Or, to put this another way: In the beginning was the hole (whole?).
But you would have had to have been amongst the stoned at Dieppe this year to catch this allusion
Confused yet?- What this is about is kites with holes in them.
There is a thing about wings and things, that if you make a hole or slot part way across the chord to let some pressure bleed from the lower surface (high pressure area) thru to the upper surface (low pressure area), then flow remains attached over the wing even when it's at a steep angle to the flow direction (called angle of attack). This allows more lift to be generated from the same wing area. Basically what happens in the absence of a slot is that when a wing is angled at more than 15 or 20 degrees, that part of the flow that passes over the top surface breaks away and becomes turbulent instead of doing what it ought (which is curve downwards, and speed up to create a low pressure area that contributes to lift). Andreas`s Holy Pilot Plus Manta Ray - 2012
This is why those birds for whom generating more lift sometimes matters a lot, have gaps between their tip feathers. These 'slots' enable them to develop more lift at lower flying speeds and with less wing area.
Aircraft use this effect by having extendable leading and trailing edge members that create slot effects for take-offs and landings. These extensions not only increase the 'plane's wing area, but because of the slot effect, substantially increase the 'lift coefficient' (ratio of lift to area, other things being equal). Slotted Pilot Kite
But there is an extra drag price to pay for this increased lift:
With the best combination of leading and trailing edge extensions, lift can double at the same flying speed- but drag more than doubles.
For an aeroplane, it's worth it though: more drag requires that the engines provide more thrust for as long as the flaps are extended, but the extra lift generated allows the 'plane to take off and land at lower speeds than would otherwise be possible.
Can kites benefit form using slots?
Historically many framed kites have used slots (for examples, the Steiff Roloplan, and Wau Bulans), but why they used them is not so clear; maybe for aerodynamic reasons, but in some cases probably more for graphical effect.
But there seems to be a good case for kites to use slots. If slots add more drag than lift (which they do), then by my current theories of kite stability they are generally a GOOD THING! This is because most kites struggle to generate enough stabilising drag, and many need tails, drogues or other drag creating additions so that they can fly. And therefore something minimalist like a slot seems to me to be a better palliative than having to have a big bucket of air hanging out the back. Single Slot Per Cell Pilot Kite
Three Slots Per Cell Pilot Kite
And in the detail (along with the devil apparently) slots have an extra advantage for kites (should they then be considered unholy?):
As explained above, the prime function of slots is to improve airflow and therefore lift at high angles of attack.
Which for kites is when the wind is light. In stronger winds, kites fly at lower angles of attack, (unless they have brutal bridling) when slots have diminishing effect.
Therefore slots should increase lift at low wind speeds without generating line-breaking pull when the wind goes berserk (which is just about every day here at present).
But can this be true in practice as well as in theory? (in theory, practice and theory are the same but in practice they're not).
Well, for the last few months I've been testing this by making and flying a series of identical sized pilot kites (yes, very reductionist).
And the answer is: I don't know. Not yet anyway.
The slotted pilots do seem to have significantly more pull in mid range conditions, but it's surprisingly difficult to be sure about this in the extremely turbulent winds we have here at present (and all strong winds tend to be turbulent), even when flying each new variant against a 'standard' 8sqm.
In the lower range they seem to retain the advantage of more pull but also seem to fly at a generally lower angle (which is a bit odd and might be just bridling differences).
In the very light they are running about 50/50 in the 'which stays up longest' competition- nothing in it either way so far.
One unequivocal disadvantage is that they are VERY noisy in winds above 50km/hr. When there's wind I leave kites up over-night and Gavin (who lives in the kite factory beside our kite field) gives me detailed accounts on the mornings after as to how they've 'entertained' him all night. 3 Bridle 8sqm Stackable Pilot - Sardinia 2012
And a technical point here; the prototypes I've made so far take air from inside the kite (that is, they bleed out only through the top skin). Andreas Fischbacher, who has been experimenting with slots for longer than I have (see photo of one of his above his Manta Ray), bleeds the air from the lower surface thru sewn in passages. An advantage of bleeding out some of the inflation air is simplicity of construction, a disadvantage is that this leakage impedes initial inflation (but not noticeably if the slots are narrow). The jury's not yet in on which is best (actually it hasn't yet been empanelled even).
An encouraging thought for this development is that the first few I've built are not clearly worse than our standard 8sq.m Pilots by wind range (8km/hr to 75km/hr plus) and seem to have quite a bit more pull (which is advantageous)- and the standard 8sq.m's have had the cumulative benefit of 15 years development, so set a pretty high bar.
I'm sure I haven't yet found the optimum dimensions and placement for the slots, or the camber and bridling that best supports these yet to be optimised slots. More work needed therefore.
But for now, what I can offer is plans for a developed 3 bridle 8sq.m Pilot*. I'm delighted with this design- it's simple, reliable, has lots of pull in the light but less in very strong winds (compared to our standard 8sq.m), and a wind range without adjustment equal to anything I've ever made. The plans with this newsletter also cover an 8 bridle version of this kite- which is the most reliable kite in turbulent conditions that I've ever flown.
Upper and Lower Panels.
8 Bridle Side Flare Dimensions.
3 Bridle Side Flare Dimensions.
Click for 6sqm version
Liannawati (Bandung, Indonesia)made one by scaling up from the 6sq.m Pilot plans on the peterlynnhimself website and flew it at Chengdu and Changsha earlier this month. (It's the pink one in the Changsha photo.) On average it flew better than any of the pilots I had there, especially in very light wind. It was more often than not first up and last down- though this may be more a tribute to Lianna's dedication than to the kite. Either way I'm happy! 3 Bridle 8sqm Stackable Pilot Changsha 2012
It also survived this year's Taiwan typhoon, one of the only kites to do so apparently- which says heaps for its high wind end.
So, 3 bridle or 8 bridle version; take your pick!
And it's this basic design that will sport slots sooner or later if the current developments prove successful.
Peter Lynn, Ashburton, New Zealand, November 1 ' 2012
*It's actually 8.5sq.m flat, probably a bit less than 8sq.m when flying.- and please take note , this design doesn't scale. (no soft kites do). If you make a different size it will behave differently- especially with regard to the volatile instability/superstability balance (see Single Line Stability at peterlynnhimself).