Hi everyone,
a friend of mine posted a picture of his takuma kit and I noticed that the leading edge of the wing was corrugated.
For the life of me, I can't figure out why this would help. Does anyone have a long-winded fluid dynamic answer?
takuma.com/en/foils/208-kujira-1440-wings-set.html
To bring you an example of this from the sailing world, the "whale fin" design has been used by a top naval architect Juan K on the rudders of grand Prix maxi yachts like Rambler 88
www.sailmagazine.com/boats/tech-talk-rudder-nodules/
in the article above designer Paul Bieker (one of the guys involved in designing Americas cup foils that hit 50+ knots and all round guru of foil / hull design.) says:
"The proven effect is an increase of the maximum lift angle of attack, i.e. a more gradual stall. The non-proven effect is the rotating turbulence from the tubercules creating 'fences' along the suction side of the foil [on boats with a single rudder], which may delay ventilation."
Select to expand quoteMHSA said..
To bring you an example of this from the sailing world, the "whale fin" design has been used by a top naval architect Juan K on the rudders of grand Prix maxi yachts like Rambler 88
www.sailmagazine.com/boats/tech-talk-rudder-nodules/
in the article above designer Paul Bieker (one of the guys involved in designing Americas cup foils that hit 50+ knots and all round guru of foil / hull design.) says:
"The proven effect is an increase of the maximum lift angle of attack, i.e. a more gradual stall. The non-proven effect is the rotating turbulence from the tubercules creating 'fences' along the suction side of the foil [on boats with a single rudder], which may delay ventilation."
+1
*All this in theory, as far as i understand it :) *
The tubercles change the foil section profile.
It is fuller on the high sections and flatter in the low sections.
So their stall Angle of Attack is different and the high sections will stall later,giving a very progressive mush out feel before the stall vs a sudden drop.
Plus the mentioned ventilation fence effect, there was a slow mo vid of a kujira breaching the tip where you could see the air trying to flow from tip to root and being stopped or delayed by the channels.
Can i actually feel this?
Without comparing to a non tubercled Kujira it is very hard to say.
It is a forgiving foil for it's glide ratio and it handles tip breaches very well, it does work great but i guess we will never know if it is thanks to the tubercles or in spite of them.
Other foils without tubercles (SPG Gamechanger for example) seem to achieve this good breaching behaviour without the indentations.
Tip washout,geometric or aerodynamic, is probably a critical factor in tip breaches.If the tips are "unloaded" air will not be sucked in or will do so very slowly allowing recovery.
The supertiny winglets are annoyingly pointy and i doubt they add anything being so small but i have not dared chop them off,only rounded them a bit.
www.researchgate.net/publication/258246710_THE_EFFECT_OF_LEADING_EDGE_TUBERCLE_GEOMETRY_ON_THE_PERFORMANCE_OF_DIFFERENT_AIRFOILS
The effect of the tubercles on stall behaviour is not due to the change in foil profile as i believed and posted above (in an intuitive simplistic brain fart).
Kind complex matter when you dig in a bit, involving span wise vortexes etc...
From reading the article i understand that the high sections stall later (like i said,ignorant's luck),but the reasons are way more elaborate.
Quite a few studies on this and not all reach same conclusions.
There seems to be a consensus that tubercles give better stall behaviour and that they increase drag.
I read the opposite that it reduces drag.... People say it has a lot of lift for its size. Guess the only way to find out is to try. It's definitely caught my eye.
Select to expand quoteAlysum said..
I read the opposite that it reduces drag.... People say it has a lot of lift for its size. Guess the only way to find out is to try. It's definitely caught my eye.
I agree -- very little drag and a lot of glide. I use the 1440, 1210 and 980 so I'm biased -- I really like them for winging.
Here's some info -- you decide:
From the article i posted,heavy reading... :)
It does say that it should allow for a smaller wing to be used,which would have overall less drag/weight than a bigger one without tubercles.
But at same sizes,tubercles=more drag.
Another study says that they found decreased drag only in the post-stall regime.
Anyway,it works very good.Yesterday i rode the 1210 in small ,gusty beachbreak conditions and i could not believe the super slow speed gybes i was getting away with on the inside.It just kept lifting :)
I have the Helium and like it. I haven't even tried the SS Infinity 76 after I got the Helium. I ordered a Kujira 980 wing set this week and hope to have it while there's still some season left.
Everyone who has tried these foils seems to like them and they are sold out almost everywhere.
The concept reminds me of the goofy windsurfing fin and sail designs from the late 1980s. Usually a much simpler shape emerged that was superior to the weird shape in every way, but as far as the Kujira humpy wings goes, I guess we'll be wiser in maybe 2 years or so?
I'm honestly surprised not other companies are playing with vortex generators. Easy enough on a molded wing.
