*By Paul Fender*

For some time now I have been wanting to get actual chronograph readings on arrow velocity at different yardages. I knew that the smart thing to do would be to build some sort of barricade in front of the chronograph so I didn’t accidentally shoot it and blow it up once I got out at some decent distances. After all, Doo-Doo happens, right?

I finally came up with a quick and easy way to cobble together a barricade that would enable me shoot across my chronograph, and that would effectively protect it while not utterly destroying any errant arrows. Good thing I did this too. Sure enough, I shot the barricade once. Oops.

What follows are the velocities I measured from 2 yards out to 35 yards. That was the longest distance I could get in my back yard while still having room for the chronograph and the barricade in front of the target butt.

Yards Velocity in Feet per Second (fps)

2 204

10 196

15 188

20 181

25 174

30 170

35 167

Right off I noticed something that at first seemed odd. The first, closer distances measured showed larger drops in velocity than what was measured at longer distances. I then realized that this actually just makes sense. Drag is a major player in an arrow’s trajectory and how its velocity changes over time. The force that drag exerts to counteract forward motion is proportional to forward velocity. That is, the faster the arrow goes, the greater the force that drag exerts against the arrow. So, initial deceleration will be greater than the deceleration later in the arrow’s flight. This is of course a big part of why an arrow doesn’t follow a “true” parabolic ballistic trajectory.

Once I had that information I then realized that I could take things a step further and look into something else that I had been wondering about. I could figure out how fast the fletching was making my arrow spin at a particular yardage.

I built a “trap” out of a cardboard box and two sheets of paper. I opened up the box and taped the sheets of paper across the open ends. This put the 2 sheets of paper 18.5 inches apart. I then smeared red lipstick onto one fletch of one of the arrows that I had used for the velocity measurements. I just arbitrarily decided to figure out rate of rotation at 20 yards. So, standing 20 yards from my “trap”, I shot the lipsticked arrow through it. This left clearly defined marks on both sheets of paper. Using a protractor, I then was able to measure that in the time it took for the arrow to pass through the sheets of paper, it had rotated 85 degrees.

I’ll save us all from having to trudge back through the math, but using the velocity originally measured at 20 yards, and knowing that the arrow spun through 85 degrees as it traveled 18.5 inches, I was able to calculate that the arrow was spinning at a rate of approximately 1660 revolutions per minute. That figure is of course rounded off. After all there is no point in trying to maintain some semblance of a precision of measurement that simply does not exist.

Of course, we’re now left with the question, “But what does all this really mean?” To be perfectly honest, not much. There is no real “conclusion” to be drawn from this. None of it is either “Good” or “Bad.” As they say, “It is what it is.” I just found it rather amusing to be able to finally answer a couple of questions that had been bothering me for a while.

Paul,

This is great data, and something I was threatening to do myself. I will send you a workup that I did of your data as a spreadsheet. It confirms something I see when I try to model the ballistics of arrows in arch4 – the drag coefficient falls of very rapidly in the first 20 yards. I’m guessing that this is due to the fletchings being pushed back and forth in the airstream due to “paradox” effects.

If you get bored, you might do the exact same experiment with bare shafts. My prediction is that the change in drag coefficient will still be there, but greatly reduced. My predictions are almost always wrong…

Regards,

Jim Conger