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[Jeremy McCreary]

Following Iacopo's lead, I took some slow-motion video of my hand twirling a low-drag LEGO top of (by Iacopo's standards) small to moderate AMI. Twirls appear to start with just the thumb and forefinger, but the wrist soon starts to roll and bend so as to increase relative thumb-finger velocity. These slight wrist motions stop when the fingers stop, right after launch.

Is this wrist involvement consistent with Iacopo's empirical SUC? I think so. Since SUC slope steepens progressively and then levels out quickly just prior to launch, the net torque-speed curve (TSC) torque-time curve (TTC) must do the same at constant AMI. As the wrist enters the twirl, it adds powerful new muscle groups to those already at work. And as Iacopo suggested, the driving finger muscles may get stronger, too. The net torque at the stem would then increase with time due to applied muscle power alone, and the slope of the SUC would follow suit.

But muscle power is surely only part of the story, because the lever arms through which the various muscles act also change as thumb and finger positions and configurations evolve over the course of the twirl. Alan suggested that these lever arms generally become more favorable with time, and like Iacopo, I'm inclined to agree. Hence, net torque at the stem -- and therefore the SUC slope -- would have not just one but two independent reasons to grow through most of the twirl.

What about the very end of the twirl? When SUC slope and net torque finally tail off just prior to launch, perhaps the hand is in part preparing for a release favoring fine tilt control over net torque.

[Iacopo]

but the wrist soon starts to roll and bend so as to increase relative thumb-finger velocity.

This is very true. In my hardest twirls I even tend to exaggerate a bit the movement of the wrist and I think it helps.

[Aerobie]

A few weeks ago, I tried to stir up some maker interest in EDC tops.  I've been pursuing this and today I made a 1.5", 51 gram, top which I twirled for 20:30.  I'll bet Iacopo can twirl it longer.  Are any of you top makers motivated to beat that?  Let's call it the 50 gram division.

Alan

[Iacopo]

51 gram, top which I twirled for 20:30.  Are any of you top makers motivated to beat that?  Let's call it the 50 gram division.

I think I could try, when I will have some more free time.
Now I am taking measurements of tip friction and air drag of different tops of various sizes, the collected data could be useful for improving the design. But it is taking much time.  I will post all these data.

Look at this top, it weighs 51 grams, like the your, but it is made of tungsten, (the spindle is made of aluminum and the tip is a ceramic ball); it has been twirled for 49:27.   Really impressive.  I suspect that the use of denser materials for the flywheel is especially advantageous for littler tops.  The top ends spinning in upright position, if it toppled down it would have spun for less time.  Even so, this is a really long spin time.

[Aerobie]

Wow!!!!  You've won the contest already!  What is the diameter of the wheel and the ball?

Alan

[Iacopo]

Wow!!!!  You've won the contest already!  What is the diameter of the wheel and the ball?

Oh, it's not me !
George Sherwood, the owner of the top, in the comments below that video in YouTube, says it has been made by Dave Kemner.  Dave Kemner has a website where he sells his tops. 

https://www.kemnerdesign.com/collections

But I have not seen this tungsten top in his website.  You could try to write him, or George, to ask for more info about this top, if you want.

George says it weighs 51.2 grams.  So I think the diameter should be about 30-35 mm.
I don't know the diameter of the ball tip, but if you look at the behaviour of the top when it is spun, it doesn't seem a large ball.  I would say maybe a 5 mm ball, or something so.

 

[Jeremy McCreary]

A few weeks ago, I tried to stir up some maker interest in EDC tops.  I've been pursuing this and today I made a 1.5", 51 gram, top which I twirled for 20:30.  I'll bet Iacopo can twirl it longer.  Are any of you top makers motivated to beat that?  Let's call it the 50 gram division.

That 20:30 is a good spin time by any standard. What does the top look like?

The challenge is a 50-gram EDC-style top with an even longer spin time?

[Aerobie]

I checked the Kemner site.  They don't currently have any tungsten nor any with lightweight (relieved) center hub.  All their wheels are currently solid cylinders.

