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Catch Your Drift

Posted by UniBoffin at 17:17 on 21st May 2010 in UniBlog

Catch Your Drift

I suppose it’s just possible that some particularly dedicated and long-suffering reader out there will recall that, a year ago almost to the day, I unjustly suspended a conceptual Sword of Damocles above the heads of those who would unsuspectingly sit at this humble feast of darting science.

This literary Weapon of Mass Destruction was referenced toward the end of my “Level Best” blog when I sadistically threatened to delve into something called “trajectory turn-over rate” and added that, if I felt “really evil”, I might even mention “g cosine theta over v”.

Well, the bad news is the day of reckoning has, albeit belatedly, arrived!

You might possibly imagine this is because I’m worried that I’ve been going too easy on you folks recently by being far more comprehensible than any mad scientist who values his acid-stained lab-coat image should be. However, that’s clearly ridiculous. (Comprehensible? Me? Perish the thought!).

The real truth is that Unicorn and I have actually been hard at work on something that I hope you’d find a good deal more interesting than most of the stuff I blither on about here. Unfortunately, as much as I’d like to, I can’t tell you anything about that just at the moment, so I’m perversely going to go to the other extreme and discuss something of almost no practical use to you at all What can I say but “Enjoy”?!

So, I’ve talked about spinning darts before (most notably in “The Spin Doctor”), but something I have carelessly neglected to mention is the fact that spinning a dart will certainly throw it off target.

Before you panic and run around kicking the furniture shouting “Why wasn’t I told? That could be the reason why I might well get dropped from The Dog and Duck Seconds”, let me hastily qualify that statement by telling you how much deviation we’re talking about here – and the answer is generally less than 1mm.

And the culprit behind what could be an innovative if implausible excuse for just missing that double 11 to avoid the team being relegated to Division 3? Yep, you guessed it – the dreaded “g cosine theta over v” finally plummets directly on to your unprotected head. This toxic device is the mathematical formula for that long-threatened “trajectory turn-over rate”, the rate at which the gravitational acceleration g acting on a projectile travelling at velocity v changes its trajectory angle theta.

By definition, a “stable” projectile like a dart tries to stay aligned with its trajectory, which means that it also wants to change angle at this same rate. But, if the projectile is spinning, there will be a gyroscopic effect at right angles to the trajectory which will result in it deviating sideways. And this effect is called drift (strictly speaking, it’s called yaw of repose drift, but don’t ask me why or we could be here all night – although I will mention that it’s absolutely nothing to do with why a spinning golf, soccer, cricket, tennis, or any other sort of ball tends to swerve in flight).

For spin-stabilised projectiles such as bullets and artillery shell the drift can be very large – in the latter case hundreds of metres – and will be in the same “direction” as the spin if we define “right-hand” spin as clockwise looking from the rear.

By comparison the drift of a relatively slow-spinning “statically stable” projectile such as a dart will be tiny and in the opposite direction to the spin. Thus “left-hand” spin (probably the most usual for right-handed players – their thumb pulling down on the left side of the dart as they release) will cause drift to the right.

Of course, as I said, unless you have fingers like an electric drill you’d need a micrometer to verify that fact. Nonetheless, if you want a piece of unsettling darts trivia with which to put Big Tel off his throw during that “friendly” play-off for the last place in the team, you could do worse!

Well, that’s about it again, but if you’ve reached this point intellectually unscathed and keen for yet greater challenges to your scientific boredom threshold, let me precariously dangle two more ballistic WMDs above you – a malicious pair of twins my spellchecker used to insist on incorrectly correcting to “procession and mutation”.

Less masochistic readers will just have to hope Dionysius’s single horse-hair holds for at least another year!


Q&As
Hays Patterson and John Part
Just one Q to A this time; Hays (as well as thinking of entertaining acronyms!) patriotically asks if I’ve ever worked with John Part, to which the answer is no, although it would be an honour so to do. As to whether I could help him, I would like to think that giving any player the kind of a scientific insight I can provide may offer them a fresh perspective which could enable them to improve. However, I must add the proviso that there is a limit to the difference technical input can make. I never lose sight of the fact that darts is first and foremost a game of skill in which confidence and mental attitude tend to play much bigger roles than aerodynamic optimisation!

There is 1 comment to this post

Posted by Joe Pearce at 13:36 on 2nd June 2010

I've noticed that when i have absolutely no spin, to maybe a single rotation to the board that the tail end of my dart tends to whip around violently. however if i do have a little bit or rotation it tends to enter the board at a straighter angle. would this be because i am using the smaller sigma and pear shaped flights? if i was using a larger flight with a little more drag would it negate the fact that naturally i have very little spin.

also on a side note, does unicorn have any kind of beta testing for these new things you have been working on? It is something i would be really interested in. :)

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