BB Rotation

[krap101]

Hey guys,

I've been looking around for a way to measure angular velocity on a bb, and it seems like the only way is to use a high speed camera (without something super duper expensive). I have a camera that shoots at 1000fps (really low resolution), and I'm trying to figure out the order of magnitude of the rpm of a bb (revolutions/minute). At 400 fps, and assuming no slip, I come up with 40,640 rps or 2438400 rpm, which seems too high, so the assumption of no slip must not be true. I believe the camera will be able to pick up rotations up to 30-40000rpm, but not 2.5million...

Is there rpm data somewhere on the net?

Thanks


Here's the math.

Assuming no slip, the point of contact is seen as instantaneously at rest, meaning you can see the center of the bb as rotating about that point. That means that the center of mass is moving at 400fps, 121.92m/s, with a lever arm of 3mm, and angular velocity is v/r=> 40640 rps.

[audi_bhoy]

Basically you want to know the backspin speed? Assuming your 1000fps camera is fast enought not to skip a spin, you should be able to figure it out, by setting a start frame, then looking frame by frame until you get the same angle as the start frame, then calculate from the number of frames the time it took, and finally report it on a RPS scale.

[CDN_Stalker]

I want to see a high speed camera capture a BB (or similar) spinning at different rpm (as is such with different velocities) in a wind tunnel with smoke streams to show/prove my theory about BB backspin effects.

[Fong242]

The bb starts from 0 fps, not 400. You will have to factor in the acceleration.
Fong

[krap101]

Originally Posted by Fong242

The bb starts from 0 fps, not 400. You will have to factor in the acceleration.
Fong

Excellent point. Definitely something that needs to be considered. After thinking about it a bit more, the bb is at rest within the hopup, in contact with the bucking/nub, so that means that its velocity is nearly zero when in contact. Good news for me I suppose.

Originally Posted by CDN_Stalker

I want to see a high speed camera capture a BB (or similar) spinning at different rpm (as is such with different velocities) in a wind tunnel with smoke streams to show/prove my theory about BB backspin effects.

I think I can gain access to a wind tunnel, but what theory would that be? An easier experiment would be to drill a hole in the center of a bb, and use water flow and dye (attaching the bb to some sort of motor)

I'll try getting some footage on Sunday when I test out my new batch of nubs.

[CDN_Stalker]

That hop up decreases drag in a similar way to the way a boattail bullet has less drag than a squared ass one (like a pistol bullet) does. And also that higher velocity causes higher backspin rpm seriously affecting the BB when moving through variable air currents, hence the irregularities with higher velocity in variable conditions. Basically I want you to prove a lot of what I've witnessed and sorted out why it happens, and why a 360fps BB is more consistant and accurate than a 400fps one, and such. BB weight has a huge affect on all (more hop up, more obstruction, higher velocity nets higher rpm and randomness), and cant be done in controlled environments as we are looking at.

[Blackthorne]

We might be able to tell you that in March....


Bastard Labs is starting a comprehensive series of tests on some Pyrex barrels we sourced, and with a high speed camera shooting between 1000 and 100,000 FPS. I was actually going to talk with a contact we have in a US government agency (no I actually really can't say) and cross reference with Stalker to build the test rig that will hold it all.

We are trying to answer allot of questions, but the main one is does the BB touch the barrel during launch, if so how, and if not why. Our fluid dynamics consult said it should be different between propane and normal air as well. (due to density differences)

At 100,000 FPS we should be able to see rotation, but the question then becomes marking the BB. We have some 2 color BB's being drawn up that fit the bill.

Originally Posted by CDN_Stalker

That hop up decreases drag in a similar way to the way a boattail bullet has less drag than a squared ass one (like a pistol bullet) does. And also that higher velocity causes higher backspin rpm seriously affecting the BB when moving through variable air currents, hence the irregularities with higher velocity in variable conditions. Basically I want you to prove a lot of what I've witnessed and sorted out why it happens, and why a 360fps BB is more consistant and accurate than a 400fps one, and such. BB weight has a huge affect on all (more hop up, more obstruction, higher velocity nets higher rpm and randomness), and cant be done in controlled environments as we are looking at.

This is one of the one we might be able to help with. My own theory is that the faster BB needs so much hop it really starts the BB wailing off the walls and messes up the trajectory. Might be solvable with either a new hop design or an "Ultra Tight Bore", something we are also looking at.

[kennysteez]

2 color bb's for consumer use plz and thank you

[krap101]

Originally Posted by CDN_Stalker

That hop up decreases drag in a similar way to the way a boattail bullet has less drag than a squared ass one (like a pistol bullet) does. And also that higher velocity causes higher backspin rpm seriously affecting the BB when moving through variable air currents, hence the irregularities with higher velocity in variable conditions. Basically I want you to prove a lot of what I've witnessed and sorted out why it happens, and why a 360fps BB is more consistant and accurate than a 400fps one, and such. BB weight has a huge affect on all (more hop up, more obstruction, higher velocity nets higher rpm and randomness), and cant be done in controlled environments as we are looking at.

I've gotten into a bit of a scuffle at ASM over bb dynamics theories, but lack the raw data to support my theory and/or create a new one. The premise behind it is that a vast majority of hopup nubs out there currently are cylindrical (symmetrical about one axis), meaning that there is only a point of contact (for stock nubs) or one line of contact for nubs like the SCS/PCS (with some extra because of the coefficient of restitution/compression of the nub, bucking, and bb). Because of this, a higher force is needed to achieve the necessary angular velocity, and hence a higher angular acceleration. A higher force over the short time of contact yields a high impulse, which I believe leads to "slippage" between the bucking and the bb, which then causes inconsistencies in spin. This is the reason why (I believe) the g-hop and the firefly flat hop have been so successful, because you expand a "line" to an area, reducing angular acceleration, and therefore slip, while still achieving the same angular velocity.

