The behaviour of spinning objects is complicated. Spinning objects behave in a way that seem at odds with 'normal common sense'. When you put one end of your toy gyroscope on top of your Eiffel tower instead of falling down it goes round and round. Weird! All sorts of anti-gravity and perpetual motion machines have been invented based on using gyroscopes. (unfortunately they don't work).
The common gyroscope myth is that if you spin something (arrows included) then you get 'gyroscopic stabilisation' i.e. the object stays steadily pointing in the same direction. There are things around called 'gyroscopic compasses' and bullets are spun with rifled barrels to confuse the issue.
In reality when you spin a free flying object it 'destabilises' it and makes it wobble about.
Toss a pencil from one hand to the other, trying not to impart revolution to it with your hand. The pencil stays more or less pointing in a constant direction. Now do the same only this time spin the pencil between your fingers as you release it. The pencil ends up corkscrewing around all over the place. - gyroscopic de-stabilisation!
Gyroscopic 'de-stabilisation' occurs significantly where you have a torque applied to the spinning object (this is how the gyro/Eiffel tower toy works, gravity supplies the torque and so the object goes round and round in circles). With arrows you fit fletchings to provide a torque to straighten the arrow up (stabilise it if you like). The most important effect the fletchings have as regards arrow groups is removing any rotation (angular momentum normal to the shaft axis) the arrow has when it leaves the bow. The faster this is done the smaller the change in arrow flight direction and hence the smaller the arrow groups. Because of the fletching torque the gyroscopic effect in principle tries to make the arrow revolve like the gyro toy. Any spin acts against the fletching action in stabilising the arrow and so any spin will have detrimental effect on arrow group sizes.
The problem of spin destabilisation of an arrow has been reported many times in books, articles etc. It is often incorrectly described as the arrow flight becoming "unstable" from losing too much speed at longer distances - "the excessive drag story". The result is that the increase in the size of arrow groups at longer distances being larger than the general trend. A better explanation, which does not require re-writing Newtons laws of motion, is in terms of the arrow spin to speed ratio. As the arrow travels its speed drops and the spin rate increases so the spin/speed ratio increases. If this ratio becomes too large then the arrow will start to weave around as the gyroscopic effect on the arrow (higher with higher spin rate) is no longer dominated by the fletching torque on the arrow (which drops as the speed drops). This effect usually has a catastrophic effect because as the arrow starts weaving it loses speed at an even higher rate so it weaves even more and so on - a runaway effect. Visually the arrow appears to fly quite nicely for 70 metes or so and then goes haywire. A related phenomenon is the arrow that appears to fly straight most of the way and then appears to start fishtailing. Again this not due to the arrow becoming unstable at lower speed. How much an arrow fishtails (it's amplitude) depends on the arrow speed. As the arrow speed drops the fishtailing amplitude increases.
Last Revision 1 July 2009