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[Iacopo]

I have tried blowing with a straw.

If I blow under the flywheel, and the top is spinning on a concave mirror, the side of the top where I blow is sucked down, like if the air passing below it causes a local depression.

I agree with Alan who suggests this is Bernoulli effect.

The side of the flywheel that sinks down is 90 degrees later, because of gyroscopic effect, then the top walks moved by the rolling ball tip.

The effect is evident when the top spins in a concave mirror, but it doesn't work (to me at least), if the top spins on a flat surface.
I suspect that, since the bottom of the top is flat, and the mirror is concave, the air should be constrained to expand, when it is blown under the top, channeled between the two divergent surfaces, which is what causes the depression.  When the air reaches the center of the mirror, the surfaces become parallel, and then convergents, so probably the air depression is lost at that point.

[Aerobie]

Iacopo's description sounds like air flowing under the toroidal wheel creating Bernoulli suction pulling the wheel down towards the mirror.  Then the mirror actually tilting 90 degrees of rotation later, due to rules of gyroscopic precession.
Alan

[Iacopo]

Iacopo's description sounds like air flowing under the toroidal wheel creating Bernoulli suction pulling the wheel down towards the mirror.  Then the mirror actually tilting 90 degrees of rotation later, due to rules of gyroscopic precession.
Alan

I used the top you sent me, which has flat bottom.  It worked anyway.
Yes, the flywheel goes down, not where there is the Bernoulli suction, but 90 degrees later in the direction of spinning.
(But only if the top spins on a concave surface. If the top spins on a flat surface instead, it seems like there is no Bernoulli suction).

[Aerobie]

In a free stream, a curved surface is needed to develop suction at zero angle of attack.  But when a flat surface is near another surface, the air velocity increases as the air flows through the narrowed space between the two surfaces.  It's this velocity which is in the Bernoulli equation, not the shape of the surfaces.  So suction does occur between flat surfaces.

Alan

[Iacopo]

In a free stream, a curved surface is needed to develop suction at zero angle of attack.  But when a flat surface is near another surface, the air velocity increases as the air flows through the narrowed space between the two surfaces.  It's this velocity which is in the Bernoulli equation, not the shape of the surfaces.  So suction does occur between flat surfaces.

Alan

I looked at the Bernoulli equation, but it is too complicated for me. Even intuitively I admit that I can't understand how it works exactly.

[Aerobie]

All you need to know is that pressure decreases with velocity.  Many of us view this as counter-intuitive.

Alan

PS  This is the pressure which is measured with a sensing aperture at right angle to the flow.  When the sensing aperture "looks" into the flow, the pressure increases with velocity.  This is "dynamic pressure" and is used to measure airspeed as with a Pitot tube.

[Iacopo]

All you need to know is that pressure decreases with velocity.  Many of us view this as counter-intuitive.

Alan

PS  This is the pressure which is measured with a sensing aperture at right angle to the flow.  When the sensing aperture "looks" into the flow, the pressure increases with velocity.  This is "dynamic pressure" and is used to measure airspeed as with a Pitot tube.

I have found some videos about the Venturi tube, and an explanation I could grasp a bit, even if the effect isn't very intuitive.  I understand that the the "dynamic pressure" is something different.

I tried blowing with a straw under the flywheel of my tungsten top spinning on the table.  This top has low scrape angle and there are only about 2 mm between the flywheel and the table.  With this top the effect is strong; as soon as I blow, the top tilts rapidly. The suction seems stronger.   

[Aerobie]

Balancing:

Lately I'm working with tops which have very low topple speed.  I watch them at low speed leaning towards the heavy spot without any aids (such as video).  Then they come to rest with the same heavy spot touching the mirror.  If I hold the top vertical (not spinning) and release it, it falls to the same heavy spot.

This is surprisingly easy.

Incidentally, my tachometer only works down to 60 RPM, but I find it easy to clock a single revolution below 60 RPM with a stopwatch.  RPM = 60 / seconds for one rev.

Alan

[Aerobie]

I have some copper foil tape.  I weighed a large piece and determined that it weighs 0.136 grams per square inch.  Then putting little bits of it on the high side of a top I was able to calculate that a 0.125" hole, 0.015" deep (on the low side) would balance a top.  It worked. 

[Iacopo]

they come to rest with the same heavy spot touching the mirror.  If I hold the top vertical (not spinning) and release it, it falls to the same heavy spot.

This is surprisingly easy.

When I start balancing a top, first I simply put it on a horizontal flat surface, without spinning it;
the top will stay in contact with the surface by the tip and by a side of its flywheel.
I roll the top a bit, a few times, and every time the top ends with the same side of the flywheel down, in contact with the horizontal surface; this is the heavy side.

This is very easy too, but for fine tuning I then always use brush and paint, which is more accurate.
When I want very accurate balance, in the end, I use the brush with the top spinning slowly.

[Aerobie]

Is your top lying on its side?  If so, this would find the heavy side of the circle defined by the perimeter, but would not be influenced by the eccentricity of the tip.

alan

[Iacopo]

Is your top lying on its side?

No, it is lying on the tip and on the side.

[Aerobie]

So the tip and the side are touching the surface and the axis departs from vertical by the scrape angle.
Any special efforts to level the surface?  Putting a ball bearing on it would be a sensitive test of level.
Alan

[Iacopo]

I never made special efforts to level the surface.  I simply use the surface of the table, which is smooth enough and decently level.
But using a good surface like a glass pane, perfectly level, would help;  I don't know how much accuracy could be reached in this way, but it is interesting to try.

To use a bearing ball as a level is a very good idea !

[Aerobie]

I leveled the flat side of a mirror that I'd installed thumbscrews on.  A ball helped as a level.  Even more sensitive is a spinning top.  In fact it's too sensitive.  Iacopo's method of setting the top on its tip and edge is a great start to find the heavy point.  As he said, eventually you want more sensitivity.  So I twirl and wait for fall.

I've made several "no-fall" small tops lately and have to balance them to achieve "no fall".  As I mentioned in a prior post, I stick on bits of copper tape to balance.  Then I calculate how much weight I've added to reach balance and bore a little hole on the opposite side.  All of this is based on calculations because my scale can't resolve the tiny weights I'm working with.

Incidentally, despite machining to a precision of one or two ten thousandths of an inch, these 50g tops have wobbled slightly below 200 RPM.  The wobble disappears nicely when I balance them.

Alan

Update 1/30/18  I just ordered a micro scale that measures to .001 gram for only $20 from amazon.  I should have done that years ago.