As to the difference between 360 and 400, I think the need for more hopup for the 400, causes more "slip" as well as the increase in difficulty to make a "perfect" bb at higher weights. I believe Scarecrow mentioned something about the fillers to make the bb heavier also tend to cause air bubbles, but I haven't stayed up to date with the newer bb's (this might have been fixed) Air bubbles off center are like an unbalanced rotating shaft, causing forces to go all over the place, and at higher rpm, this effect is increased. (Maybe try the zirconium bb's, to see if this reduces/removes this inconsistency).

As to the wind tunnel, it seems like it would be unnecessary, unless you want to simulate a crosswind, or unless you want the bb spinning on a stand against oncoming wind.

Originally Posted by Blackthorne

We might be able to tell you that in March....


Bastard Labs is starting a comprehensive series of tests on some Pyrex barrels we sourced, and with a high speed camera shooting between 1000 and 100,000 FPS. I was actually going to talk with a contact we have in a US government agency (no I actually really can't say) and cross reference with Stalker to build the test rig that will hold it all.

We are trying to answer allot of questions, but the main one is does the BB touch the barrel during launch, if so how, and if not why. Our fluid dynamics consult said it should be different between propane and normal air as well. (due to density differences)

At 100,000 FPS we should be able to see rotation, but the question then becomes marking the BB. We have some 2 color BB's being drawn up that fit the bill.



This is one of the one we might be able to help with. My own theory is that the faster BB needs so much hop it really starts the BB wailing off the walls and messes up the trajectory. Might be solvable with either a new hop design or an "Ultra Tight Bore", something we are also looking at.

Are you planning on releasing the data? I was working on building my own testbed, but it seems like if you guys can do it better and more effectively, then it might as well be you. I was planning on doing something similar with an acrylic barrel, 1000fps camera, and somehow mark the bb without effecting the center of mass (too much). I was thinking something like a sharpie with maybe 8 or so dots. At 1000fps, I estimate the maximum rpm I can measure is 30,000 rpm, where going any higher would lead to aliasing (same reason wheels look like they're going backwards), causing error in the data.

RiotSC over at ASM has conducted a similar test, but I'll get back to you on the results.

[Fong242]

You could also include in your tests the "Tanio Koba"-style twist barrel while at it

[krap101]

Originally Posted by Fong242

You could also include in your tests the "Tanio Koba"-style twist barrel while at it

I do have one sitting around, and could try it out on sunday, but that would be difficult to achieve for the clear barrel test.

[ThunderCactus]

I thought TK barrels didn't actually provide any lateral spin, but rather the spiral barrel grooves simply allow the air to remain laminar and uniform beside the BB, making it easier to keep the BB centered in the barrel. As opposed to straight grooves where air in one groove could be traveling faster than in other grooves.

Also, it's worth testing short barreled GBB pistols versus short barreled AEG's. I'd like to see if my theory is true as well. We had a big discussion about it in another thread, but for those who didn't catch it, essentially I theorize that gas guns have better range and accuracy not only due to higher air pressure to center the BB down the barrel, but because it leaves the hop rubber at a faster velocity, increasing the spin. And that it would explain why a GBB shooting 300fps with .30s BBs would get a better trajectory than an AEG shooting 300fps with .30s. And that's absolute velocity measured with a .3 on both guns.

[Styrak]

Originally Posted by krap101

Here's the math.

Assuming no slip, the point of contact is seen as instantaneously at rest, meaning you can see the center of the bb as rotating about that point. That means that the center of mass is moving at 400fps, 121.92m/s, with a lever arm of 3mm, and angular velocity is v/r=> 40640 rps.

Maybe I'm missing something here, but wouldn't the spin velocity change based on hopup material used and also have no relation to how fast the BB is travelling (at least not a linear relation)?

[CDN_Stalker]

The most perfect and longest range I've seen BBs go were my M24 shooting at just under 340fps (0.20g) and floating 0.30g BBs out past 300ft, my 'stock' TM MP5 yet shooting 360fps (0.20g) and getting 0.28g out past 200ft, and my M24 again after many years shooting 500fps (0.20g), getting the amazing consistancy at all ranges, shooting at 395fps (0.20g) and 0.36g combination. Shooting my M24 at higher velocities gave all kinds of short range hook and flyers, no matter what I did to the gun, led me to see that hop up has a negative effect at higher speeds, increased backspin, more unstable when meeting higher densities of air (aka. normal air currents on a calm day)......... etc.

Nutshell, which I've pointed out for years, there are sweet spots for each BB weight, and upgrading your gun thinking it'll perform better isn't always the case, of which I've seen and dealt with guns that were expected more of, but failed miserably with the owner thinking they did something wrong in part choice, etc. I don't put any spell on my BBs, at least no more than the average fellow shaman does, so I have seen over the years similarities in weight vs. speed effects, and my interest level background (and work education) in aerodynamics got me thinking lots about BBs with backspin moving through air (of which led me to determine that drag is reduced greatly, as well as the known of lift being generated through the Bernoulli effect)...........

[Dynamo]

ok.. this is really easy to do with a highspeed camera..it just takes some planning. take a BB, draw a line around or even just take a sharpie and color one whole hemisphere in black. take a piece of cardboard and draw some vertical lines with 1inch spacing. take the BB and manually place it in the opening of the hop chamber so that the line/hemisphere is lined up horizontally. then proceed to shoot the BB past the lined cardboard and film it. when looking at the film, you can track the BB as it passes over the lined cardboard and count how many lines it passes for every frame, as well as count the number of revolutions the BB makes. no need to calculate acceleration from the hopup down the barrel, as the BB will start to decelerate the moment is leaves the end of the barrel.