[rntlee]

I know, they don't work...but they also can't work!

Consider the following compiled experimental data on lift coefficients:




Line 1 pertains to sphere's with a smooth surface. V/U is the ratio of the spin velocity (V) to the linear velocity (U). According to this data, most spin-induced forces in paintball (excluding super-high rpm spins like the Apex causes) occur in the negative lift region of the chart (the portion up to about 0.4 V/U). This means that a lower rpm backspin would actually impart a negative lift force and cause a decrease in range.

You would actually want a "topspin" bolt to increase range.
....hah, I'm bored as you can plainly see .

This post has been edited by rntlee: 25 December 2011 - 12:35 PM

[brycelarson]

huh, interesting find.

[Molybdenum]

Cookybiscuit, on 25 December 2011 - 05:26 PM, said:

-Jack Rice-

He would probably tell you that the number is above 0.4, and there fore is out of the negative range.

This post has been edited by brycelarson: 26 December 2011 - 09:50 AM

[rntlee]

well, with a 280 fps initial velocity, the initial spin would need to be around18000 rpm to result in a net positive lift force. The lift coefficient would begin negative but switch to positive as the ball's velocity decayed.

This post has been edited by brycelarson: 26 December 2011 - 09:51 AM

[Cutterj]

Big words make me dislike alien

[MrEeske]

rntlee, on 25 December 2011 - 12:34 PM, said:

but they also can't work!

A salesman's priority isn't to make the best functional product, just the best selling one. Sensational fads like cross-drilled brake rotors (obsoleted by modern day ceramic pads) and backspin bolts exist just to make a quick buck. HOWEVER, I'm glad to see someone shining the torch of science to bring illuminance the [potentially false] performance rumor of these bolts.

rntlee, on 25 December 2011 - 12:34 PM, said:

You would actually want a "topspin" bolt to increase range.

Careful with how you say that; the data tells nothing about what may happen with the opposite spin direction. A spin in the opposite direction may have even worse performance, with respect to lift. By the way... what axis is the ball spinning around? From what is shown, we can only assume that the rotation is backspin. The data doesn't say whether the spin is foreword, backward, or even if it's being spun like a bullet.

[drg]

So spin-induced lift doesn't exist?

[rntlee]

MrEeske, on 25 December 2011 - 09:22 PM, said:

rntlee, on 25 December 2011 - 12:34 PM, said:

You would actually want a "topspin" bolt to increase range.

Careful with how you say that; the data tells nothing about what may happen with the opposite spin direction. A spin in the opposite direction may have even worse performance, with respect to lift. By the way... what axis is the ball spinning around? From what is shown, we can only assume that the rotation is backspin. The data doesn't say whether the spin is foreword, backward, or even if it's being spun like a bullet.

It actually does if you look into the data. The "Magnus effect" is a well documented force affecting spinning spheres. In the graph I posted above, the area below the "0" line is called the "anti-Magnus" region, the area above is the Magnus region. The axis of spin isn't important as the force is relative to the direction of spin. If the spin is off-axis left or right to the direction of travel, the force will act to move the ball to the left or to the right (an Apex "hook shot" is an example).
In the anti-Magnus region, the direction of the net force is reversed, whereas a backspin will now induced a negative rather than positive lift and a topspin will induce a positive rather than a negative lift.

It's interesting to note that this data is the result of measured experiment, not calculation. It's still not certain why smooth sphere's exhibit "anti-magus" properties at low V/U. I'm not aware of any mathematical models that predict it, though I'm definitely no expert.

[rntlee]

drg, on 25 December 2011 - 09:42 PM, said:

So spin-induced lift doesn't exist?

No, it definitely exists...but the direction of lift just isn't simply "positive for backspin and negative for topspin". It's dependent on the ratio of the rotational speed of the surface of the ball to the linear velocity of the ball. (for smooth sphere's travelling in a viscous fluid)

This post has been edited by rntlee: 25 December 2011 - 11:38 PM

[Kermit]

shit, not this again...


Interesting find though, I'd like to see more without alien being brought into it

[drg]

rntlee, on 25 December 2011 - 11:09 PM, said:

drg, on 25 December 2011 - 09:42 PM, said:

So spin-induced lift doesn't exist?

No, it definitely exists...but the direction of lift just isn't simply "positive for backspin and negative for topspin". It's dependent on the ratio of the rotational speed of the surface of the ball to the linear velocity of the ball. (for smooth sphere's travelling in a viscous fluid)

So couldn't a backspin bolt potentially work, if it achieved the requisite spin?

[andrewthewookie]

Yes, but only if they actually put spin on the paintball. So far, there haven't been any tests that have shown them to do so.

[brycelarson]

Please leave alien out of this unless it's in a purely neutral conversation. No insults, no name calling.

drg - yes, spin systems can work. Its based on the spin rate ratio to velocity. It's the reason that an Apex set on the higher settings dip before rising. The spin rate (more than 25k) isn't high enough to produce lift until the velocity of the ball has dropped somewhat.

We have high speed footage of the apex - it consistently produces extremely high rotational speeds and always in the same direction. The low blow bolt systems we've filmed produced rotation at speeds and directions that roughly matched any normal bolt system - in other words <2400 at random speeds in random directions.

[MrEeske]

rntlee, on 25 December 2011 - 11:04 PM, said:

The "Magnus effect" is a well documented force affecting spinning spheres.

I am acutely aware of the Magnus Effect and how to apply it to a sphere. As I was once a tournament-participating tennis player of several years, playing with spin on tennis balls was a necessity in winning matches (e.g., top spin, back spin, side spin).

