Arrow Speed - Quick Question for the Brain Trust

[TxAg]

http://www.youtube.com/watch?v=LfnQ884zFlM&feature=youtube_gdata_player    


Has anyone tried this? You download it to your iphone or ipad. It is called Achrono. Did you find the readings accurate?

Here is what I got. What do you think?

I tried it out tonight. I have never chronographed a bow before. I figured this was a cheap way and might be fun. From the reviews I've seen, it is accurate within 2-3 fps as long as you measure consistently.

Tell me if this sounds reasonable....

I shot about 20 arrows. I consistently got readings in the 180s. I didn't do the math, but I would say 183 fps was the average. I measured the distance exactly and measured the sound readings consistently. This was with a 45@29 66" Toelke Whip shooting 12gpp arrows with a 9 strand ultracam string. I was expecting something around 170 fps.

For the record, I love this bow and don't care if it shoots 200 or 150, but I'm interested to know if the 183 sounds legit.

[pdk25]

No brain trust here, but sounds a little high to me.

[TxAg]

That's what I was thinking. It certainly isn't a scientific test, haha

[pdk25]

I just crunched some KE numbers quickly, and I think that would put you at 200.5 fps at 10 gpp, and I think that is a little more than you are probably getting.

[TxAg]

Sounds like your math fits with my initial thoughts of low 170s. Thanks.

One of these days, I'll shoot through a real chrono just for kicks.

Anyone else have a thought on the matter?

[khardrunner]

Really an interesting way to chrono. You could do the math yourself and figure out if it's rigged to go higher.

I might steal some equipment from school and try to rig up something super accurate based on this. I LOVE the idea!

I would think that the problem areas are:
1)finding the exact instant the sound starts
2) same with sound ending
3) knowing the exact distance the arrow is flying in that amount of time

the rest is simple math

[TxAg]

The distance was no problem....just used a long tape. The sound starting and stopping was tough. I went from peak to peak to make it easier visually.  If you try it, let us know what you come up with

[khardrunner]

Will do. The smallest difference there can be HUGE on the output.

[ScouterMike]

I wonder if the program takes into account the fact the arrow velocity is about 1/6 the speed of sound. The positioning of the mic in relation to the bow/target would affect the results especially if at either end. A center position between would kind of cancel this out.