Chronographing an arrow without a chronograph

[Zedd]

I'm one of those fella's who read the posts, but rarely reply choosing to use the search button to get answers to usually stupid questions of mine...I think there is a word for guys like me. Yes, I am a troller. My background is physics (former Navy-Nucular Tech) and figured with available archery ballistics calculators available on line, I could get a fair arrow velocity/momentum at distances given arrow mass and three points of trajectory.

• 614 grain arrow, measured on a hornady bullet scale
• zero @ 10 yards, group size 3 1/2" (I use a sight)
• 4" @ 15 yards (using the 10 yard pin), group size 4"
• 10 /14" @ 20 yards group size (using the 10 yard pin) 5 1/4"

All I have to do then is change the velocity in the input table until I match the charted trajectory. This is giving me a 614 gr arrow speed of 187 fps. First point, obviously making sure my groups are accurate is important. My groupings are fairly consistent (though I hope to get better as I practice. Second point, the scale is calibrated so I am confident of the mass of the arrow. All that being said, it seems like the velocity for such a heavy arrow is very high. I have shot distances in between these and the arrow definitely seems to be following this trajectory. Is there something wrong with my logic, or am is it possible to be getting such good performance from such an inexpensive bow?

[bradsmith2010]

we would need to know how far you are drawing the arrow
and what the weight of the bow is at that draw length
and what kind of string you are using
and what kind of bow it is,,
a 60# wood bow, might shoot that arrow,, 160 fps,,

[Zedd]

Samick Sage, 50# draw at a true 28" draw length. I am using fast flight string, I also have otter fur for string silencer.

[bradsmith2010]

i am not familiar with that fiberglass bow,,
but I would think that a 500 grain arrow would be doing good to hit 187,, so to answer your question,, it seems a bit fast for the arrow weight,,
shoot it though a chrono graph and see what the difference is,,
I am guessing 170 with the 600 grain arrow,,

[willie]

figured with available archery ballistics calculators available on line

without knowing the underlying assumptions of the calculator, it's hard to tell if you are duplicating a trajectory with the same parameters? can you link to the calculator you are using?

I can think of some aerodynamic effects that might influence arrows , and it would be interesting to see (if possible), what the calculator presumes

[DC]

I know nothing about the physics of the thing but it seems to me that there is a lot of room for discrepancies with a 10-14 " drop and a 5" group. If you juggle the figures a bit what do you think you would get for a maximum and minimum speed? It seems to me that with the group being 30-50% of the drop...... How many times did you try it? Can you come up with a way to map the trajectory of one shot? Maybe shooting through paper sheets or something.

[loon]

wonder about possible accuracyprecision of slow motion video or audio recording

[Zedd]

without knowing the underlying assumptions of the calculator, it's hard to tell if you are duplicating a trajectory with the same parameters? can you link to the calculator you are using?
I can think of some aerodynamic effects that might influence arrows , and it would be interesting to see (if possible), what the calculator presumes

Crap, I thought I included this...http://www.outdoorsden.com/archery/archbal.asp. Sorry about that.
 Realizing this is the 'Primitive Archery' forum, calculus has been with us since at least the 1600s, so I consider it in the purview of the site, though it probably is not in the interest of many. I do appreciate the feedback coming in so fast (and politely). One aspect this ballistics calculator does not say it addresses is if the calculation is 'simple math' or if it relies on differential calculus to determine the trajectory, i.e., the ballistic coefficient of the arrow. That being said, the actual trajectory matches the arrow I am slinging. That does not necessarily mean the velocity deriving the chart is 'true'.

[Zedd]

I know nothing about the physics of the thing but it seems to me that there is a lot of room for discrepancies with a 10-14 " drop and a 5" group. ....

That was a typo on my behalf, that should be 10 1/4" drop with 5" groups, i.e., the arrows were roughly withing 2 1/2" of a point 10 1/4" from the target. You have, however, hit on the main weakness of this 'method' of determining speed of the arrow, that being my own accuracy (or lack).

[Zedd]

The last time I chronographed this bow, it recorded about 174 fps. It was owned by the Archery target club and members told me it was not accurate.

[bradsmith2010]

if it was shooting several shots in that range,, it was probably accurate,, if that was just one random shot, hard to tell,,

[willie]

and members told me it was not accurate.

not accurate, or not precise?

to make a claim for inaccuracy, one would have to have consistent readings that deviated from a otherwise known velocity.

perhaps they meant not precise. They could have assumed their shooting was more consistent than the readings they read, or it could be they are not taking the time to weed out poor data from chrony operator errors.

Might I suggest to the OP, that he email a link of this thread to the calculator author?

[bradsmith2010]

Willie I like your line of thought

[Zedd]

and members told me it was not accurate.

They could have assumed their shooting was more consistent than the readings they read, or it could be they are not taking the time to weed out poor data from chrony operator errors.

Might I suggest to the OP, that he email a link of this thread to the calculator author?

They said 'not accurate', though I know what you mean. Even compound shooters need to have consistent drawing standards to keep consistent velocities. In the end, the trajectory table is very consistent with what I am shooting. The real question I am asking is how close to the velocity am I really getting.

[bradsmith2010]

not close,,