I have not had a lot of time for writing lately due to several issues, but I did have a chance to combine some learning at my day job with a little bit of “arrow” dynamics. Because the weather has limited my shooting, I decided to do a little bit of computational fluid dynamics (CFD) and look at what is happening at the tip of the arrow. Does tip choice have much to do with with the flight of the arrow? Today we’ll look at a couple of popular target tips, the Easton parabolic and Goldtip EX-Pull. At a later date broadheads will be added to the mix.
The analysis was done with a .290″ outside diameter shaft and arrow speed of 300 fps. My objective was to gain a better understanding of what is happening as the arrow slices through the air. First up are the flow trajectories:
This image shows the flow lines that particles of air follow as the arrow flies through them. It’s obvious that the EZ-Pull is more disruptive to the streamlines as the sharp edges cause a much larger jump in the particles.
The next image shows the pressure distribution; the turquoise is standard air pressure, green/yellow/orange/red are incrementally higher pressure, and the blues lower pressure:
What happens as any high speed object moves through a compressible fluid such as air is that a higher pressure area builds up at the front. This creates a force that resists the forward movement of the object. In the case of the EZ-Pull tip that has a backwards cutting angle, the blue areas show where a low pressure builds up. The low pressure in this case actually pulls backwards on the head and has the same effect of slowing down the object just as the a high pressure up front. The net effects of all the pressures added up are what comprise the total drag on the object.
Total drag forces for each of the heads was .00997 lbf. for the parabolic and .01413 lbf. for the EZ-Pull. That translates to 40% more drag for the EZ-Pull! Granted the forces aren’t huge but every little bit adds up and contributes to the deceleration of the arrow as a whole.
This little study is far from a complete picture of what is happening in flight but did show some interesting things. In the future I’ll be doing the same for some different broadhead types as well as looking at the tail end of the arrow and various fletching types.