I remember that video....

Just so you know, my 6A suffered that type of shimmy on it's very first landing (last Sept.). I let the nose down to quick, and the nose wheel nut needed to be torqued a bit more, as they loosen when new.

I'm now 91 hrs past that, and haven't had a landing shimmy since.

L.Adamson
 
I made a copy of one video from about this timeframe. I'm not sure if it is the one you mention as this one shows fore & aft oscillation of the nosegear spar and very little shimmy or left to right motion of the actual nosewheel.

If interested I'll put it up somewhere.
 
Clearly the fore and aft oscillation has nothing to do with fork nut preload.

I'm curious about a detail. Can someone post a few photos of a 7A engine mount and landing gear strut sans engine?
 
rvator51 said:
Here is one I posted back in 2006.

Nosegear shimmy
Click to expand...

That's the same one I have a copy of.

For the fore & aft oscillation shown, important info would be what type of nose wheel installed, whether there is a 'spacer' between the bearings on the axle bolt, torque on the axle bolt, rolling friction, etc.
 
GrayHawk said:
For the fore & aft oscillation shown, important info would be what type of nose wheel installed, whether there is a 'spacer' between the bearings on the axle bolt, torque on the axle bolt, rolling friction, etc.
Click to expand...

None of those things matter if the wheel/tire assembly is truly round, and if the there is no variation is rolling resistance with each revolution.
 
DanH said:
....I'm curious about a detail. Can someone post a few photos of a 7A engine mount and landing gear strut sans engine?
Click to expand...
Dan,

I don't know if these photos of the engine mount fitted to my -6A can help satisfy your curiosity. I suspect this engine mount is very similar in design to the -7 series nevertheless, the shimmy condition has been known to occur in the -6 series too.

11rwrvd.jpg

op8n6p.jpg
 
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Thanks Rick. I was thinking abut truss stiffness for the nose gear tube. Easy to make a structure strong enough, but sometimes difficult to make it stiff enough.
 
DanH said:
None of those things matter if the wheel/tire assembly is truly round, and if the there is no variation is rolling resistance with each revolution.
Click to expand...

True, if the world was perfect, we would not have any nosegear problems.

But the world in not perfect, the tires & wheels are not perfectly round, the rolling resistance is sometimes high and not uniform, the landing strips are not smooth and without bumps.

So we have people changing their nose wheel to a better design to minimize these effects. We have people adding spacers in between the bearing 'biscuits' so they can torque their front axle nut to the specified value without causing the rolling resistance to go sky high & uneven. And we have people avoiding rough grass strips, rabbits (yes a rabbit in an NTSB report), and chuck holes so as not to aggravate things.

So I repeat for the video's case, it would be nice to have more details on those things.
 
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nose wheel

After watching the nose gear strut going through the vibrations of taxi and take off I see nothing out of the ordinary. For such a simple design it works efficiently. A single metal rod that is tapered and bent where needed to absorb stresses without BREAKING through its many years seems to be an engineering marvel. It supports the engine, wheel and airframe without a shock absorber. Why its amazing. Compare it to the training fleet.

Tad Sargent "Stripes"
Team RV
7A
 
GrayHawk said:
That's the same one I have a copy of.

For the fore & aft oscillation shown, important info would be what type of nose wheel installed, whether there is a 'spacer' between the bearings on the axle bolt, torque on the axle bolt, rolling friction, etc.
Click to expand...

2nd generation nose gear for RV-6a. standard Vans nose fork (old style). No spacer installed. Tire dynamically balanced. Different variations in air press and break out force played with.

RV-6a now has Vans latest nose gear/nose fork combo with the new Matco spacer installed. Shimmy has decreased dramatically although it still has a little if I can recall correctly what the builder told me.
 
The first video is not a shimmy problem, but something much worse. The nose gear is an undamped spring. What appears to be happening is that there is a forcing function that is operating at one of the natural frequencies for this particular gear. The forcing function can be a combination of rough runway, varying coefficicients of friction through various sectiions of the runway, and or an out of round tire. The mass of the tire is also critical. In this situation, if the forcing function is not removed, the system will continue to build engery until it fails, most likely a nose over. Just an engineers perspective.
 
At anything above 20 mph, the nose wheel should be in the air. With the proper CG, there would be no probelm with this speed. But the wheel does need to spin with little seal drag or too tight bearing set.
 
YUP

ferret said:
The first video is not a shimmy problem, but something much worse. The nose gear is an undamped spring. What appears to be happening is that there is a forcing function that is operating at one of the natural frequencies for this particular gear. The forcing function can be a combination of rough runway, varying coefficicients of friction through various sectiions of the runway, and or an out of round tire. The mass of the tire is also critical. In this situation, if the forcing function is not removed, the system will continue to build engery until it fails, most likely a nose over. Just an engineers perspective.
Click to expand...


