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

1. Oct 1, 2007

### Skhandelwal

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?

2. Oct 1, 2007

3. Oct 1, 2007

### rcgldr

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

Last edited: Oct 1, 2007
4. Oct 2, 2007

### 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?

5. Oct 2, 2007

### arildno

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.

6. Oct 2, 2007

### arildno

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.

Last edited: Oct 2, 2007
7. Oct 2, 2007

### Staff: Mentor

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.
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.

8. Oct 2, 2007

### rcgldr

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

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

Last edited: Oct 2, 2007
9. Oct 3, 2007

### Skhandelwal

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.

10. Oct 3, 2007

### 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..

11. Oct 3, 2007

### rcgldr

no.
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):
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.

Last edited: Oct 3, 2007
12. Oct 4, 2007

### 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.

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.

Last edited: Oct 4, 2007
13. Oct 4, 2007

### 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?

14. Oct 5, 2007

### rcgldr

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

15. Oct 5, 2007

### 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.

Last edited: Oct 5, 2007
16. Oct 5, 2007

### quetzalcoatl9

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.

17. Oct 6, 2007

### rcgldr

A bit off-topic, but since this continues and the the OP got his answer already ...

The velocity of air molecules, 502m/s, is much faster than the speed of sound, 343 m/s (at 20 degrees C). Also the molecules aren't "smashed". I'm not sure if the outer electron shells are "compressed" significantly more than they would be at sub-sonic speeds. At super-sonic speeds, effective angle of attack is going to be very small. Lift will still be the result of accelerating air downwards (and drag related to accelerating air forwards). The main issue is designing the airframe and engines to deal with the shock waves generated at the leading edges of the aircraft. For the SR-71, adjustable cones at the intake make use of the shock waves to slow the air down, and in addition, use large internal pipes to bypass a significant part of the compressor stage turning the engines into partial ram jet engines. In older aircraft, spikes at the nose were used to initiate shock wave generation. Note that scram jets operate just fine with super-sonic intake and output.

Regarding "laminar" air flow, it doesn't really exist. Even "laminar" air foils are named so only because they maintain laminar air flow for more of their chord length than other air foils. At low Reynolds numbers, such as model gliders, the laminar flow to turbulent flow transition is an issue so they use "turbulators" (tape or a rough leading edge) to prevent this. One of many links about this:

http://www.dreesecode.com/primer/airfoil5.html

Last edited: Oct 6, 2007
18. Oct 6, 2007

### arildno

Sure turbulators are used in order to dissipate energy so that you get "streamline attachment" to the wing faster. This is a crucial feature in how lift is GENERATED, but once that is done, calculations based on inviscid flow are extremely accurate in predicting, say the pressure differential across the wing.

One could equally well say that "pure" objects don't really exist, since we disregard their wave-like nature as well.

It depends on the relevant scale of accuracy what bits and pieces of physics we need to take into account to make a realistic model.

For pipe flow, the laminar Hagen-Pousseille model is often sufficiently accurate.

19. Oct 6, 2007

### codec9

ping pong ball

I think I can explain the ping pong ball using forces. The trick is to realize that air around the ball effects the ball and that the ball also effects the way the air moving.

So when a ball with no spin is flying through the air, say without gravity, there is just one force pushing against the ball straight on in the opposite direction the ball is moving.

When there is a perfect side-spin, say counter-clockwise, the air moves around the ball with the spin. So the air is moving counter clockwise, too. Imagine you are on the ceiling looking down at the ball below. Now let's consider three forces: at the forward tip or "nose" (very unofficial term) of the ball, on left side, and on the right.

At the tip there is still a force going right against the straight path the ball is moving in, so no change there. The only thing that can weaken this straight force is another straight force. The force won't weaken or anything due to a sideways force if there is one, that will just be added.

On the left the air is moving past the ball. So the straight forces not hitting the exact nose of the ball but between the nose and the very left side are basically being helped by the air spinning in the same direction.. you see? What's important is that they are working together on the left.

