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Author Topic: Flywheels and fairings and spokes, oh my!  (Read 8306 times)

ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #105 on: March 10, 2021, 02:03:19 PM »

https://videoportal.uni-freiburg.de/video/Supraleiter-auf-Moebiusband/f489e06ec021b42a23780fe7fbf861d2

cooool video of super conductor on moebius strip.
That is nothing new of course and not really directly related to the topic of this thread. But have a look at those strings where the möbius strip hangs from: If we tension them, we have our minimalistic spokes and a moebius flywheel. Jeremy please do a quick calculation of the generator for that and put it into you spreadsheet! Is it better or worse then the square?
No, not serious. Regardless if it is better or worse for spin time, it would be cool to have a moebius flywheel top. Any 3-D printing specialist around?
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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #106 on: March 10, 2021, 02:53:34 PM »

Jeremy please do a quick calculation of the generator for that and put it into you spreadsheet! Is it better or worse then the square?
No, not serious. Regardless if it is better or worse for spin time, it would be cool to have a moebius flywheel top. Any 3-D printing specialist around?


Piece of cake! And yes, we have some very adept 3D-printers.

That hanging Mobius goes way beyond cooool, my friend. It's gorgeous, too! Elegant kinetic sculpture, fascinating geometic model, and spectacular science demo all in one. Hats off to the artist.
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Playing with the physical world through LEGO

ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #107 on: March 10, 2021, 04:00:16 PM »

Now I want a flywheel like one of these:


I can't even really tell if you have to go around two times or four times here to get back to the start. Both variations are a possibility.


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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #108 on: March 10, 2021, 04:20:53 PM »

Twisting toroids, Batman! These would look great. And if anybody could make a suspension top out of them, it'd be you.

In fact, these look like great testbeds for evaluating suspension system designs.

Welcome to aerodynamic hell.
« Last Edit: March 10, 2021, 04:26:42 PM by Jeremy McCreary »
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ta0

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Re: Flywheels and fairings and spokes, oh my!
« Reply #109 on: March 10, 2021, 06:31:44 PM »

I hadn't seen the superconductor running on a Mobius band before. It's super cool!  8)

I'm guessing the moment of inertia is intermediate to that of a vertical track and a horizontal track (corrected for the added mass given by the longer path), which for a thin track would not be very different to the mass times the radius (at the center) squared. The complicated part is the air drag  . . .
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ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #110 on: March 11, 2021, 03:46:07 AM »



I'm guessing the moment of inertia is intermediate to that of a vertical track and a horizontal track ....


Probably a very good guess. Those two are also on my sketch for the evolutionary T-shirt. But Jeremy has not presented any results on these. So they might be  quite mediocre?
« Last Edit: March 12, 2021, 02:50:20 PM by ortwin »
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ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #111 on: March 12, 2021, 03:02:08 PM »

I hadn't seen the superconductor running on a Mobius band before. It's super cool!  8)





https://youtu.be/8tFsrGRwOOM 


So this one is not as cool, since it does not need any cooling, but it is running as long the sun is shining.




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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #112 on: March 12, 2021, 06:41:11 PM »

Now I want a flywheel like one of these:

I can't even really tell if you have to go around two times or four times here to get back to the start.

Made some twisted toroids using flexible LEGO axles and mounted them on appropriate cores with rigid spokes...



Below is the smaller one on the right at speed...



The twisted toroid on this one turns out to be a trefoil knot.


(Public domain image from Wikipedia's trefoil knot page.)

If the white axles were the edges of a continuous ribbon, the ribbon would form a 3-pi Mobius strip.

The other one turned out to be too hard to balance, so I scrapped it for this soft suspension top...



This one and the trefoil top only stay up 10-15 s, but I like them anyway.

« Last Edit: April 02, 2021, 01:07:54 AM by Jeremy McCreary »
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ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #113 on: March 12, 2021, 07:07:52 PM »

Made some twisted toroids using flexible LEGO axles ......
LEGO, really? You? Looks like licorice candy sticks to me. Although I did not know they come in white also.

No, really cool stuff. The last one looks as if you were trying to simulate a Curtain-Ring-Top. ;)


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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #114 on: March 12, 2021, 07:52:36 PM »

Made some twisted toroids using flexible LEGO axles ......
The last one looks as if you were trying to simulate a Curtain-Ring-Top. ;)

I was!

Working now on a video exploring some of the engineering challenges involved in suspending a toroid from an inherently unstable core using only chains, cables, strings, etc. Interesting stuff.

Sent you a PM about it.
« Last Edit: March 12, 2021, 07:56:01 PM by Jeremy McCreary »
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ortwin

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Re: Flywheels and fairings and spokes, oh my!
« Reply #115 on: March 13, 2021, 03:04:46 AM »

Sent you a PM about it.


Something must have gone wrong there. My post box is still empty.
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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #116 on: March 13, 2021, 06:10:05 AM »

Sent you a PM about it.
Something must have gone wrong there. My post box is still empty.

