Success in Paralympic archery is a matter of millimetres21 September 2012
Dr James Park
by James Park
The accuracy obtained by top-level archers – at both the Olympics and
Paralympics – using modern archery equipment would astonish most non-archers.
Archers stand or sit 70m away from a target which has a centre scoring ring
(the “ten ring”) just 12.2cm wide.
In a major competition such as the Paralympics, the Olympics or the World
Championships, an archer using a recurve bow needs to hit that ten ring roughly
once every two shots in order to be competitive.
With the more technically sophisticated compound bow (using eccentric pulleys
to modify the bow’s draw force) an archer needs to hit the centre ring at least
twice out of every three shots. And, they need to do so even when it is windy
and the arrows are being blown around.
Most non-archers will not be able to keep a pointer at arm’s length aligned
with the centre ring of that target at that distance, let alone having to do so
when holding the substantial draw force and mass weight of the bow. It’s hard
Major archery competitions are decided by very small margins. Often it is only
a single point in several hundred points that matters. If we can cleverly
select and optimise the archer’s equipment, we can obtain a small score advantage
which might provide a considerable competitive bonus.
We could do this through trial and error. But it’s far more effective and
satisfying to use mathematical models of the equipment. Developing and testing
those models has been the subject of research carried out in the Department of Mechanical and Aerospace Engineering at
The thing that messes most with an outdoor archer’s score is wind drift. If an
archer is 70m from the target, a moderate breeze can easily move the arrow by
several target rings.
Where possible the archer tries to shoot between wind gusts. But because there
is a time limit for each group of six shots, it is inevitable that the archer
will need to sometimes shoot in strong wind.
The archer usually sets the bow’s sight for the average wind drift and then
aims off the centre of the target for each shot. They judge the offset
depending on the wind strength and direction before the shot, and using
knowledge from the movement seen for past shots. It’s an error-prone approach.
But if the effect of wind on the arrow can be reduced, it also reduces the
The wind drift is directly related to the aerodynamic drag of the arrow.
Understanding the various components of the drag and minimising each of them
will help. Drag affects the arrow point, the arrow shaft, the arrow’s fletches,
and the arrow’s nock (which attaches the arrow to the bow string). Of those,
for a typical arrow, the shaft drag dominates – it contributes approximately 74
per cent. The fletches contribute 13 per cent, the nock 9 per
cent and the point 4 per cent.
The shaft drag is primarily due to the shaft’s comparatively large surface
area. It can be minimised by using a shaft of small diameter. Most competition
arrows are constructed from carbon fibre composite material with a minimum
diameter of approximately 5mm.
The fletch drag is due to both their surface area and to their projected edge
frontal area. The fletch area needs to be big enough to stabilise the arrow
(stability is primarily obtained through the lift from the fletches rather than
drag). Given a certain fletch area it is then best to use a low profile in
order to minimise the pressure drag from the edge projected area.
In order to overcome small imperfections in the arrow (such as an arrow that’s
not quite straight) it is best to angle the fletches to spin the arrow about
its longitudinal (length-ways) axis. This costs a small amount of drag while the
arrows gets up to its full spin rate. Once spinning, the fletches are
effectively edged on to the wind. To further minimise the pressure drag it is
desirable to use a very thin fletch.
Drag from the arrow’s nock is primarily due to pressure drag from the wake –
the area of turbulence left behind as the arrow speeds through the air. It is
best to select a nock with a small diameter. The nock has to fit on the string,
which means the choice of shape is a bit limited, but it should have at least
some degree of aerodynamic shaping to reduce its drag.
In most cases the arrow’s point is only a small contributor to the drag. A
typical “bullet shaped” point is pretty streamlined.
By carefully optimising each of the portions of the arrow, we think we can give
an archer a 5 per cent wind drift advantage over their competitors. It’s
little, but it goes a long way.
But even the best technology won’t save a bad shot. We can help, but the archer
still needs to shoot well and to deal with the considerable pressures of
Dr James Park works in Department of Mechanical and Aerospace
Engineering at Monash University.
This article originally appeared in The Conversation.