Earlier this week I fired up the impulse engine on StarShip Boffin (I’m still working on an eco-friendly anti-matter drive – anyone know where I can get organic dilithium crystals?) and chugged at warp factor 30mph down to Unicorn HQ. Where I found so much frantic activity underway that I could have sworn I saw (not just heard!) the Doppler shift as people rushed past.
However, as the Big Boss wouldn’t be very happy if I stole the marketing department’s thunder (they can get quite tetchy if you do that), for the time being I’m going to leave you in suspense about exactly what else I saw. Let’s just say some very interesting things have now emerged from the drawing board (OK, CAD/CAM system) and may soon hit the streets. Just don’t tell anyone I told you!
So, instead of risking a slap from the hand that feeds, I’m going to play it safe for this blog by trying to answer the gratifyingly more expansive selection of comments and questions which my last one provoked – many thanks to Bob, Paul, Warren, Ole, Lonnie and Clarence, it is definitely nice to know someone’s still out there!
These guys have raised some good points and I hope you’ll find the following Q&As interesting enough to bring you back next time, when maybe I’ll put on an old (preferably tomato-sauce coloured?) shirt and spill some of those Unicorn beans!
Q&As
Bob and Plastic Shafts, etc.
Bob makes some good points about the number of non-aerodynamic factors that can affect the choice of a barrel, shaft or flights (eg, the possibility of aluminium shafts being worse than plastic for causing bounce-outs). Much of what he says has validity, but to incorporate many such parameters into uniLab (it already has barrel shape, weight and grip type) would complicate its use and arguably dilute the pure science on which it’s based. Still, I’ll be bearing his suggestions in mind and will be returning to his comments about barrel nose shape and the grip on Sigma darts in future blogs.
Paul and Curved Flights
Paul describes how he has tried bending his flights into a spiral in an attempt to reduce bed-blocking and wonders about the aerodynamics consequences. In fact there have been a lot of studies on this shape of flights (well, shape of fins, anyway) as it is a handy one for missile designers because it allows the fins to be stored wrapped-around a tubular missile and then spring-up after launch.
One aspect of wrap-round fins (as they are called) is that the lift they produce will usually be less when they are the “u” way up than the “n”, which will cause it to be different on each side of the missile. Thus a yaw angle will cause an “aerodynamic rolling moment” (remember the phrase from last time?) that, as the missile yaws one way and then the other, will alternate between clockwise and anticlockwise. Over a relatively long trajectory, this can cause the “spin-yaw resonance” I mentioned.
However, as I also said, the flight path of a dart is too short for resonance to be a real problem and the consequences of using spiral flights should thus be none too dramatic, although they might induce some undesirable inconsistency in the spin and yawing motion (the latter partly because their curve slightly reduces their span and hence lift).
I’d be interested to hear about Paul’s own experiences in this regard and whether he did notice any reduction in bed-blocking – personally I’m not sure that it would help much, except possibly by weakening the material of the flights.
Ole and Spin
Well, Ole is really risking me going into technical overload now with his question about spin stabilisation. Laudably, I’ll resist pontificating too much on this complex subject, but I will say that the mathematics of spin-stabilisation has been well-defined since Fowler, Lock & Gallop (plus a poor bloke called Richmond who often gets forgotten) produced their 1920 report on “The Aerodynamics of a Spinning Shell”.
To explain briefly how spin stabilisation works, when an aerodynamically unstable projectile (see my early “Difference Between Chalk and CGs” blog) such as a shell yaws upwards (say), the yaw causes lift which acts in front of the CG and tips the shell nose up. If it’s spun fast enough, the shell then responds like a gyroscope to this tipping and “precesses” to yaw sideways. This in turn causes sideways lift and thus a downward gyroscopic response which negates the original upward yaw. For this “negative feedback loop” in yaw to work and keep the shell pointing more-or-less straight, the gyroscopic effect created by the shell’s spin must exceed the aerodynamic tipping effect due to yaw. The ratio of these is defined mathematically as Sg, the “Gyroscopic Stability Factor”, which has to be more than 1 for the shell to be stable.
As Ole guessed, for a dart without flights (or an on-board fly-wheel) to satisfy this criterion, it would indeed need to spin like a dentist’s drill! Mind you, flights might beat spin hands down for dart stabilisation, but technically they’re not the only option. Tie some ribbon to the shaft and you’ll have a “drag-stabilised” dart - give it a try (but maybe not in front of your mates!)
Finally, Ole, on your other question about spin, the flight surfaces on each side of a dart at yaw will generate lift forces which, even with a free-spinning shaft, will largely cancel out in terms of rotational effect, but still combine to push the dart back straight.
Clarence and Sigma Soft-Tips
Glad you like the darts, Clarence, and if you want to use them with a conventional board I’d recommend trying the shorter Unicorn Converta points (catalogue number 78590). If you’re currently using the Sigma Pro shaft and flights successfully, there’s no reason why this combination then shouldn’t still work well, but you could also have a go with Sigma One shafts and Q Slim flights (if you can get them – Hi-Lites Slim will do if not). Let me know how you get on!