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Posted: Mon Dec 08, 2008 12:21 am
by D_Hall
jimmy101 wrote:About the same as the 70,000 in the OP.
I'm very suspect of that number. Watching the video it's hard to tell precisely what it is they're using to measure that speed, but I have a sneaking suspicion that the device is counting dimples as they go by rather than actual rotation. If so....
[pause while Dave finds a golf ball]
...Then the number would actually be on the order of 2,200 rpm. A very reasonable number, IMHO.
To get a bent barrel to impart spin doesn't that require some blowby? A tight fitting ammo wouldn't get much spin from the bent barrel.
True, but the same could be said of ANY system. You simply aren't going to see backspin in a barrel with a tight fitting projectile regardless of how you're getting it to actually spin.
Posted: Mon Dec 08, 2008 11:39 am
by psycix
But DR, what keeps the golfball in its place before you launch it?
When adding air behind it the golfball will get pushed up the barrel, even if there is air escaping at the back.
Posted: Mon Dec 08, 2008 11:56 am
by jimmy101
D_Hall wrote:
To get a bent barrel to impart spin doesn't that require some blowby? A tight fitting ammo wouldn't get much spin from the bent barrel.
True, but the same could be said of ANY system. You simply aren't going to see backspin in a barrel with a tight fitting projectile regardless of how you're getting it to actually spin.
Unless you use an O-ring top hopup at the muzzle, or a barrel that isn't cut square, or a barrel that slightly increases it's ID right before the hopup at the muzzle.
I guess you could also use a bent barrel + loose fitting ammo with a tight fitting sabot or wading.
Posted: Mon Dec 08, 2008 3:21 pm
by DR
psycix wrote:But DR, what keeps the golfball in its place before you launch it?
When adding air behind it the golfball will get pushed up the barrel, even if there is air escaping at the back.
With the barrel mounted in a fixed position and at an angle, <i>gravity</i> would keep the ball in place, and prior to launch, the ball is resting on the concave part of the 2" x 1" Reducer.
Posted: Mon Dec 08, 2008 6:12 pm
by Hotwired
D_Hall wrote:jimmy101 wrote:About the same as the 70,000 in the OP.
I'm very suspect of that number. Watching the video it's hard to tell precisely what it is they're using to measure that speed, but I have a sneaking suspicion that the device is counting dimples as they go by rather than actual rotation. If so....
[pause while Dave finds a golf ball]
...Then the number would actually be on the order of 2,200 rpm. A very reasonable number, IMHO.
I wouldn't put my name down to say that was 70krpm but 2.2k does seem to be a very low number considering the very high pitched whine it was giving off. Rotary air tools whose upper rpm doesn't need to be limited such as angle grinders and cut off saws can do 30k.
I don't think it was counting dimples but rather the dark lettering as it spun about, you can see the numbers jump on the meter, probably because the airflow is being directed by hand and the ball has no fixed axis to spin on. But I wouldn't be surprised if it was doing at least several tens of thousand rpm.
Posted: Mon Dec 08, 2008 9:57 pm
by TurboSuper
I'm inclined to agree with Hotwired on this one: those photo-tachometers work based on reflection, so it seems more likely that it was picking up the lettering rather than the dimples.
70k seems a bit far-fetched, but I could see it easily doing 40k, just by looking at how long it took the ball to stop rolling...