Interesting. I'll try again.
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Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #117 on: April 02, 2021, 12:01:28 AM »

Experiment: "von Braun space station" top with and without fairings*



Test top: High-AMI, low-CM spoked flywheel top with 2 easily removed rigid (HDPE?) disk fairings of negligible thickness

Base: Concave lens, modest curvature, lubed with skin oil. Taken at the horizontal flywheel's lower outer edge, the top has 22 mm of ground clearance on the lens and 12 mm on a flat surface.



Measurements: Best spin time of 3+ runs from 1,010±10 RPM and 0° tilt to first audible scrape.

Top A. Bare top, short test stem, no fairings


Top B. Top A + single 17.2 g fairing below flywheel (bore sealed from below)


Top C. Top A + single 11.6 g fairing above spokes (bore not entirely sealed from above)


Top D. Top A + both of the fairings above


Variables
1. Total mass (M): 137 - 165 g
2. CM-contact distance (H): 24 - 26 mm
3. Total AMI (I3): 7.1e-4 - 8.2e-4 kg m²
4. Total TMI at tip (I1t): 4.5e-4 - 5.2e-4 kg m²
5. Critical speed (ωC): 10.8 - 11.4 rad/s (103-109 RPM)
6. Scrape angle (θmax): 15° on lens, 8° on flat surface

Controlled parameters (reasonably constant from top to top)
1. Flywheel, spoke, and core assemblies, max radius 84 mm
2. Release speed (ω0): 106 rad/s (1,010±10 RPM)

Best spin times
Top A (no fairings, on lens) .................. 185 s
Top B (lower fairing only, on lens) ......... 188 s
Top C (upper fairing only, on lens) ......... 203 s
Top D (both fairings, on lens) ................ 333 s
Top D (both fairings, off lens) ................ 290 s



Can lightweight fairings improve a spoked flywheel top's spin time? You bet! Fair the flywheel and spokes both above and below, and you get a whopping 64% gain in spin time over the better 1-fairing case (Top C) and an 80% gain over the 0-fairing case (Top A).

With just the lower fairing, Top B stayed up only 2% longer than Top A. But with just the upper, Top C consistently stayed up 10% longer than Top A — and that with an 8 mm air gap still present between the upper fairing and flywheel.

Fairings vs. mass properties: The simple shapes and the few internal air voids involved allowed me to calculate mass properties with decent accuracy. Turns out, the fairings had little impact on CM height: In mm, Top A = Top D = 25, Top B = 24, and Top C = 26.

The moments felt the fairings more. When both were present (Top D), they accounted for ~17% of the top's total mass, ~12% of its total AMI, and ~13% of its total TMI about the tip. However, all 4 tops ended up with nearly identical TMI/AMI ratios at the tip (where it counts).

Fairings vs. critical speed: Estimated critical speed combines all of a top's key mass properties into a single figure of merit with a big impact on spin time. In RPM, Top A = 104, Top B = 103, Top C = 109, and Top D = 107.

So for all the differences in mass properties, little change in critical speed. No good explanation for Top D's dramatic success there, so it must have been aerodynamic.

Ground effect? Most of the 43 s lost when Top D came off the lens must have been lost to air resistance. Why? Well, Tops A-D were already in their death spirals at the 8° scrape angle available off the lens. Takes at most another 5 s to reach the 15° scrape angle available on the lens. The rest of that 43 s had to be aerodynamic, cuz nothing else changed.

With Top D off the lens, there was still a 12 mm air gap beneath the lower fairing. Would a narrower gap have reversed the trend? Hard to say, but it would definitely have made the top harder to twirl on a flat surface.

Suppressing a top's centrifugal pumping action may still be a good thing to try, but top aerodynamics are clearly way more complicated than that.

Play value: I really enjoy this space station top. Plan to hang on to the fairings, but prefer the top without them. It's big and bold, loves to sleep, and stays up 130 s with a single twirl without fairings. When I want more spin time, I'd rather go to a fun starter instead.



* Acknowledgments: Thanks to Iacopo for pushing me to do this experiment. Sorry it took so long. Also thanks to ortwin for pointing out that von Braun beat me to this top's basic design 70 years ago. (And some Russians long before him.) Not sayin' Werner had it easy, but no gravity or air resistance to contend with in orbit.

https://youtu.be/5JJL8CUfF-o
« Last Edit: April 02, 2021, 02:01:11 AM by Jeremy McCreary »
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the Earl of Whirl

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Happiness runs in a circular motion!!!

Jeremy McCreary

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Re: Flywheels and fairings and spokes, oh my!
« Reply #119 on: April 02, 2021, 01:16:38 PM »

https://www.google.com/amp/s/www.space.com/amp/gateway-foundation-von-braun-space-station.html

Very cool! I'd go for a week with a suitcase full of tops to test in the various (fake) gravity environments available.
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