Broadhead Arrow Flight

Broadhead Arrow Flight Written by Tom Brissee

The following article concerning Broadhead Arrow Flight was written by Mr. Andrew Middleton from the United Kingdom (England). Its focus is the study of the forces which affect broadhead-tipped arrows and how changes made in style of broadhead and broadhead-weight can affect arrow flight. Mr. Middleton requested that I make it clear that he is not a bowhunter, as it is not legal to hunt with bow and arrow in England at this time.

I read this article and felt that it would be valuable to bowhunters who strive to perfect their arrow flight while using fixed-blade broadheads, as well as expandable broadheads. While the testing documented in this article was done with a crossbow, the information given can be applied to standard bow shot arrows as well. Andrew Middleton has granted permission to me to provide this information to you via the Strictly Bowhunting website. I hope that you learn something from this article about perfecting your broadhead-tipped arrow flight.

Tom Brissee

Copyright © 1997-2001 by Andrew Middleton (analytical content and explanation) and Zee (other content and design). All rights reserved.

Questions or comments concerning the analytical or explanatory contents of this web site should be referred to andrew.middleton@dial.pipex.com.

The close correspondence between a simple ballistic model of arrow flight and where arrows actually strike shows that arrows do not 'fly' any more than does a bullet. The role of air resistance is to retard the projectile rather than buoy it up.

However, fitment of a broadhead can lead to a consistent but different strike point. This leads to all kind of advice on how the archer should 'tune' his broadheaded arrows and bow.

I have carried out some measurements on broadheaded arrows as a prelude to trying to develop a ballistic model that extends to predict broadhead behavior. Results have been rather interesting, suggesting that 'tuning' for broadhead flight addresses symptom rather than cause, and may be doomed to partial success.

I initially tried 3 types of broadhead, all 125 grain heads. One was a conventional, solid 2 blade head. Another was a 3 blade heads with vented blades. The third was a 3 blade open-on-impact mechanical. Later I added a 150 grain 2 blade broadhead of smaller cross section.
(Editor's note: Please see image above.)

First, all were tried with my longbow shooting 2115 E75 29" arrows with 5.5" feather fletching at 185 ft/sec. To the limits of my accuracy, I can only say that they all flew just like field points, and with no evidence of 'floating' such as I have observed with other's arrows. However I note that many archers use lighter broadheads, smaller fletching and lighter shafts (e.g. carbon). The significance of these
differences will become clear later.

I then turned to my standard 'measurement apparatus', a telescopically sighted crossbow shot from a rest. This had the major advantage that I could clearly observe the arrows in flight and could measure small changes in accuracy.

First, I found that a 16" 2219 field bolt with 5" plastic vanes was incapable of delivering any of the broadheads, except the open on impact, with any acceptable accuracy (errors in excess of 2 feet at 50 yards). The problem was very evident through the telescope. The arrows were following a spiral track through the air, and if caught by a gust of wind tended to swerve. Even the open-on-impact did not strike at exactly the same point as a field point, although it appeared stable and was accurate. The open-on-impact strike point difference (1" lower at 15 yards, then pro-rata with range) was not explained by weight change compared with a fieldpoint. I also tried varying the tiller on the crossbow, and found it absolutely critical (unlike for fieldpoints). Basically, if the bowstring movement varied from straight-line track by more than 0.5mm, it had a marked effect on arrow flight. The broadhead group could be shifted by adjusting the tiller, but the adjustment tended to be valid only at fixed range. Very careful tiller adjustment caused the bolt, initially, to depart without a visible 'spiral' motion although it tended to return to that pattern within 30 yards of flight. It's difficult to get 0.5mm tiller accuracy even on a target crossbow. Good luck to all you people with compound bows and high performance cams!

In the newsgroups, broadhead flight trouble is often associated with high speed compound bows. To test the theory that the poor stability was associated with high arrow speed, I fitted the crossbow with a low power prod. This reduced the broadheaded arrow speed from 260ft/sec to 200 ft/sec, a value comparable with my longbow. However the effect on accuracy was nil. I now believe the association with compound bows may relate to cam rollover effects, which would tend to introduce what amounts to tiller error.

