Top Spin

[Black Hole]

Is it just me, or does the effect spin has on the aerodynamics of a ball (or cylinder) seem counter-intuitive?

Hit a tennis ball with top spin (ie, the racquet is inclined with the top edge forwards so that it imparts a forward rolling spin on the ball when hit), and the ball will have a trajectory which scoops downwards as it slows down, hitting the ground after a shorter distance than it would if it had no spin. Conversely, back spin makes it lift and travel further. Similarly golf balls with hook and slice.

Look up the Wikipedia entry about it (Magnus Effect) and that shows some confusing diagrams which indicate it is to do with how the wake is affected by the spin.

My own "wet finger in the air", ie intuition, would have said the spin produced a drag on the leading face which creates a force in the opposite direction to the Magnus Effect, which would make top spin climb not drop. Maybe there is such a force, but the effect is weak compared to the opposite force of the Magnus Effect.

 

[Trev]

And what about the idea the faster the air travels over the surface, the lower the pressure, therefore more lift the surface produces due to the reduced air pressure. i.e. Top spin should make the ball rise. But, like your theory, that is contrary to what you say actually happens.

 

[Black Hole]

Are there any tennis or table tennis players here to confirm it?

 

[MikeSh]

It makes sense to me, but I do have an aviation background.

And what about the idea the faster the air travels over the surface, the lower the pressure

That's not actually the cause, though it may look like it.
Consider a simple aerofoil section - an arc of a circle with a flat bottom, like an orange segment. As it travels through the air the molecules that go round the curved part have to travel further than the ones going below, since they all have to meet up again at the other end. So the 'curved'ones have to speed up. The only way to get free air to accelerate is to apply a pressure differential. The air all around is at atmospheric pressure, so the only way to get a pressure differential to speed up the curved molecules is to have a lower pressure above the curved section. Thus lift.
With a circular section the molecules travel the same distance - no lift. However, if the circle spins, say backwards, then the friction will drag some of the air that would have gone under the section up, so it now goes over the top - the long way round. Since some of the air is now traveling further it has to speed up, just as in the aerofoil, and a low pressure is created and thus lift. For a forward spin the lift is down.
People have proposed actual wings or sails made with a spinning tube, though I don't know if any got to production.

 

[Black Hole]

 

[Trev]

In that video at 46 seconds, if you assume that the ball is travelling from right to left, it shows that backspin would cause the ball to dip. However if the ball was travelling from left to right, it would be top spin, and still dip.

 

[gomezz]

What about reverse swing of a cricket ball which is not spinning but has one relatively rough side and one relatively smooth side? Seems to be more prevalent with overcast conditions with presumably the air holding more moisture so making for a more grippy "fluid".

 

[MikeSh]

In that video at 46 seconds, if you assume that the ball is travelling from right to left, it shows that backspin would cause the ball to dip. However if the ball was travelling from left to right, it would be top spin, and still dip.

Your assumption is wrong - the air is travelling right to left, if the air was static the ball/tube would be travelling left to right. If you reverse those directions but retain clockwise rotation the air will be deflected over the top of the ball/tube and the lift will then be up instead of down.

 

[Black Hole]

What about reverse swing of a cricket ball which is not spinning but has one relatively rough side and one relatively smooth side? Seems to be more prevalent with overcast conditions with presumably the air holding more moisture so making for a more grippy "fluid".

The Wikipedia article about the Magnus Effect says it's not relevant to the swing of a cricket ball. I guess that has more to do with drag.

 

[Trev]

Your assumption is wrong - the air is travelling right to left, if the air was static the ball/tube would be travelling left to right.

Ah yes. I see it now, top spin would make the ball go down. Thanks Mike.
But will this really drive ships more efficiently?

 

[MikeSh]

But will this really drive ships more efficiently?

I can't speak to efficiency but if it was cost effective we'd see them sprouting up all over. That video was posted 7 years ago, so I suspect it's an idea who's time has not yet come.

 

[Black Hole]

...or may never come. The advantage of a sail is that it doesn't need an engine.

 

[MikeSh]

The advantage of a sail is that it doesn't need an engine.

True, but it does require a lot of rigging and operation, meaning either man-power or multiple remote controlled motors. A solid 'wing' might be better, but it's going to be big, heavy, expensive.
And they are all only useful when the wind is in a cooperative mood, which means that the opportunities for a commercial vessel to pay back the cost of ownership are limited.

 

[EEPhil]

And they are all only useful when the wind is in a cooperative mood, which means that the opportunities for a commercial vessel to pay back the cost of ownership are limited.

Oh, the temptation to make some comment about wind turbines...

 

[MikeSh]

Oh, the temptation to make some comment about wind turbines...

Perfectly correct. The difference is that they are static and can turn to use whatever wind is available. Commercial shipping doesn't have those luxuries.
I believe wind turbines generally get about 70% usability (but don't quote me) - a ship would get far less, probably nearer 25%.

 

[gomezz]

If you build enough wind turbines then that will surely significantly affect the movement of the air over the planet thus the weather and the climate and even the spin of the planet.

 

[Trev]

Are you implying that they could add to global warming gomezz? We're all doomed.

 

[dandnsmith]

Perfectly correct. The difference is that they are static and can turn to use whatever wind is available. Commercial shipping doesn't have those luxuries.

You could, obviously, use the turbine mode with axis vertical and convert the rotary motion into driving propeller shafts.

I have a nagging feeling that I've seen an article about such a (real) sailing vessel - but cannot remember how the drive was organised.

 

[Black Hole]

Hmm. Concept: wind turbine, forward facing, geared to screw propeller. Can the wind blowing directly onto the bows drive the ship directly into the wind?

The external force acting on the boat is backwards. Does the physics allow for the boat to move in the opposite direction to the applied force? I can probably work it out, but for the moment I am confused.

 

[EEPhil]

If you build enough wind turbines then that will surely significantly affect the movement of the air over the planet thus the weather and the climate and even the spin of the planet.

I would expect a change in wind movement at low altitudes. I doubt whether this would affect the wind at higher altitudes.
Now if we had lots of solar farms reflecting heat and light, that might screw up the climate.

Are you implying that they could add to global warming gomezz? We're all doomed.

Have you been reading the latest Private Eye?