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600 fps -new results

A place to ask general spud cannon related questions.
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Unread postAuthor: Velocity » Fri Apr 27, 2007 7:25 pm

hi wrote:ya, and another thing is that maybe at 10 feet away you loose 10-20 fps because of drag. it may not be as much as 20 fps, but maybe 5-10. i have no proof of you loose that much, but its possible.

edit- so the muzzle volocity is not 602, the speed at 10 feet is 602 fps.


I'm a complete novice at all areas regarding physics, but I think it would be even more than just drag which slows down the projectile. A chronograph measures the time which it takes for an object to pass from point A to point B. It then uses this figure as the velocity reading.

However, this number is not actually the muzzle velocity (no matter how close you put the barrel to the chronograph); rather, it is the average rate of change from point A to point B, not the instantaneous rate of change at point A; the latter would be the muzzle velocity.

This idea kind of brings up another question: Is there any means to measure the exact muzzle velocity of any type of spudgun or such, considering that you do not have an accurate formula of the speed of the projectile?

Sorry if this post is kinda random/completely wrong; I was just thinking aloud...
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Unread postAuthor: sjog » Fri Apr 27, 2007 7:41 pm

I've heard of a more modern way I think it had video as well. Not sure.
But it gave a better break down, because it was being used to measure shotguns and their pellet pattern's
Chrony for me is fine, the two sensors are only about 15" apart.
I was curious as to the speed in general , now I'm getting hooked on making higher numbers. I've got the chrony for as long as I want.
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Unread postAuthor: frankrede » Fri Apr 27, 2007 10:50 pm

I have little experience with chronographs, but would using a clear barrel that you put in the chronies "testing" area work?
I don't know if that would work but it could give you a more accurate reading from the muzzle.
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Unread postAuthor: Velocity » Fri Apr 27, 2007 10:59 pm

frankrede wrote:I have little experience with chronographs, but would using a clear barrel that you put in the chronies "testing" area work?
I don't know if that would work but it could give you a more accurate reading from the muzzle.


Now that is a very interesting idea if it turns out that it would work. You could position the muzzle exactly halfway in between the two sensors, and hopefully the muzzle velocity would be recorded (I think it would be a bit lower than the true muzzle velocity though, because the potato is not at full speed as it is approaching the muzzle and after it leaves the muzzle).
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Unread postAuthor: hi » Fri Apr 27, 2007 11:49 pm

i doubt that it would work because it would think that it is like radar, it sends out radion waves and messures the return time.

ps- "i would think" means that i have no idea how the thing works, but if i was working for a company that made cronographs thats how i would build it. i may work entirly differently, but its worth a try.
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Unread postAuthor: sjog » Sat Apr 28, 2007 11:42 pm

Chronograph is a speed trap. Two sensors "see" the projectile. Clocks the time it takes to pass between photo electric "eyes". They are about 16" apart.
It has to see movement to clock it .It will not see thru clear barrel. I dont think??
I'll take pic of inner workings and post if any one interested.
I shot over it twice today 585 fps when it was warm out, then I shot at sunset, cooler temp. 601fps same as the other day.
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Unread postAuthor: jimmy101 » Mon Apr 30, 2007 10:52 am

As far as I know all shooting chronies use a pair of gates and measure the time of flight between gates.

It is possible to build a chrony that doesn't use two gates or that doesn't even work in the time domain. I believe most speed guns use doppler shift instead of time. The gun actually measures the frequence shift of a signal bounced off the target and returned to the gun.

Below is a graph of what the typical velocity versus distance from barrel might be for a 2" spud starting at 600 fps. (This was calculated for a horoizontal shot, drag proportional to velocity<sup>2</sup> and a drag constant based on a terminal velocity of 105 fps which is the same as a baseball or golfball.)

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10 feet from the barrel a 600 fps velocity will have dropped by about 17 fps.
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Unread postAuthor: sjog » Mon Apr 30, 2007 8:04 pm

jimmy101 When I borrowed chrony the owner( he uses it for pistols, loads his own ammo ) he told me to shoot from 10'. When other people suggested getting closer ,I tried from 4-5" with the same results.
the only change in speed had to do with tempurature.
When I shot at 49- 52 degrees it went 595-600 fps at 62-64 degrees 585-587fps. I've shot over the chrony about 6-8 times, the results have been the same. Getting bored with it.
But now that you are interested I will shoot tomorrow . I will shoot from 3' and 15' to see if there is a difference. It may be a warm day but I can try for an average speed from 3 distances.
You have me thinking , results could be interesting.
I will post tomorrow evening 9 pm eastern.
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Unread postAuthor: jimmy101 » Tue May 01, 2007 11:03 am

SJOG: I don't think I would worry about the difference in velocity measured at a few feet versus say 10 feet. The difference just isn't all that great.

Just pick a distance that works well for you and always use about that distance. If you like ten feet then anything in the range of say 8 to 12 feet is fine.

http://www.competitionelectronics.com/media-files/pro-tach_op_instructions.htm has some info on the care and feeding of the Pro-Tach chronometer. Like most shooting chronys, it reads in FPS.

The numbers you have posted so far are; 584, 587, 596, 593, 483*, and 602. If you omit the 483 value (since it was fueled differently) then the average velocity is 592 +/- 7 FPS (error is the standard deviation of the values). That is just a ~1% shot-to-shot variability. For a spudgun that is remarkably good. I don't think you need to invoke changes in temperature (or atmospheric pressure) to rationalize this level of variability, it is just the nature of a homemade device firing hunks of veggies.

A small change in the ammos mass has a significant affect on the muzzle velocity. If a 100g spud fires at 600 FPS then you would expect a 105g spud to fire at ~586 FPS (assuming identical friction).

You are shooting spuds?

In my experience spuds a pretty variable. They have different masses and different frictions. When Latke did his chrony tests he was seeing standard deviations of ~10% for spuds (IIRC). 10% is a much greater level of variability than is the difference between having the chrony at 5' versus 10'. It is also a much greater variability than what you are seeing.

BTW, that really is a great looking gun.
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Unread postAuthor: sjog » Tue May 01, 2007 7:17 pm

604-3
590-8
589-16
590-20
590-25'
don't think you need to invoke changes in temperature (or atmospheric pressure) to rationalize this level of variability, it is just the nature of a homemade device firing hunks of veggies

I only shoot golf balls And yes you're right It is a crude device.
Thanks for the compliment. Thanks again for the info on Pro-Tach
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Unread postAuthor: iknowmy3tables » Tue May 01, 2007 10:09 pm

well I'm almost comppetly lost in the 4 pages of pst but I hope this helps:
the mph convertion is worthless so here are some comparisons. for a projetile the size of a golfball you did really well
cheapo airsoft mini AEG 150fps
cheapo airsoft spring gun 210fps
decent airsoft and good paintball guns 250fps
upgraded pimp'n AEG 320fps
.38 Special 600 fps
speed of sound 1100fps
low recoil 12 gauge skeet round 1400fps?
.22 rimfire cartridge 1200 - 1500 fps
.22 centerfire cartridge 2400 - 3000 fps
.22 Swift 4000 fps
.221 Fireball 2650 fps
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Unread postAuthor: jimmy101 » Wed May 02, 2007 10:58 am

Sjog:

Ah, i see, golf balls. I was wondering how you were getting such good shot-to-shot reproducibility.
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Unread postAuthor: sjog » Wed May 02, 2007 5:09 pm

I thought the .45 was at the low end for speed. High caliber --low velosity.
At 960fps
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