When a ball spins to left, shouldn't it curve right?(hurricane vise)

~~Skhandelwal~~ · Oct1-07, 04:22 PM

Basically, what I mean to say is that lets say I am playing ping pong...I put a lot of sidespin on the ball.(from back to forward vise, not up to down...how hurricane spins, not how clock runs)

If the ball is rotating to the right, shouldnt it travel to the left b/c wind is the median. Similar to how the tire travels on the road.

Am I being clear?

**russ_watters** · Oct1-07, 05:03 PM

If you look at the ball from the top down and it is rotating counter-clockwise, it will curve left.

http://library.thinkquest.org/11902/physics/curve2.html

**Jeff Reid** · Oct1-07, 06:21 PM

This exact example can be seen in the 2nd sequence in this video clip, the ball curves "left" quite a bit. The players are Jan-Ove Waldner and Kong Linghui.

tt2.wmv

~~Skhandelwal~~ · Oct2-07, 12:16 PM

Russ,

First of all, the baseball curves b/c its surface is uneven.
I am talking about a table tennis ball.(high quality ones have internal seam so they are perfectly smooth)

Also, from the article, how come the air hitting the top of the wings of the airplane is faster?

**arildno** · Oct2-07, 12:23 PM

Originally Posted by Skhandelwal

Russ,

First of all, the baseball curves b/c its surface is uneven.
I am talking about a table tennis ball.(high quality ones have internal seam so they are perfectly smooth)

Also, from the article, how come the air hitting the top of the wings of the airplane is faster?

No, the pressure above the wing is less than below; that means the air will accelerate more strongly along the upper surface than along the lower surface, since the pressure difference between a location in front of the plane and a location above the wing is greater than between that frontal location and a similar location beneath the wing.

Thus, the net effect is that the air above the wing will get a higher velocity than air going along the underside.

**arildno** · Oct2-07, 12:30 PM

Around the wing, the air spins (caused ultimately by the viscosity of the air). Thus, flight is more related to the Magnus effect (the plane "curves" upwards!), the Bernoulli effect is a mere amplifying side result of the circulation that goes around the wing.

The Bernoulli effect, often coupled with the fallacious "equal trasit time"-principle yields a totally wrong estimation of the circulation strength, and hence lift, for the plane.

To understand a little better why a plane maintains flight, it is sufficient with geometrical streamline arguments for an inviscid fluid:

Assume the simplest case, in which both the upper and lower wing surfaces have positive effective curvatures, i.e, the centres of their osculating circles lie BELOW the surface.

Now, if we go way up vertically from the upper surface, we get into the freestrem, where the pressure is p.
Similarly, if we go way down vertically from the lower surface, we also hit the free stream, with the same pressure p.

Now, consider the pressure situation along the upper surface:
In order to traverse that curve, you must have a downwards acting centripetal acceleration, which means that the pressure along the upper surface, pU, must be less than the free-stream pressure p.
That is, pU<p

Similarly, on the lower surface, you also need a downwards centripetal acceleration, and the pressure there, pL must therefore be GREATER than p, i.e, p<pL

Combining these inequalities yields the desired result: pU<pL, hence lift.

**russ_watters** · Oct2-07, 05:01 PM

Originally Posted by Skhandelwal

First of all, the baseball curves b/c its surface is uneven.
I am talking about a table tennis ball.(high quality ones have internal seam so they are perfectly smooth)

A high quality table tennis ball still has a certain amount of friction and since it weighs virtually nothing, it only requires a little bit of friction (and thus lift) to have a big effect on it's flight path. I also suspect that you can spin a ping pong ball faster than a pitcher can spin a curveball.

Also, from the article, how come the air hitting the top of the wings of the airplane is faster?

I'm not sure exactly what you mean, but air flowing over the top surface of a wing flows faster because it has a longer distance to traverse. I'm not sure I like the way they tried to draw that analogy. With lift, the rotation theory is generally treated separately from Bernoulli's, but essentially the idea for a wing is that the wing's shape makes the airflow rotate, similar to the way a rotating ball does.

