THE HEAVIER ARROW HAS LESS WIND DRIFT MYTH

One of the pieces of nonsense you frequently come across in archery is "a heavier arrow has less wind drift". Factually it may often be true but it's a very misleading comment so I'll try to put a more realistic spin on it.

The approach is to compare the performance of two arrows using a (fairly simple) arrow flight simulator. Don't believe the results quantitively but it will hopefully put across the idea. The two arrows (picked out as equivalent from the Easton Shaft Selector) are an X10-550 and an ACE-570. Both arrows are 29" long with identical points (120 grain) and nocks (6 grain). Bareshaft arrows are used in the simulation as were trying to compare shaft performance only.

Estimated values for the arrows are total weight X10 = 343 grain, ACE = 309 grain; FOC for X10 = 16.6%, ACE = 18.5%. The asumed relative arrow launch speeds are X10 = 200fps and ACE = 208fps, the ACE being faster because it's lighter.

drift1 If we shoot both arrows perfectly with zero windage adjustment in a 5m/s crosswind then the following picture illustrates the simulated arrow flight paths. The blue line is the X10 flight path and the red line the flight path of the ACE.

Clearly in the 70 - 90 metres distance range the X10 has less wind drift than the ACE. There are two factors at work here. The X10 is 34 grains heavier than the ACE so for the same drag force accelerating the two arrows sideways the X10 would have a lower sideways acceleration. However the heavier X10 shaft (lower FOC) is aerodynamically a poorer performer then the higher FOC ACE so the X10 will incur more drag force accelerating it sideways. In this case the effect of the increased mass of the X10 outways the effect of the higher sideways drag force on it so the result is less wind drift than for the ACE. This may not always be the case. (The obvious example is the McKinney 2 arrow - a lighter shaft which will have lower wind drift than the X10 with a similar flight simulation). You can also see that the above simulation predicts that the ACE will have less wind drift then the X10 in the 30-50 metre distance range.


Where the basic misunderstanding generally seems to exist is by oversimplification of the mechanics. Remembering your school physics distance = half the acceleration mutiplied by the square of the time and the time of travel equals the distance divided by the speed.
Applying this to wind drift with;

Then the down wind acceleration = force/mass = f/m
the windrift distance s = 0.5 * (f/m) * t2 and arrow flight time t = d/v
Putting the value for t in the expression for wind drift gives you:
s = [ 0.5 * f / v2 ] * d2/m

Very often (e.g. in pin tape programs) the actual arrow flight aerodynamics are ignored and the simple assumption is made that f = k * v2 where k is a constant i.e. k = f/v2. The end result for the wind drift equation is that
s = [ 0.5 * k ] * d2/m

So our schoolboy physics gives you that the larger the mass the less the wind drift and that the wind drift is proportional to the square of the target distance. Both conclusions are incorrect. The problem is that the expression in the [ ] brackets is not a constant as both f and v are fairly complex functions for both of which arrow mass (or rather the arrow mass properties) is one of the parameters. The above flight simulation clearly demonstrates that the assumptions about wind drift variation with mass and distance are both incorrect.

It's at this point the argument often stops - the heavier arrrow has less wind drift! Note that this argument conveniently forgets that an equivalent aluminium arrow, although its considerably heavier than the X10 would have considerably more wind drift than the X10, so in this case the lighter arrow would have less wind drift. Clearly saying that a heavier arrow has less wind drift is being oversimplistic

drift2 Let's move into the real world. If an archer shoots a perfect arrow in a wind and the arrow hits left in the blue the archer doesn't spend the rest of tournament peppering the blue, he adjusts the windage (or aims off) so the perfectly shot arrow hits the X. The following picture illustrates the arrow flight paths with the windage correctly adjusted.

As you can see, the actual wind drift has now become irrelevent as a perfect shot with either arrow will hit the X. (The same is true with the effect of gravity. You "aim off" to allow for gravity just as you "aim off" to allow for wind. With Lunar Archery and a different gravity force you would have a different aim off just the same as you have a different aim off with a different wind strength. Absolute values of "wind drift" or "gravity drift" are not very important.)


What does matter is not the 'wind drift' but what group sizes the archer is going to get with the two arrows. Which of the two arrows shooting in the wind is going to more 'forgiving' to the archers' variability which at the end of the tournament is going to give him the highest score.

drift3 The following picture illustrates the flight paths of two arrows where the windage has been perfectly adjusted and the archer has shot a less than perfect arrow.

For this particular shot the ACE gives a better result than the X10, it hits nearer the middle. With further simulation it becomes clear that in this case the lighter ACE arrow gives you smaller groups than the X10 and the lighter arrow is the best performer. Note the emphasis on 'in this case'. Even with the same two shafts if anything changes, launch speed, wind conditions, arrow length point weight etc. then you are back to square one as regards wind performance. It is impossible to predict in a simplistic way which of two arrow shafts is going to be better. Flight simulation may give some indications but actually shooting arrows and measuring their relative performance is the only realistic method.


In the above example two different shafts were compared so more than one variable was being changed. Suppose instead we compare the X10 above and same X10 with an additional say 20 grains of mass. The result as regards wind drift and arrow performance depends very much on how the extra mass is added (one of the oversimplifications in the expression 'The heavier arrow'). If we add the 20 grains extra to the shaft then the absolute wind drift of the arrow remains much the same - the extra mass is cancelled out by the increase in the drag force drifting the arrow. The grouping performance of the heavier shaft is worse than the lighter shaft. If we make one arrow 20 grains heavier than the other by adding 20 grains to the point then the heavier arrow has less wind drift than the lighter arrow and the heavier arrow groups better than the lighter arrow. So how you make the heavier arrow heavier makes a big difference

So the next time you hear (as you invariably will) that 'a heavier arrow has less wind drift' hopefully you will be aware that this an oversimplistic approach which does not take into account the aerodynamics of the arrow and may be true or false depending on what two arrows you are referring to and how one arrow is made heavier than the other.

All the above assumes that the strength and direction of the wind is constant. What happens if they are not?

To a large extent depends on the type of bow. With a recurve bow how "forgiving" an arrow is to archer form variation is a major factor in the overall arrow performance as regards tournament scores. Wind drift for recurve archers with variable wind strength is considered on another page.. For compound bows arrow aerodynamic performance, i.e. how "forgiving" it is, appears to be of very little interest. As long as the arrow is stiff enough so the bow remains a point and shoot device, overall weight and diameter, i.e. absolute wind drift, seems to be the only consideration, at least with the average club archer.

Last Revision 1 July 2009