Broadhead vs Field Point POI - Again

[nchunterkw]

I hesitated to post this but figured what the heck, a lot of us aren't hunting right now and need some excitement.

Did some shooting yesterday getting ready for an upcoming pig hunt.
Bow: 2011 Vortex
Trigger: TT 2 stage
Scope: Hawke XB1
String: Excal FFF
Arrow: Easton FMJ, brass insert, Blazer Vanes, 150gr FP, toal wt ~~575gr

I setup at 20 yds (verified with my range finder)
Shooting from a bench with front and rear bags
Sighted in for dead on with center crosshair.

Moved to 40yards (verified with the range finder)
Same bench and bags
Sighted in for dead on with my 40yd crosshair using only the speed ring for adjustment.

I then changed out the FPs for 150gr, 2blade Magnus Stingers and re-checked zero at 40 yards. The 4 arrow group was consistently about 0.75" to 1" low.

Went back to 20 yards with no scope adjustments, and all 4 arrows were still dead on with the BHs installed.
Went back to 40, made one adjustment to the speed ring and all is well - for me - not the pigs hopefully

THEORY #1:
The ballistics coefficient of the arrow with the BH installed is higher than the arrow with a field point (ie BH has more drag). While this may not be noticeable at 20 yards, it results in POI differences as distance gets further out.

THEORY #2:
Because the BH is a bit longer than the FP, there is a slight difference in FOC causing the arrow to fly at a slightly different trajectory, which again is only noticeable at longer distance.

Let the discussion begin.

[vixenmaster]

Fixed BH's tend to have a higher wind resistance than F-pts or Mech.'s. Yup abit of drag

[Kegbelly]

Fixed BH's tend to have a higher wind resistance than F-pts or Mech.'s. Yup abit of drag

Exactly what VM said, more surface area on the broadhead creates a little extra drag. I have to go up 25 grains on field points to match POI from my 4-blade stingers when I shoot 30 yards or farther.

[nchunterkw]

As I said, drag was my first thought too. But as a test engineer I'd like to run an experiment to kind of prove that. My thought is to have 2 points that weigh the same, with pretty similar ballistic coefficients, but with different lengths. That should change only the FOC ever so slightly. I'd like to see if that changes the trajectory. Just for fun. I thought of using something like a bullet point versus only the ferrule of an old Thunderhead. Something like that. Any ideas?

[Kegbelly]

I may be wrong but I think the ballistic coefficient is largely related to the length of the projectile, but I'm thinking in terms of bullets, not arrows, so I really don't know how it would affect arrow trajectory. Seems like it would change FOC just a little, but enough to change POI? Good question, try it and see what happens, would be interesting to see your results.

[Bullzeye]

I would think the ballistic coefficient works the same for both bullets and arrows, BC basically means how well a bullet/arrow cuts through the air and maintains velocity, simply put, its a measure of air drag. BC is what determines trajectory, and for a bullet the BC will change due to its shape and speed, so that is why you would use a pointed bullet instead of a round nosed bullet for long distance shooting, I assume its the same for an arrow. So the more streamlined your arrows are, the higher the BC. In bullets there is also a slight change in BC if the bullet has a boat tail or flat base, so in an arrow I would think the vanes would also be a factor in BC.

[Fred T]

Thanks NC for sharing your results.If the snow every leaves I plan on doing a similar comparison of FP's vs Muzzy's and Slick Trick's.

[nchunterkw]

Bullseye
You are right in your assessment of BC. At least that is how I understand it. And while BC affects trajectory due to drag as you point out, it is not the only thing. FOC does as well. As I understand it a higher FOC results in more trajectory meaning more drop at distance. Two arrows with the same weight but different FOC will result in two different flight paths. I'm trying to put together the parts for an experiment. I need two FPs with similar BCs but of different lengths. Like a short bullet point versus something about 1 1/2" long. I think that would simulate a FPS versus a BH on an arrow but without changing BC. Just tinkering I know but to me this is interesting.