On the right the air is moving with the ball. It's going along the ball's path but that is also against the air which is sitting there and trying to be inert. So we have the force of the air due to the spin banging up against the air ahead which is sitting still, effectively pushing with a straight force against the top right corner of the between the nose and the very right side. So these forces are working against each other in opposite directions.. you see? That's important.

Now to where it gets a little weird, there is basically more pressure on the right. The air is more dense because the two types of air we are talking about--the moving air just around the ball and the normal air in the room which is staying still (or yes, pushing against the ball because the ball is moving)-- are smashing together on the right. The left on the other hand is smooth sailing. Nothing but free flowing air over there.

Now because it is less dense on the left compared to the right, the ball goes over there. Why? That is just how pressure works. I don't know why.

20. Oct 6, 2007

### codec9

I didn't realize you were originally talking about top-spin (as opposed to back-spin), Skhandelwal. With top-spin the "easy" side where it is less dense is the BOTTOM and the "harder" side where the air is more dense because it is banging against each other is the TOP. So, the ball moves to the bottom.

When you do top-spin the ball arcs downward more than it would from just gravity. When you do back-spin/bottom-spin the ball could arc up or just float along because it is less dense on top.

21. Oct 7, 2007

### craghunath

clarification plz

Hi
I am raghunath, your thought is intersting..
Can you please be more explicit...

letz take the ping pong board as the refrence and when the ball spins specify the direction of angular momentum vector.

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22. Oct 7, 2007

### codec9

I explained it spinning on the side, counterclockwise if you look from the top. So if you hit the ball I am talking about, and you are right handed, you would hit it on the right side most likely or else move the paddle from the left side of your body across to the right side and beyond.

23. Oct 7, 2007

### quetzalcoatl9

i never said that the mean velocity of the gas was equal to the speed of sound propagation (only that they were RELATED - see McQuarrie's derivation in "Statistical Mechanics" for the exact relation, pg. 391 in my copy).

why would you say that the molecules are not smashed? that's what causes the shock wave. i certainly would not say that the gas molecules are flowing smoothly. electronically, yes there will be electrostatic repulsion and Pauli exclusion taking place but that need not concern us if we make the reasonable approximation that the air molecules are inert van der Waals spheres - none of that really matters here. for me, "smashing" is the antithesis of "flowing" and denotes that the air is being pushed away from the wing at a supersonic velocity. the air is "exploding" from the leading edge of the wing.

what you are referring to are rather clever engineering techniques for taking a shock wave (travelling supersonically) and "guiding" it into being sub-sonic with a more streamlined flow. that doesn't address my main point (well, it somewhat supports it) which is that supersonic flight occurs by an entirely different mechanism than subsonic flight...this is why so many pilots died shortly upon breaking the sound barrier. it is easy to understand why this happened when you realize that a sub-sonic wing could hardly be worse for the mechanism that i am alluding to.

24. Oct 7, 2007

### quetzalcoatl9

are these the little triangular things that one typically sees on the fuselage of supersonic aircraft?

25. Oct 8, 2007

### rcgldr

Although ping pong balls don't travel near the speed of sound, I'll answer, but this part should be split off into a seperate thread about supersonic flight.

The molecules aren't smashed, but the amount of jerk and acceleration are very high on any surface producing a shock wave on an aircraft. Wiki mentions that the air can't send "information" faster than the speed of sound, so air ahead of a super-sonic aircraft doesn't "respond" until contact by the aircraft's leading surfaces that produce shockwaves.

Still the principle of lift is the same, the net downwards acceleration of air results in a reactive upwards force causing lift (and the net forwards acceleration of air results in a drag force). Also with good design, the shock wave is used to do some of the forwards acceleration of air, so that a significant part of the leading edges of surfaces don't involve a transition of speed greater than the speed of sound, although this effect diminishes as mach number increases (shock wave angle gets very small).

I defer to these articles:

http://en.wikipedia.org/wiki/Supersonic

http://en.wikipedia.org/wiki/Shock_wave

In the second half of this short video, a F14 does a supersonic flyby, where the "crack" (as opposed to a boom) can be heard:

f14flyby.wmv

Last edited: Oct 8, 2007