James Park
05-10-2002, 09:53 PM
A number of people have asked me how Accurate Sights calculates the arrow velocity, and how accurate that velocity will be.
First: how accurate should the velocity be? I expect it to be withing about 1 ft/sec, assuming the input data you enter is carefully measured. The input data is important, and if you make a mistake with it the velocity could be significantly in error (although the sight settings will probably still be reasonably accurate, which is useful). My experience is that the calculated velocity (with good input data) will probably be more accurate than the standard archery chronograph.
Second: how is it calculated?
The first thing I do is to model the shape of the sight setting positon vs distance to the target. This includes calculating the expected angle of elevation of the arrow for a particular target distance, arrow velocity and drag, as well as calculating the parallax due to your eye being above the nock rather than directly behind the nock. The drag is a very important component of this because it is quite high (the drag can easily be 1 or 2 G's, which is very significant). The drag is quite a complex calculation and requires the data on the length, mass and diameter of the arrow, together with fletch type. The parallax needs to make use of the peep-arrow and peep-sight distances, and so these need to be reasonably accurate, as otherwise the sight settings for short distances will be in error.
Once I have the above sight setting profile for a given arrow velocity and target distance I can calculate the sight gap for two given distances and a given peep-sight distance. Then it is simply a matter of comparing the actual sight gap for those two distances with the calculated sight gap and adjusting the arrow velocity until these match exactly. Note that any error in the peep-sight distance will translate directly into a velocity error, it is important to get this correct (although fortunately even if you get it wrong the calculated sight settings will still be quite good).
We now have the real arrow velocity and from there we can do wonderfull things!!
First: how accurate should the velocity be? I expect it to be withing about 1 ft/sec, assuming the input data you enter is carefully measured. The input data is important, and if you make a mistake with it the velocity could be significantly in error (although the sight settings will probably still be reasonably accurate, which is useful). My experience is that the calculated velocity (with good input data) will probably be more accurate than the standard archery chronograph.
Second: how is it calculated?
The first thing I do is to model the shape of the sight setting positon vs distance to the target. This includes calculating the expected angle of elevation of the arrow for a particular target distance, arrow velocity and drag, as well as calculating the parallax due to your eye being above the nock rather than directly behind the nock. The drag is a very important component of this because it is quite high (the drag can easily be 1 or 2 G's, which is very significant). The drag is quite a complex calculation and requires the data on the length, mass and diameter of the arrow, together with fletch type. The parallax needs to make use of the peep-arrow and peep-sight distances, and so these need to be reasonably accurate, as otherwise the sight settings for short distances will be in error.
Once I have the above sight setting profile for a given arrow velocity and target distance I can calculate the sight gap for two given distances and a given peep-sight distance. Then it is simply a matter of comparing the actual sight gap for those two distances with the calculated sight gap and adjusting the arrow velocity until these match exactly. Note that any error in the peep-sight distance will translate directly into a velocity error, it is important to get this correct (although fortunately even if you get it wrong the calculated sight settings will still be quite good).
We now have the real arrow velocity and from there we can do wonderfull things!!