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[Jeremy McCreary]

Been thinking about how to increase spin time in a Simonelli-like single-twirl top. One possibility, based on this old post, would be to try a disc-like rotor of radius R greater than a typical Simonelli with a thickness (axial length) of under R/10.

Something like this ?



The diameter is 76 mm and thickness 6 mm.  0.0000802 kg m².
The problem is that when the thickness is little, the AMI too is little, relatively to the air drag.
The top can spin for 20 minutes or little more.
Spin decay:  starting speed 2000 RPM, spin time every 200 RPM lost: 27.9 s at 1800 RPM - 61.3 s - 102.0 s - 152.3 s - 216.7 s - 304.1 s - 430.6 s - 638.4 s - 1060.8 s at 200 RPM - 1257.0 at 150 RPM. Topple down at 150 RPM, spin time 20 m 57 s.
Starting speed by multiple twirls up to 2200 RPM. Starting speed by a single twirl up to 1000 RPM.

Ah, one of my favorite visual designs from you! Quite elegant.

Yes, roughly like that shape, but wider and even thinner with a smoother bottom surface. And with more metal in the flywheel to end up with a substantially larger AMI.

Alas, a 21 min spin is far from ta0's challenge of 60 min with a single twirl. But I wonder...

Q1. How much single-twirl launch speed would you lose with substantially more AMI?

Q2. Could you get any of it back with a different stem profile?

Q3. Could you reduce the CM-contact distance even more?

Just something to think about if you ever want to try a new profile.

[Iacopo]

Q1. How much single-twirl launch speed would you lose with substantially more AMI?

Q2. Could you get any of it back with a different stem profile?

Q3. Could you reduce the CM-contact distance even more?

Q1: Difficult to say.

Q2: The stem is a bit short, with a longer one I believe that the top could be started at maybe 1200 RPM, instead of 1000, and spin for maybe one minute longer.

Q3: Yes, if I reduce the topple speed from 150 to 50 RPM the spin time could be maybe 10 minutes longer. And the top could spin for maybe 28-29 minutes, with a single twirl, (starting from about 1000 RPM). Which is not, even so, extraordinary, for a top with the tip recessed. My best tops in this category spin for 45 minutes, (50 minutes with the lead flywheel), with a single twirl.

I am not sure about the effect of making the flywheel even thinner and heavier, with more brass.

The top we are talking about is the Nr. 33, in the graph below, (about the percentage of RPM retained after each minute of spinning).
It is very different from my other tops;
it is by far the worst one at high speed, losing 20% of the RPM after just one minute spinning, when the other tops lose only 4-10 % at the same speed.  Too much air drag for that AMI.
But that same top becomes my best one, below 200 RPM. The air drag is no more a problem, and having the best AMI/weight ratio makes it the most efficient at that point. 

I know that you remember, Ortwin made some extraordinary tops, made like a space station, in some way similar to the Nr. 33, (as for the very good AMI/weight ratio), but using spokes instead of a full disk, so not only he had excellent efficiency at low speed, but also not too much air drag at high speed, and he had some fantastic single twirl spin times.