A nice low minimum RPM might contribute to total spin time.

Couldn't agree more! Each of my tops seems to have a maximum attainable release speed for each spin-up method (hand, string, etc.). Hence, best spin times are strongly limited by the minimum speed

*w*_{min} for stable sleeping or steady precession. And the strongest single influence on

*w*_{min} is CM height, followed closely by axial moment of inertia.

This isn't just theoretical. Many of my tops have readily adjustable CM heights. The correlation between CM height and spin time is obvious.

But moving the body of the top closer to the tip increases the aero drag shear on the bottom surface, which would cause end-of-spin to occur sooner. So, as I've written before, "There's no free lunch".

I may have some good news here: As long as the ground clearance

*c* beneath the rotor exceeds

*d*, the boundary layer thickness over the lower rotor face, the supporting surface won't affect the drag on the rotor.

This effect is well known to engineers who worry about windage on rotors in tightly fitting shrouds. Taking advantage of it in top design calls for an estimate of

*d*. For a top with laminar swirling (von Karman) flow over a flat lower face of radius

*R*,

*d* = 5.4 sqrt(

*v* /

*w*) = 5.4

*R* / sqrt(

*Re*),

where

*d* and

*R* are in meters,

*v* ~ 1.5e-5 is the kinematic viscosity of room temperature air in m^2/s,

*w* is the angular speed in rad/sec, and

*Re* is the dimensionless rotational Reynolds number given by

*Re* ==

*w* *R*^{2} /

*v*This formula for

*d* is valid for

*Re* < 300,000. Counter-intuitively, the boundary layer thickens as spin-down proceeds.

For your 47-gram tungsten top spinning upright at 2,000 rpm,

*R* = 0.50 in = 0.0127 m

*w* = 209 rad/s

*Re* = 2,252 (well within the laminar regime)

*d* = 0.0014 m = 1.4 mm

**Upshot:** You can reduce CM height until

*c* ~

*d* without incurring a drag penalty, but twirlability will suffer if

*c* gets too small.

One measure of twirlability is the "grounding angle"

*a* between the stem and the vertical when the top is leaning on its rotor:

*a* = tan

^{-1}(

*c* /

*R*)

The smaller the grounding angle, the less wiggle room the twirler has to avoid a rotor strike. For your 47 g tungsten,

*a* is only 6° when

*c* =

*d* = 1.4 mm. That's pretty tight for a twirl by hand but easily doable with your string-launch system.

A simple spin-up aid like the one below really helps with tops with small grounding angles, as it lets one hand control the stem angle while the other applies the spin-up torque. This division of labor tends to benefit both spin time and the likelihood of getting a sleeper from the get-go.

https://www.youtube.com/watch?v=SL9KBh2z6uQ