Incidentally, my 20:30 top has a 5mm white ceramic ball.  I usually go for steel or tungsten carbide, but grabbed ceramic for this one. 

I think that it would be impossible to reach the time in that video.  To run that long, the energy input would be about 6 times the input of my 20:30 run.  I doubt anyone can twirl that hard.  My longest string twirl of a 400g top falls far short of that video.

Alan

[Iacopo]

I checked the Kemner site. 

At the bottom of this page I have found a picture of this tungsten top where you can see its ball tip:

https://www.kemnerdesign.com/blogs/news?page=1

[Aerobie]

The Kemner tungsten top is approximately like my most successful design.  My design is basically a cylinder with a groove on either the upper or lower surface to eliminate weight in the center.  The "hub" thickness at the bottom of the groove is about 1 mm. 

I began top-making with Delrin hubs, but a single material and 1 mm bottom thickness is about the same weight.  Although the same weight, the grooved design has more "wetted" surface.  But the performance is similar, so I think the air in the groove rotates with the top.

I've not explored rounding as much as Iacopo has.  Most of my tops are square edged.

Lately I prefer the groove on top, so I can machine the groove, center hole (for the ball) and stem with the same setup.  This achieves very good concentricity and balance.  Then I part it from the bar stock and turn the top around.  I can face the bottom so it's parallel to the top of the rim within .00001" (100 millionths).

This photo shows 1.75" OD and 1.5" OD tops.  Both wheels are about 0.25" thick.

Alan

[Jeremy McCreary]

This photo shows 1.75" OD and 1.5" OD tops.  Both wheels are about 0.25" thick.

Very cool tops, Alan. The base is pretty cool, too. Did you make it?

Lately I prefer the groove on top, so I can machine the groove, center hole (for the ball) and stem with the same setup.  This achieves very good concentricity and balance.  Then I part it from the bar stock and turn the top around.  I can face the bottom so it's parallel to the top of the rim within .00001" (100 millionths).

This suggests a spin-time experiment with something useful to say about the air flows around more or less cylindrical top rotors -- including rotors like yours and Iacopo's. One of you may have tried it already.

Experimental setup: Reasonably realistic, I think...
o Sleeping top on perfectly smooth, flat, horizontal "ground": Constant or slowly decaying spin rate about a vertical stem (no precession or wobble of any kind).
o Still ambient air: No air currents other than those stirred by the top itself.
o Dominant rotor: The rotor controls AMI, TMI, CM height, and all aerodynamic effects with no significant tip or stem contributions.
o Top "U": Hub is flush with the upper face of the outer metal "ring" and recessed below.
o Top "L": Hub is flush with ring's lower face and recessed above.
o Equal CM heights and TMIs: The hub is so much lighter than the ring that Top U and Top L are effectively equal here.
o Otherwise identical: Tops U and L are the same in all other respects -- including mass, rotor length, AMI, surface roughness, and rotor rounding.

Experimental question:
o Which has the longer spin time, Top U or Top L?
o Does spinning them on the same tall, narrow pedestal change anything?

[Aerobie]

Jeremy,

Thank you, but I need translation of your abbreviations.  In general, I'm unsure of your intent in this and several earlier posts.

I've mentioned in a prior post that the performance of the groove on top vs groove on bottom is about the same.  At one time I expected lower aero shear drag for groove on bottom.  But if it's lower, it's damn close.  I think the air in the groove rotates with the top, so the bottom surface still "looks" about flat.

Alan

PS  That's a standard mirror with my added thumb screws.  I use the screws to either level it, or tilt it slightly every few minutes during a spin - to move the top to some fresh lube.  These mirrors are 2X magnification, which is hard to find, but gives longer spin time than higher magnification surfaces, which grip the ball at a wider radius.  They are about $12 on amazon.

[Iacopo]

Which has the longer spin time, Top U or Top L?
o Does spinning them on the same tall, narrow pedestal change anything?

