallel with the spring.  The weight wiggles up and down, and in this example the excitation force is applied to the weight.  The spring can be a torsion spring and the weight can rotate back and forth, but the behaviour is essentially the same.  

 

Here is how movement of the weight responds to the excitation force as you vary its frequency.  Ignore the top right chart which shows phase angle, and focus on the larger chart showing the magnification factor.  Frequency is on the bottom axis and the magnification factor of the weight movement is shown on the vertical axis.  The different curves show different levels of dampening.  Omega sub n is the natural frequency of the system.  It is defined by the stiffness of the spring and value of the mass.

 

First message: the greater the dampening, the lower the magnification factor.   When the excitation frequency is right on the natural frequency, we call it resonance.  If the dampening is low and resonance occurs for a very long period (which can be millions of cycles for a crankshaft or less than a second for wing flutter) the structure can fail. 

 

So dampening is good for minimizing vibration at resonance. 

 

Staying away from the natural frequency is good, but for us, not possible.  Since we are landing at (say) 80 knots and slowing to zero, the excitation frequency, if it comes from tires, covers a broad range down to zero.  It is hard to change the natural frequency (goes as the square root of spring constant over mass) so it is better to zip through the natural frequency quickly to prevent the vibrations from building up.

 

Complications

 

The key factors are the mass, the stiffness of the spring, the amount of dampening, and the frequency and amplitude of the exciting force.  The complication arises in from the fact that we do not have a simple spring mass system.  Rather we have somewhat flexible strut mounted on a less flexible engine mount which is mounted on a big mass (the airframe) which is not really a fixed base like the foundation of a building, and can be wiggled as well.  Also, the excitation can come from multiple sources from outside the wheel/strut assembly (main gear brake wiggles which push the nose wheel from side to side) or from within the wheel/strut (nose wheel imbalance and from the self-centering forces arising from the caster angle of the strut).

 

More complication: the self-centering force arising from the caster angle is like a spring.  It applies a restoring force that tries to push the nose gear to an equilibrium position.  The restoring force from the caster angle can be BOTH like a spring (restoring force gets larger as the wheel turns more) and an excitation force since the tire interacts with the ground and can collect energy from the ground/tire forces when things start to wiggle.  And the tire patch also offers some dampening.  The picture below gives an inkling of the complications.  (Please forgive my artwork.)

 

Clearly we have lots of springs, forces, and masses (including torsion springs and masses) in the system.  Add the fact that portions of the airframe like to wiggle, and it gets even more difficult to analyze.  But understanding the simple model is enough to help diagnose the problem.  Excitation forces, springs, masses, and dampening are the keys.  Let’s review each in turn.

 

Springs and Masses

 

Not much we can do here.  Basically we would like to adjust the natural frequency of the strut assembly to be so high (or so low) that real world excitation forces will not cause resonance to begin.  Fuggedaboutit.  You can not change the spring stiffness or masses enough to make a difference. 

 

With three exceptions: ES aircraft have a nose wheel pant which increases the torsional moment of inertia which reduces the rotational natural frequency of the nose gear compared to no nose pant. Moving the frequency down lower may make the torsional natural frequency come closer to the strut bending natural frequency.  If they lay on top of one another, you are more likely to get trouble.  But I speculate, and the reports suggest other causes are more likely.  

 

The second exception is the length of the strut.  Extended, it is more flexible (lower natural frequency) than when retracted.  On my 182 RG the rare nose wheel shimmy could be stopped by pushing the stick and shortening the strut.  That raises the natural frequency.  Whether you want to raise or lower the natural frequency depends on where it is, and how fast you are going.  The reverse may be true on the ES.  Any reports from pilots?

 

One possibility: the longer wing and larger tail surface of the ES make the airframe moment of inertia about the vertical axis larger than it will be on the IV.  The ES is more resistant to being rotated around the vertical axis.   If the oscillator behavior is sensitive to the rotational tendency of the airframe (the strut and the airframe wiggle together and could create coupled oscillators), the longer wing would lower the oscillator frequency a bit.  But this seems like an unlikely source compared to other possibilities.   And besides, there is nothing we can do about it except be aware of it. 

 

The third exception is fuel load.  Having more fuel increases the moment of inertia and reduces the natural frequency of the fuselage around its vertical axis.  And sloshing fuel adds dampening.  But I think this is a long shot and besides we can do nothing about it.

 

 

Excitation forces

 

We have lots to work with here.  Get rid of them.  Not entirely possible, of course, so minimize them.  What could they be? 

 

1)     Nose wheel imbalance or out of round or uneven nose tire wear are obvious causes and should be examined. The excitation force will generally be the rotational speed of the tire and that will start from about 2200 RPM (36 cycles per second) down to zero (14 inch OD tire, 80 knots touchdown speed).  Anecdotal reports suggest the problems usually occur in the 20-40 knot speed range (to be verified) suggesting frequencies of 9 to 18 cycles per second.  Those kinds of numbers feel intuitively correct. 

2)     Main gear imbalance: same thing.  Imagine if both tires are out of balance and they get 180 degrees out of phase so that they alternate in pulling forward and backward.  The main gear imbalance can cause the nose to wiggle from side to side, and if that forcing frequency is just right, you get resonance.

