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Author Topic: Flywheels and fairings and spokes, oh my!  (Read 7280 times)

Iacopo

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Re: Flywheels and fairings and spokes, oh my!
« Reply #15 on: February 22, 2021, 08:40:49 AM »

No enclosure, 322 s (5:22)
Bell enclosure, 350 s (5:50)
Straight enclosure, 362 s (6:02)*

You have a 12 % improvement of the spin time with the straight enclosure, not bad. 
Even with all that empty space in the enclosure the advantage is still evident.

Maybe even a tube, open above and below, could work.. ?  I will try something.
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ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #16 on: February 23, 2021, 03:42:39 AM »

In this context of "fairings" I remembered a detail about my Quark Spinning Top that might be of some relevance here.




The body has groves or wells. Why it has those and why they are on the lower side of the top I can only guess. Probably related to an effective production process or/and design aspects.



As visible in the next picture, the groves are partially covered with a sticker "Quark by Micro Logic".




Actually what I mean is not visible that clearly in the picture: The inner grove of the body is covered completely by the sticker, alright. The outer grove is covered only partially by the sticker. The grove is about 3 mm wide but the sticker is hanging about 1 mm over the rim.
Could this be just poor design? Or is some aerodynamic reasoning involved ? Could that lip formed by the overhanging sticker yield some advantage?  I am reluctant to rip the sticker off just now and do comparative measurements.






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Iacopo

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Re: Flywheels and fairings and spokes, oh my!
« Reply #17 on: February 23, 2021, 06:59:35 AM »

The body has groves or wells. Why it has those and why they are on the lower side of the top I can only guess. Probably related to an effective production process or/and design aspects.

Is the body made of metal ?  Maybe the grooves are for making the body lighter.  The top spins longer if the body, (the central part of the top), is lighter.
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ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #18 on: February 23, 2021, 07:16:38 AM »

No, the body is injection molded plastic. If the groves would be from above, the center of mass would be lower at least.
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Iacopo

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Re: Flywheels and fairings and spokes, oh my!
« Reply #19 on: February 23, 2021, 08:45:14 AM »

Some more tests:
I made some cardboard enclosures and tested them with the top Nr. 29.

The best one is the littlest one, there are only 2 mm clearance all around the top.  Advantage 17.4 %, (from 1600 to 1500 RPM).
That with 5 mm clearance all around the top was good too.  Advantage 14.2 %.
The largest one, with 15 mm clearance above and below, and 5 mm clearance at the sides, was less effective.  Advantage 12.6 %

I also tried a simple ring around the flywheel, clearance 5 mm.
This too reduced the air drag, but very little.  Advantage 1.7 %.

« Last Edit: February 23, 2021, 08:48:51 AM by Iacopo »
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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #20 on: February 23, 2021, 08:52:58 AM »

No, the body is injection molded plastic. If the groves would be from above, the center of mass would be lower at least.

Agree about center of mass, but removing central mass increases AMI per unit mass, which is also good for critical speed in the same way spokes are -- and come to think of it, maybe in a more aerodynamic way.

The exposed grooves could also reduce total drag in other ways, I suppose, but not in a way I'm familar with.

In the end, however, they covered the grooves with a fairing (sticker). So maybe not.
« Last Edit: February 23, 2021, 11:41:04 AM by Jeremy McCreary »
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Playing with the physical world through LEGO

ta0

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Re: Flywheels and fairings and spokes, oh my!
« Reply #21 on: February 23, 2021, 10:01:07 AM »

In this context of "fairings" I remembered a detail about my Quark Spinning Top that might be of some relevance here.
The body has groves or wells. Why it has those and why they are on the lower side of the top I can only guess. Probably related to an effective production process or/and design aspects.
The grooves cannot have any aerodynamic purpose as they are covered by the sticker. In fact, I didn't know that they were there. Perhaps they provide mechanical compliance for press fitting the metal flywheel.

The best one is the littlest one, there are only 2 mm clearance all around the top.  Advantage 17.4 %, (from 1600 to 1500 RPM).
That with 5 mm clearance all around the top was good too.  Advantage 14.2 %.
The largest one, with 15 mm clearance above and below, and 5 mm clearance at the sides, was less effective.  Advantage 12.6 %
Those are some outstanding numbers!  :o

I wonder if several circular ridges over the surface of the top would disturb enough the von Karman pumping mechanism to overcome the increased surface area.  :-\
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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #22 on: February 23, 2021, 10:38:33 AM »

@Iacopo: Impressive results!

My lightweight tops and relatively shallow concave bases would never work with even 5 mm of clearance, as the tops are too easily pulled off-center by aerodynamic forces at their highest speeds.