Next, I tried very large flights on the bolts. I fitted them with 5.75" replica medieval shield fletchings. Not only do these have 50% greater cross sectional area than 5" plastic vanes, they also exhibit greater lateral rigidity. This set-up looked absurd but produced several very positive effects with broadheads. I'm now a supporter of feather fletchings.

1/ The broadheads all, on average, now struck as did fieldpoints, after correcting for their weight (125 grain compared with 150 grain fieldpoint).

I also checked out these feather fletched bolts with fieldpoints and found them accurate (2" group at 50 yards) but the drag was very high. This drag effect would be unimportant at ranges up to 40 yards (the odd inch or two). However at 100 yards it would cause an extra 3 foot drop.

The problem was clearly one of arrow stability in flight, and not a different 'flight pattern' for broadheads. Given that the broadheaded arrows were following a spiral flight track, there would be no correct 'tune' to apply to a bow which would be valid for all ranges.

Next I tried the effect of changing the broadhead weight by adding lead collars. The effect was dramatic. An extra 25 grains almost removed the spiral flight track effect. The bolts flew straight for about 30 yards before tending to spiral, and the tendency to veer off was gone. However reversion to plastic vanes was not feasible; both greater weight and large flights were required.

The final experiment was to make up 150 grain 2 blade broadheads with reduced blade area. Rather than a 30 degree rake on the blade, I used 45 degrees. These flew without visible instability and struck exactly as fieldpoints.

Later, I conducted similar trials with 20" 2117 bolts which have a significantly higher Moment of Inertia due to their length. They were not significantly better than 16" bolts. My aerodynamics model is beginning to help me understand what is happening; this lack of benefit from longer arrows is predicted by the model. The model suggests that too much steerage can also be a hazard as well as too little and the relationship between arrow length, weight distribution, flight and broadhead size is very complex.

Any broadhead will reduce the stability of an arrow because it applies 'steerage' to the front of the arrow in response to wind gusts etc. In 'control engineering' terms, a broadhead applies 'positive feedback' in that a change in arrow aspect in flight will self-reinforce with a broadhead fitted.

To counterbalance the 'positive feedback', there are the arrow's flights. The bigger these are, the better they will be able to counteract the broadhead. Big feather fletchings apply more steerage than plastic vanes.

And finally, the 'Moment Of Inertia' of the arrow. This is the inertial resistance of the arrow to turning e.g. pitching. The heavier the arrow, the higher the MOI. Heavy weights at one end are much more effective than a distributed weight such as a denser shaft. For a given weight, long arrows are much better than short ones. In control engineering terms, a high MOI will reduce the 'bandwidth' of the feedback loop and so contribute to arrow stability. Adding a 25 grain weight to the arrow point can have a dramatic effect.

So, when your broadheads don't go where you want yet is the same weight as your fieldpoints:

a) try and get the tiller etc. of your bow spot on, because an unstable arrow will be provoked less by a clean release. However, this will help rather than fix the problem unless your arrow is totally stable in flight. If your arrow is that good, you'll probably not have noticed any tiller problem anyway.

b) use (very) big fletchings. Feathers give better lateral stability than most plastic vanes.

d) use broadheads with minimum windage i.e. vented blades, and few blades. Remember that a large blade may cause more damage to the target, but only if you hit.

e) keep the arrow centre of gravity well forward towards the point. It's amazing what an extra 25 grains can do to a marginally stable arrow.

Finally, on the accuracy of broadhead construction. It did seem to be true that cheaper or slightly un-true broadheads flew badly, but only on arrows that were on the borderline of stability. On arrows with big fletchings and heavy points, the odd bit of broadhead asymmetry did not
seem to matter. In the end, my home made heads (1mm steel sheet soldered into slotted field points) were the most accurate, because they were that critical bit heavier. They were nothing like as precisely made as the bought-in broadheads. However they grouped within a 4" circle at 50
yards on a windy day in an open field, which was not a lot worse than fieldpoints under those poor conditions.

I hope you find this useful ! As usual, the 13th century archers had it right, we have to watch that our superior materials and fabrication techniques do not lead us to adopt poorly optimized configurations and then wonder what has gone wrong.

I am still working on the model of arrow stability in flight. I hope that, once complete, it will provide some firm guidance into how to 'tune' arrows. For now, I think it has to come with too many 'health warnings' for me to publish it."
--Andrew