3)     Main gear braking action can be a problem for the same reason as main gear imbalance, although the forces can potentially be much larger and independent of rotational speed.  Imagine a brake disk that is of variable thickness.  You will get wiggles as you brake and they will get worse the harder you press. Reports suggest that going to high energy brakes can help.  Brake fade has been reported on low energy brakes. Brake fade comes from high temperatures which create a lot of gas emission from the brake pad which lubricates the pad/disk interface.  Essentially gas pressure holds the pad off the disk.  Changes in the surface texture on the disk can cause variability as the disk rotates because the gas escape and friction can vary.  High energy brakes substitute materials that reduce gas emission rates, pad designs that let the gas escape and rotors that can take higher temperatures without bad behavior.  ES brakes maybe more subject to brake fade than the IV because the rotors and calipers are inside the wheel pant and can not cool as well.  If the low energy brake disks warp from the higher level of heating it could well contribute to increasing any exciting forces coming from the main gear.  Add this to wheel/tire imbalance, get everything in the right phase, and these forces could be large enough to cause problems, and also cause the airplane nose to bob up and down a bit as well.

4)     Nose gear caster angle creates self centering restoring forces that can be both spring forces for an oscillator, and exiting forces for that same oscillator.  The caster angle clearly has to have some effect, and I suspect the tire pressure does as well since a wiggling nose tire is making S turns down the runway.  As the nose wheel makes its S turns, more caster angle will result in larger vertical forces as the tire to rotates around the strut central axis.   If the strut is not vertical viewed from the front, there will be a continuing side force trying to turn the nose wheel.  This force and forces opposing it (to keep the airplane straight) could yield excitation forces. 

5)     Bumps from rocks and bumps on the runway create a one time hit to the oscillator, like ringing a bell.  If the shape of the pulse created has a lot of energy at the natural frequency of the system, it could excite the oscillator which otherwise may have remained quiescent during the deceleration. But I think it would take a long bump.  If the natural frequency corresponds to, say, 30 knots (50 feet per second), an oscillation of 10 cycles per second means one cycle consumes 5 feet of travel distance.  A half wavelength bump would 2.5 feet.  Such a bump would contain a lot of energy at 10 cycles per second.  But 2.5 to 5 feet seems to suggest a single tire rotation more than a bump.  But things can add up.

 

Dampening

 

Here again we have something to work with.

1)     Strut oil is the obvious target.  Anecdotal evidence suggests that hot struts thin oil and reduce dampening to the point that shimmy can occur.  On the IV, the strut is baked under the engine during flight, but cools somewhat during descent, and then is cooled a lot when the gear is extended.  Is 3-5 minutes enough time to bring the oil temperature in the hot strut down close to ambient while you motor around the pattern?  I think so given the 100 knot cooling blast.  So controlling strut heating by preventing hot air flow down the strut fairing seems worthwhile.  Higher viscosity oil should help as well, but watch out in Minnesota in the winter.

2)     There is dampening in the nose tire patch that contacts the runway, particularly if the nose wheel starts to make fast S turns.  A bigger tire patch from lower pressure would seem to help.

3)     There is also dampening in structure: strut, strut to engine mount connection, engine mount dampening (probably very low for the stiff welded structure) and perhaps even some effect from the elastic engine mounts as the nose gear shimmies under the engine mass.  But there is not much we can do except to check that everything is tight.

 

Checklist

 

This discussion suggests a checklist that would include the following:

1)     Tire balance, mains as well as nose wheel.  And check that they are round and not unevenly worn.

2)     Brake pulsations caused by warped or variable thickness disks that may have been overheated.  Substituting high energy brakes or maybe just a resurfacing of the brake rotor could help.

3)     Keep the strut cooler by blocking hot airflow down the strut, or increase the viscosity of the oil.  Lancair IV drivers could put the gear down earlier, particularly on a hot day, to provide for more strut cooling.

4)     Check the strut and engine mount structure for free play.  Make sure everything is tight

5)     Look for cracks on the engine mount.  These can change the spring rate and natural frequency which may worsen the tendency to shimmy.

6)     Adjust nose tire pressure, probably downward.

7)     Check strut to make sure it is vertical viewed from the front. 

 

My guess is that those with persistent shimmy problems probably have several factors at work that sometimes work together to create the shimmy.  Corrective action in several places will be required.

 

Suggestions

 

I suggest that some enthusiastic ES builder compile a big table of evidence from the ES community, those that have shimmy problems and those that do not.  Record as much information as possible in a consistent format.  I suggest listing subject columns across the top, and builder name and incident down the left side.  Multiple incidents from the same builder would be recorded separately. Column subjects would be those areas identified above: Tire balance and roundness, tire pressure, brake condition, strut cooled or not, speed when shimmy occurs, control wheel forward or aft during shimmy, OAT when problem occurred, CG location (which affects strut extension), oil used, and any stray comments.  Not every report will record all data, but something is better than nothing.

 

I bet that once this process is begun and the various incidents and conditions are recorded, the patterns will become clear and suggest which the big contributors to shimmy are, and which are smaller but still can make a difference. 

 

Be sure to share the results with all the rest of us!

 

I hope this helps.  It is a good exercise for the little gray cells.

 

Fred Moreno