**Jeff Reid** · Oct2-07, 07:21 PM

Getting back on topic, spin on a table tennis ball can reach 150 revolutions per second, that's 9000 rpm. Link below.

about table tennis

Before 2001, when ball size was 38mm (instead of the current 40mm), there was more variety in the surfaces of table tennis balls. Smooth ones (like a Peace) curved more than rough ones (like a Barna), but I don't recall either ball being very popular. The balls made now have a "matte" finish, similar to the older versions by Nittaku and Halex. The roughness of the surface of a table tennis ball has similar effect as the dimples on a golf ball (reduces the amount of curve).

When a table tennis ball is spinning and moving laterally, a very thin layer of air remains somewhat "attached" at the surface of the ball, quickly diminishing with distance from the surface, but it's enough to create a resistance to lateral airflow. This resistance results in a difference in acceleration of air on the forwards and backwards spinning surfaces, which results in a pressure differential, which in turn cause the ball to curve away from the forwards spinning surface.

Since the ball is a very thin shelled hollow sphere, it is affected a lot by aerodynamics. The 2001/2003 increase in size from 38mm to 40mm without an increase in weight caused the balls to lose more lateral and rotational energy to aerodynamic drag, they curve more, and slow down more, with less energy for the player returning the ball to deal with. The idea was to increase the length of the rallies.

Regarding the sub-topic aspect of wings, a flat plane will fly just fine, although with more drag, such as a box kite. All that is needed for lift is some air speed and an effective angle of attack. Air will be deflected from below and drawn towards the void above, with a net downwards acceleration (plus forwards acceleration, related to drag). Wings are shaped the way they are to reduce drag while increasing lift, designed for a range of air speed, and with the compromise of manufacturability, such as a flat bottomed wing (a fully cambered air foil would be more efficient). For those that think the hump has to be on top, I refer to this picture of a M2-F2 flying body glider (pre-shuttle prototype), which has a flat top and huge hump on the bottom, gliding next to a chase jet. Note the difference in angle of attack between the jet and the glider, and the fact that the upper surface of the glider is virtually horizontal.

flat top curved bottom glider.jpg

~~Skhandelwal~~ · Oct3-07, 10:38 AM

I am sorry guys...I feel really dumb. After all this explanation, I still don't get it. To begin w/...I need to know how a plane's wings look like?

Second, people still havn't answered WHY the table tennis ball curves.

**quetzalcoatl9** · Oct3-07, 05:12 PM

the assumption here is laminar flow (i.e. the velocity of the wing is much less than the mean velocity of the gas, so that the gas "rearranges itself" around the wing nearly instantly). as the velocity of the wing approaches the mean velocity of the gas (which is related to the velocity of sound propagation through the gas) the gas can no longer "rearrange itself" ahead of the wing, and so the gas molecules are smashed out of the way by the wing. this is what we call supersonic flight, and the mechanism by which a supersonic wing stays aloft is completely different from a sub-sonic/laminar/Bernoulli wing..

**Jeff Reid** · Oct3-07, 05:14 PM

Originally Posted by Skhandelwal

Do I need to know how a plane's wings look like?

no.

Second, people still havn't answered WHY the table tennis ball curves.

I thought I did:

The drag part: While traveling laterally, a small amount of air is accelerated forwards, while most of the air is seperated by the ball and flows around it. As the back of the ball passes through a volume of air, it leaves a low pressure moving void behind it, and the air accelerates towards this moving void. Since the air can't flow through the ball, there is a net forwards acceleration of air, and the reaction force of the air on the ball slows it down.

The curve part (again):

Originally Posted by Jeff Reid

When a table tennis ball is spinning and moving laterally, a very thin layer of air remains somewhat "attached" at the surface of the ball, quickly diminishing with distance from the surface, but it's enough to create a resistance to lateral airflow. This resistance results in a difference in acceleration of air on the forwards and backwards spinning surfaces, which results in a pressure differential, which in turn cause the ball to curve away from the forwards spinning surface.

More explanation, the forwards spinning part of the ball accelerates air forwards (in the direction of ball travel) more than the backwards spinning part. This will create a pressure differential, higher for the forwards spinning part, lower for the backwards spinning part. The result of this pressure differential causes the air to accelerate "outwards" and the reaction force of the air causes the ball to curve "inwards", away from the forwards spinning part. So a left spin causes a right curve, a right spin causes a left curve, top spin causes a downwards curve, and enough back spin and speed will cause an upwards curve.