At my local field the joints in the taxiway concrete are at exactly the natural frequency of the nose gear bounce for normal taxi speed (this is the slower up and down boing, not the for/aft shudder) ..I mean you couldn't place the joints like that if you tried..So of course the oscillations get bigger and bigger.

The solution is to taxi across the joints at about 30 degrees..the extra wavelength takes the vibration outside the natural frequency and danger of prop strike is averted..:)

Frank
 
ferret said:
The first video is not a shimmy problem, but something much worse. The nose gear is an undamped spring. What appears to be happening is that there is a forcing function that is operating at one of the natural frequencies for this particular gear.
Click to expand...

Well said Doug. Most "fixes" appear to be an attempt to reduce the power of the forcing function. Not unreasonable, but a shift in natural frequency might be a better approach.
 
X-Z plane satisfies both

Unfortunately gear spring deflection and fore-aft natural frequency oscillation are in the same vertical plane. I'm not sure how the natural frequency could be raised without increasing the effective spring rate. The best advice remains to keep the nose wheel off the ground as long as the elevator is effective since tire RPM and speed are a 1:1 relationship.

Any other design than a tapered nose leg would increase complexity and increase frontal area. "There's no such thing a free lunch." [ancient engineering expression]

Tires can be forcing function even when they look round.

They may look round and black and can, in fact actually be round in the unloaded condition and still cause problems. Think of your tire as a series of radial springs, compressing as the tire rolls on the surface. If the local spring rate isn't the same all the way around the tire, then the tire has a "radial force variation." Auto manufacturers specify maximum radial force variation as one of the purchasing specifications to tire manufacturers. Every OEM tire is inflated on a special 2-piece wheel and runs under load against a large drum because there is no way to know what was built until it is measured. When I was doing this work in the auto industry, the 1st order radial force variation for some tires was as low as 12 lbs. maximum! The colored wax dots on the side of an automotive tire show how close to perfect it was made. For a given tire size and model, the car companies get the best tires, the manufacturer's tire stores get the next best, then independent tire stores. Don't believe anyone that tries to tell you a "BLEM" is a cosmetic defect. What that really means is that the tire was most probably square.

FWIW, in the video, the oscillation appears to begin in the range of 28 mph or 41 ft/sec. A 16" diameter tire has a circumference of about 4.2 feet, so at 28 mph the tire is turning about 600 RPM. An automobile going 60 mph will turn 600 tire RPM at about 60 mph. This may be another argument for the slightly oversize tire that some folks flying off turf strips are using. To reach the RPM that starts the natural frequency forcing function would require a slightly higher speed when using the larger tire.

Larry
 
LarryT said:
Unfortunately gear spring deflection and fore-aft natural frequency oscillation are in the same vertical plane. I'm not sure how the natural frequency could be raised without increasing the effective spring rate.
Click to expand...

Good point. Although raising the effective spring rate might not be a big deal, a reduction in unsprung mass is the other option.

Great tire info. Given your explanation about radial force variation, I can easily buy it as forcing frequency. Rotation frequency doesn't match the leg's observed natural frequency, but it may be a multiple.

Just for fun, here's a different cause-effect theory that might explain the randomness of nose gear failures. Note Frank's report about expansion joint spacing. If a 3.1hz natural frequency is accurate based on counting/timing oscillations in the video, all it would take is a series of bumps, lumps or waves in the surface, spaced so the nose wheel rolled over them 3.1 times per second. The required spacing varies with speed. At 20 MPH it would be about 9.5 feet, 35 about 16.5 feet, and 50 around 24 feet. Any sort of bump would do (grass clumps, tire tracks, even pebbles) as long as you hit three or four of them in series at an unfortunate velocity for the spacing.
 
Repetitive external forcing function...

...is also a good point. As is reducing unspriung mass.

Here is one way to reduce unsprung mass. I talked to Remi Beringer about his wheels and brakes when he first started advertising on VAF. (Beringer makes wheels and brakes for racing motorcycles) I believe there would be a reduction in unsprung mass using his (expensive) components.

There were several features I liked about his brake and wheel products: Wheels are completely CNC machined from tubing or bar stock (i.e., no castings, no porosity), the 380/150 x 5 tire is available from Michelin as a tubeless tire with a xxxxxbutyl inner liner so it won't leak air, the brake is an internal fixed piston caliper with a floating external rotor (eliminates the rotor "hat" section). The rotor is a surface ground plate. Easy and relatively inexpensive to replace. I doubt that a tubeless tire could be used with a cast wheel without very frequent tire pressure correction. I am still on the fence about buying these components, but perhaps by the time I need to make a decision, the US dollar will have improved relative to the Euro.

Larry
 
The botom line is that the nose gear can build sufficient harmonic energy to bend the nose gear back far enough for it to grab the ground and do a nose over. This can happen in a very short period of time (several seconds). The conditions that will cause the exact harmonic induction can never be determined in advance. The only preventative approach is to do soft field take off and landings when the surface conditions are questionable, i.e. anything other than a well maintained hard surface runway.
 
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