Quote:
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.
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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.
Quote:
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.