By definition, the reaction force in the direction that the ball moves, slowing it down, is called drag. The reaction force perpendicular to the direction the ball moves, causing it to curve, is called lift.

**arildno** · Oct4-07, 05:50 AM

Originally Posted by quetzalcoatl9

the assumption here is laminar flow (i.e. the velocity of the wing is much less than the mean velocity of the gas, so that the gas "rearranges itself" around the wing nearly instantly). as the velocity of the wing approaches the mean velocity of the gas (which is related to the velocity of sound propagation through the gas) the gas can no longer "rearrange itself" ahead of the wing, and so the gas molecules are smashed out of the way by the wing. this is what we call supersonic flight, and the mechanism by which a supersonic wing stays aloft is completely different from a sub-sonic/laminar/Bernoulli wing..

No, no, no!!!
Laminar is used in contrast to "turbulent".
ALL flows are turbulent, and turbulence concerns are crucial in any flight consideration.

The stationary inviscid fluid approximation (i.e, the reign within Bernoulii is applicable) is a limiting case of minimal turbulunce presence, a rough measure of which would be a very thin wake region behind the wing.

Of course, your concerns about the assumptions of sub-sonic flight is very relevant and to the point, but that is not to be confused with the distinction laminarity/turbulunce.
There exist good laminar approximations to some supersonic flows as well, not just for sub-sonic ones.

**quetzalcoatl9** · Oct4-07, 09:21 PM

Originally Posted by arildno

No, no, no!!!
Laminar is used in contrast to "turbulent".
ALL flows are turbulent, and turbulence concerns are crucial in any flight consideration.

The stationary inviscid fluid approximation (i.e, the reign within Bernoulii is applicable) is a limiting case of minimal turbulunce presence, a rough measure of which would be a very thin wake region behind the wing.

what's the difference? you are saying that the opposite of turbulence is laminar, which i agree with. then you are saying that the "stationary inviscid fluid approximation" is what you get in the limit of no turbulence...how is that not laminar? (pardon my ignorance, i am not an engineer but a chemist)

in other words i agree with the statement that "all flows are inherently turbulent" but it is a matter of degree, at what point can we refer to the ideal as if it were "real". in the end, i highly doubt that the navier-stokes is numerically solved each time we want to consider airflow.

am i misunderstanding something here?

**Jeff Reid** · Oct5-07, 01:26 AM

Shouldn't we create yet another seperate thread on how wings produce lift, and leave this thread to spinning balls?

**arildno** · Oct5-07, 01:55 AM

Originally Posted by quetzalcoatl9

what's the difference? you are saying that the opposite of turbulence is laminar, which i agree with. then you are saying that the "stationary inviscid fluid approximation" is what you get in the limit of no turbulence...how is that not laminar? (pardon my ignorance, i am not an engineer but a chemist)

in other words i agree with the statement that "all flows are inherently turbulent" but it is a matter of degree, at what point can we refer to the ideal as if it were "real". in the end, i highly doubt that the navier-stokes is numerically solved each time we want to consider airflow.

am i misunderstanding something here?

YOU used laminar flow in contrast to supersonic flow. That is incorrect.

Supersonic flow, as well as subsonic flow, can be either (practically) laminar or turbulent.

**quetzalcoatl9** · Oct5-07, 02:49 PM

Originally Posted by arildno

YOU used laminar flow in contrast to supersonic flow. That is incorrect.

Supersonic flow, as well as subsonic flow, can be either (practically) laminar or turbulent.

i see, that is news to me but then i don't (admittedly) know much of anything about this area.

can you explain to me how a supersonic object could possibly have laminar flow? my understanding is that you will always be stuck with a (albeit very complicated) shock wave propagating from the object. how could such a thing possibly be considered laminar? i believe you (in the sense that the fluid dynamics become difficult to intuitively predict) but it is hard to imagine any sort of stream-lined layer near the wing.

it was my understanding that supersonic flight occurs successfully because the shock waves propagate in such a way that there is upward momentum transferred to the object that is "riding" the wave (actually, the wave is continually hammering the object from below), a situation that i would hardly describe as being laminar.