Hunting arrows measured in flight: Barton 2012 study

An electronic tip, screwed on in place of the normal point, records the arrow's acceleration throughout its flight, from which velocity, energy and time of flight can be worked out at any point along the trajectory. The authors used it to compare two hunting broadheads on a crossbow bolt, with 4 shots each at 45.72 metres. The difference was small: with the fixed-blade head the bolt left at 88.04 m/s and arrived at 80.91, with the mechanical practice head at 87.7 and 80.24 m/s. Kinetic energy fell from 134.83 to 113.89 J in the first case and from 134.03 to 112.21 J in the second, so the bolt kept 84.47% and 83.72% of its launch energy. Time of flight was 552.02 ms against 555.33, and mean deceleration from air drag 1,385 against 1,441 thousandths of a g.

The authors judge the 0.75-point difference in retained energy too small to choose between the two heads on that basis, and present the work as a demonstration of the instrument rather than a test of the broadheads.

The sensor (Velocitip) is an aluminium tip 9 mm in diameter and 40 mm long, with the standard 8/32 insert thread, containing a three-axis accelerometer (up to 16 g), a shock switch with a 150 g threshold, a microprocessor, memory and two coin cell batteries; it records up to 800 ms of flight with a time resolution of 0.3 ms and was drop-tested at 5,000 g. An adapter lets any point be screwed on in front of the sensor.

The tests used a 150 lb Parker Safari Magnum crossbow and Easton Carbon Power Bolts 0.51 m long and weighing 20.02 g, with plastic vanes, shot into a foam target. The heads were a Tight-Point Shuttle T-Lock fixed-blade broadhead of 6.48 g (100 grains) and a Grim Reaper mechanical practice head of 6.54 g (101 grains), which flies like the hunting version but does not open its blades. Sensor, adapter and head added about 14.8 g at the front of the bolt, for a total of over 34 g.

The data are few: 4 shots per head, with one crossbow, one type of bolt and one distance, and no information on how the crossbow was shot or on wind and temperature. The standard deviations are small, but four shots say little about how repeatable the system is, and the paper does not set the sensor's velocities against a chronograph or video, so its accuracy has not been compared with an independent method. Nor does it explain how launch velocity is derived from an accelerometer recording deceleration in flight. The configuration with the plain field point, listed among the equipment, does not appear in the results, and the weight of sensor plus adapter changes by just over a tenth of a gram between paragraphs (8.42 and 8.29 g).

One of the authors works for the company that makes the sensor, and the figures belong to the company: the paper is also a product presentation. Finally, everything was measured on a crossbow bolt carrying almost 15 g of extra weight at the front, a set-up very different from a recurve or compound target arrow.

For recurve or compound target archers the study is mainly of technical interest: it shows that the loss of speed and energy along an arrow's whole flight can be measured with a sensor in the tip rather than with two chronographs. The numbers apply to a crossbow bolt with hunting heads and do not carry over to target arrows. For bowhunters, the comparison suggests that choosing between two similar heads will not turn on energy lost in flight, which here differed by less than 1%.

The sensor itself adds weight at the front, so it changes the arrow it measures: on a light target arrow that extra mass shifts the balance point (front of centre) and very likely the dynamic spine too, so the arrow being measured is no longer the one that is shot.