In my experience, the spinning of a top with its bottom very near to the spinning surface, does not decrease significantly the spin time.  On the other side, lowering the center of mass even by just one millimeter, increases the spin time by 2-4 minutes, in tops like the mine. 
So the top "L" would win. 
Spinning them on a tall and narrow pedestal wouldn't change anything.
The advantage of the pedestal is not about aerodynamics; the advantages are that it allows to spin tops with lower center of mass, then tungsten carbide spinning surfaces, which I use in my pedestals, are better than common glass ones, they are more slippery and more wear/scratch resistant.   

[Jeremy McCreary]

That's a standard mirror with my added thumb screws.  I use the screws to either level it, or tilt it slightly every few minutes during a spin - to move the top to some fresh lube.  These mirrors are 2X magnification, which is hard to find, but gives longer spin time than higher magnification surfaces, which grip the ball at a wider radius.  They are about $12 on amazon.

Leveling screws -- what a great idea! Agree, lesser mirror curvature (magnification) improves spin time -- not only by reducing tip friction, but also by making it easier to twirl a sleeper from the very start. Greater curvature is better only when you're mainly after precession or strong tip containment.

Thank you, but I need translation of your abbreviations.

Sorry, AMI = axial moment of inertia, TMI = transverse moment of inertia about the tip, CM = center of mass. These acronyns have become fairly common on this forum. I'd normally define any others on 1st use in each post, but maybe I missed some.

In general, I'm unsure of your intent in this and several earlier posts.
I've mentioned in a prior post that the performance of the groove on top vs groove on bottom is about the same.  At one time I expected lower aero shear drag for groove on bottom.  But if it's lower, it's damn close.  I think the air in the groove rotates with the top, so the bottom surface still "looks" about flat.

Sorry, didn't recall that prior post of yours. The only intent in the last post of mine was to suggest an experiment much like the one you've already performed. Just wanted to be clear as to exactly what I was proposing. Unfortunately, it's not an experiment I can do properly with LEGO.

The experimental questions I posed were only partly about what goes on beneath the rotor. The 3D flow patterns above and below a cylindrical rotor are probably quite different -- in part, due to (i) blockage of inflow by the ground, and (ii) potential viscous coupling between rotor and ground (your aero shear drag). Each of these flows might interact differently with a rotor grooved above than with one grooved below.

Not sure I'm prepared to say what the flows in deep, steep-walled grooves like yours might look like, but it may not matter, and that may explain your null result (no clear difference in Top U and Top L spin times).

If the minimum air gap below the rotor is taller than a few mm, the viscous coupling may be negligible regardless of groove location, as the boundary layer along the bottom of the rotor may not reach the ground at the topple speeds we tend to get. (This boundary layer will tend to get even thinner at higher speeds.) For similar reasons, high-speed industrial rotors can have tightly fitting shrouds with tiny air gaps on either side and still lose very little power to viscous coupling.

In my experience, the spinning of a top with its bottom very near to the spinning surface, does not decrease significantly the spin time. On the other side, lowering the center of mass even by just one millimeter, increases the spin time by 2-4 minutes, in tops like the mine.... The advantage of the pedestal is not about aerodynamics....

These observations lend some credence to the boundary layer view. Many of my tops make it easy to change CM height without changing anything else. I also observe that even tiny changes in this all-important parameter can have big effects on spin time, presumably via critical speed.

[Iacopo]

Many of my tops make it easy to change CM height without changing anything else. I also observe that even tiny changes in this all-important parameter can have big effects on spin time, presumably via critical speed.

Recently I shortened the tip of one of my tops, reducing the height of center of mass from 7.2 mm to 5.5 mm.
This improved its best spin, from 26'20" to 30'25".
The spin decay curve is practically the same, but the toppling down speed has changed, from 178 to 128 RPM;
it takes about four minutes to go from 178 RPM to 128, so the reduced toppling down speed is the only reason of the longer spin.

Some time ago I tried to take some comparative measuraments of air drag in different flywheels, using a little electric engine. One thing I tried was to observe if there was difference of air drag between the flywheel spinning far from whatever surface, and spinning very close to it, at about 1 mm of distance.
I didn't see any difference.
Maybe the viscous coupling substitutes for the viscous pump effect, so that the air drag doesn't change significantly.