With a 3,000 RPM vertical launch on a concave base, generally takes the test top here 120-150 s to settle into quiet sleep after covering it with one of the 2 enclosures shown above. In open air, it reaches quiet sleep in 0-30 s, depending on the quality of the launch.

The motion most commonly induced or at least exaggerated by the enclosure isn't a typical precession. Instead, the tip orbits the bottom of the concavity at decreasing radius with the top at near-zero tilt.

« Last Edit: February 23, 2021, 10:59:04 AM by Jeremy McCreary »
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ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #23 on: February 23, 2021, 10:48:40 AM »


The grooves cannot have any aerodynamic purpose as they are covered by the sticker.


What no visible groves in  your Quark? I checked the picture again  that I posted earlier. Since I just borrowed it from another
post of yours in another thread I figured  this is what you have. And indeed there is no visible groove.


My theorie on all this mess now goes as follows: 


- ta0 you have the tungsten quark. That  has a smaller diameter I read. I have the brass Quark that is somewhat larger.
- the Quark top was produced with different bodies, depending on the flywheel in use. The picture in my post earlier probably shows bodies for the brass Quark
- only one sticker was designed for the Quark. It fits the tungsten top perfectly, but in my brass top it leaves the outer grove partially uncovered.



-


So, no physical finesse here it seems. Just some laziness in designing a second sticker. Like my laziness in taking a photo of  my own Quark right at the start.
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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #24 on: February 23, 2021, 11:11:25 AM »

I wonder if several circular ridges over the surface of the top would disturb enough the von Karman pumping mechanism to overcome the increased surface area.  :-\

We're clearly dealing with some very complicated phenomena here.

Agree, suppression of the energy-consuming pumping action seems like a promising strategy. We need to keep an open mind as to how that's best done. Trying to think of a way to test this interesting suggestion in LEGO.

As I keep telling my fellow LEGO club members, who wonder why I'm still making tops 6 years later...

Tops are way more complicated than they look. And for a topmaker, therein lies the fun.
« Last Edit: February 23, 2021, 11:16:08 AM by Jeremy McCreary »
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Iacopo

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Re: Flywheels and fairings and spokes, oh my!
« Reply #25 on: February 23, 2021, 01:03:22 PM »

The motion most commonly induced or at least exaggerated by the enclosure isn't a typical precession. Instead, the tip orbits the bottom of the concavity at decreasing radius with the top at near-zero tilt.

Yes, I know it, I call it "hula hoop". 
This is the reason that I don't use ball tips in my tops with a recessed tip, otherwise they hula hoop.
If you have the possibility to use a tip with a littler radius of curvature, maybe you can avoid this kind of wobble.

But large and light tops like the your are very sensible to the air drag so you can detect air drag differences even with a lower speed, as you did nicely with your latest test.
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Iacopo

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Re: Flywheels and fairings and spokes, oh my!
« Reply #26 on: February 23, 2021, 01:33:51 PM »

I wonder if several circular ridges over the surface of the top would disturb enough the von Karman pumping mechanism to overcome the increased surface area.  :-\

I would like to know that there is a surface treatment which could make the surface more slippery for the air.
But I don't know if this is even possible, (I don't know anything about this issue).
Ridges seem to increase the air drag, at least at high speed, in the test I made. 
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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #27 on: February 23, 2021, 02:23:10 PM »

The motion most commonly induced or at least exaggerated by the enclosure isn't a typical precession. Instead, the tip orbits the bottom of the concavity at decreasing radius with the top at near-zero tilt.

Yes, I know it, I call it "hula hoop". 
This is the reason that I don't use ball tips in my tops with a recessed tip, otherwise they hula hoop.
If you have the possibility to use a tip with a littler radius of curvature, maybe you can avoid this kind of wobble.

But large and light tops like the your are very sensible to the air drag so you can detect air drag differences even with a lower speed, as you did nicely with your latest test.

Hula hoop -- perfect term!

Already close to the practical lower limit of tip radius of curvature in LEGO plastic, but will investigate non-LEGO tip options in LEGO tip holders.

You're right about experimenting at lower speeds.
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ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #28 on: February 24, 2021, 05:23:55 AM »

Yesterday I did some measurements with my brass Quark, and then I ripped that sticker fairing off and did them again.There is no big difference as can be seen in the table below. The difference between sticker and no sticker is not larger as the difference in consecutive runs without the sticker. 



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Iacopo

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Re: Flywheels and fairings and spokes, oh my!
« Reply #29 on: February 24, 2021, 08:04:53 AM »

Yesterday I did some measurements with my brass Quark, and then I ripped that sticker fairing off and did them again.There is no big difference as can be seen in the table below. The difference between sticker and no sticker is not larger as the difference in consecutive runs without the sticker. 


I would trust more in the higher RPM lapses, and in the 1400-1000 RPM lapse the top with the sticker seems slightly better.

   
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