quote:
Without resorting to mathematics, I would rather suggest that the apparent wind does nothing except make the sail design more critical?
You're right, you can look at it a number of ways, but the end result is that apparent wind doesn't push you forwards.
quote:
Tip eddies reduce sail efficiency so we close the gap at the bottom and taper the top. But a kite looks like it would contain the eddies in the same way that a ducted fan does.
Errr... not really, in fact not at all! As you know, eddies form where air from the high pressure side of a wing 'rolls over the edge' and gets to the low pressure portion. As it rolls over it forms a cylinder of rolling air. The more square the wingtip, the more of an eddy you get. Hence the rounded wingtips of the spitfire, and why it was so efficient in WWII. Also hence the 'winglets' on jumbo jets these days.
Kites will also form eddies on the wingtips, although from what I can see they're getting more rounded as the designers figure things out. The eddies will just stream downwind from the tips, and won't be contained in the structure at all.
What I want to see is a kite that looks for all the world like a wedge-tailed eagle, with the spread primary feathers on the wingtips. Those feathers are a really clever invention, they mean that you get lots of little eddies instead of one big one. This means more efficiency! Imagine seeing a 10m wide massive bird of prey soaring along the seashore, with some hardcore dude slashing up the surf underneath
Regarding efficiency, the higher the aspect ratio the more efficient a wing is. So a kite that's 12m tip to tip and 1m front to back (aspect ratio of 12) will be more efficient than a kite that's 6m tip to tip and 2m front to back (aspect ratio of 3), despite their both having the same surface area! It's harder to turn, but it would let you point higher. The same goes for windsurfing sails -- a higher, narrower sail will beat a smaller, wider sail of the same area (all other things being equal of course!)
If I were designing a kite to break a speed record, it would have:
* Aspect ratio of at least ten (most kites look like they're below 3)
* Two strings instead of four (string drag wouldn't be negligible)
* Pointed (yes, POINTED) wingtips like an albatross
* Rigid bar below the kite to spread the bridle attachment points from
* High efficiency airfoil section
It's only a matter of time before someone cottons on to these points...
quote:
Regarding point (2), a tiny fin is the obvious solution?
Tony's Missile has nostrils, where is the science about that?
A small fin would reduce drag slightly, I don't know that much about how planing works though. I do know that you get drag from the boundary between the fin and the board, so a good fast fin would be narrower where it meets the board than a slow fin, not sure about area though. I'd imagine that board shape and wetted area would have a lot more of an effect than fin size though.
The nostrils? Again no idea, I'm thinking it might be a little bit of a gimmick but I've got no evidence to back me up. All I know is that aero and hydro dynamics is a real black art, you can try lots of things that 'should' work but turn out to only slow you down when you stick them in the wind tunnel.
Perhaps Tony could block up the nostrils with a rag, have a couple of GPS runs, then unblock the nostrils and have another go? I'd imagine that there would be very little difference in the speeds, but again I could be wrong.