For the record, I am a believer of the Magnus Effect. I am not arguing that spin does not induce lift; from my own experience in tennis and with education of fluid dynamics, spin very much induces lift; however, the direction of that lift is dependent upon the axis of rotation and direction of rotation. The concept of the Magnus Effect is that the rotating sphere is creating pockets of low and high air pressure, just like an airplane wing. The location of those pockets of different pressure depend, again, on the axis and direction of rotation.

With that out of the way, what I was criticizing wasn't so much your conclusions about the data, but the presentation of the data. It's a chart with squiggly lines, but what those lines represent isn't well defined. For example, why are there two lines for each sample? The chart seems to indicate that it displays both coefficient of drag (C_D) and coefficient of lift (C_L), but which line is which? Also, those numbers are entirely dependent upon the Reynolds number of the sphere... does a paintball have a Reynolds number of 94,000?

By the way, one article that data is from, The Aerodynamics of Golf Balls, is specific to golf balls. The article also assumes perfect spheres, which our paintballs are most definitely not due to their seam.

Again, I'm not criticizing the conclusions, I'm criticizing the presentation of data. Give context to the numbers. I'm not trying to kill this conversation, just making sure of the quality of presented data.

This post has been edited by MrEeske: 26 December 2011 - 01:09 PM

[cockerpunk]

interesting find, i like the graph.

can i get a source on that? looks to be from an academic paper.

This post has been edited by cockerpunk: 26 December 2011 - 03:51 PM

[rntlee]

cockerpunk, on 26 December 2011 - 03:50 PM, said:

interesting find, i like the graph.

can i get a source on that? looks to be from an academic paper.

I took that graph from Gary Dyrkacz's site... I'm not sure if it's his own compilation or whether it originated somewhere else.

[rntlee]

MrEeske, on 26 December 2011 - 12:59 PM, said:

... what I was criticizing wasn't so much your conclusions about the data, but the presentation of the data. It's a chart with squiggly lines, but what those lines represent isn't well defined. For example, why are there two lines for each sample? The chart seems to indicate that it displays both coefficient of drag (C_D) and coefficient of lift (C_L), but which line is which? Also, those numbers are entirely dependent upon the Reynolds number of the sphere... does a paintball have a Reynolds number of 94,000?

The chart does show both the drag coefficient and the lift coefficient, with C_D being the top line as it is definitely the larger force. The reynolds number of a paintball @ 280 fps is somewhere around 94 000.

MrEeske, on 26 December 2011 - 12:59 PM, said:

By the way, one article that data is from, The Aerodynamics of Golf Balls, is specific to golf balls. The article also assumes perfect spheres, which our paintballs are most definitely not due to their seam.

That's a good point...should well made paintballs be considered "smooth sphere's" ? I think so, as Maccoll obtained similar anti-Magnus readings using wooden spheres. It would really need to be demonstrated via experiment to be certain though.

[drg]

brycelarson, on 26 December 2011 - 09:48 AM, said:

drg - yes, spin systems can work. Its based on the spin rate ratio to velocity. It's the reason that an Apex set on the higher settings dip before rising. The spin rate (more than 25k) isn't high enough to produce lift until the velocity of the ball has dropped somewhat.

We have high speed footage of the apex - it consistently produces extremely high rotational speeds and always in the same direction. The low blow bolt systems we've filmed produced rotation at speeds and directions that roughly matched any normal bolt system - in other words <2400 at random speeds in random directions.

Right, of course what I'm getting at is that the OP claimed that backspin bolts "can't" work ... which is a pretty broad claim and is not supported by the evidence he presented.

By the way the spin depression/rise effect is well known to anyone who golfs. It tends to happen on lower-loft, longer clubs.

This post has been edited by drg: 28 December 2011 - 04:26 PM

[rntlee]

drg, on 28 December 2011 - 04:24 PM, said:

... what I'm getting at is that the OP claimed that backspin bolts "can't" work ... which is a pretty broad claim and is not supported by the evidence he presented.

I stand by my statement.
"Backspin" bolts will not increase range as low rpm (>18000) backspin will impart a downward force, not a lifting force. Unless you feel backspin bolts are capable of inducing this quantity of spin.
You're question concerned whether spin effects exist, which they do. Unfortunately, for smooth sphere's the forces are reversed at low V/U, which is right where the spin induced by backspin bolts would reside.

It's all an academic discussion anyway... no "backspin bolt" tested here has ever demonstrated an ability to induce any consistent backspin anyway.

[drg]

rntlee, on 28 December 2011 - 10:21 PM, said:

drg, on 28 December 2011 - 04:24 PM, said:

... what I'm getting at is that the OP claimed that backspin bolts "can't" work ... which is a pretty broad claim and is not supported by the evidence he presented.

I stand by my statement.
"Backspin" bolts will not increase range as low rpm (>18000) backspin will impart a downward force, not a lifting force. Unless you feel backspin bolts are capable of inducing this quantity of spin.
You're question concerned whether spin effects exist, which they do. Unfortunately, for smooth sphere's the forces are reversed at low V/U, which is right where the spin induced by backspin bolts would reside.

It's all an academic discussion anyway... no "backspin bolt" tested here has ever demonstrated an ability to induce any consistent backspin anyway.

Current or tested backspin bolt designs in specific barrel setups may not work in this way, but that's not to say another design or combination may not be created that does. Since you did not specify the designs to be current or anything at all, you leave your statement open in perpetuity to being proven wrong, and unable to ever be proven right. Sharpen the thinking and be specific, this is supposed to be an egghead type forum, is it not?