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 Post subject: sharp leading edge on racing fins
PostPosted: Tue Oct 23, 2012 11:24 pm 
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Ok I give up, why are the leading edges of kite and windsurf racing fins sharp?
windsurf fins have been around for ever, so I suspect it's been proven faster just can't figure out why or see any research on the topic.
Each fin I looked at aren't just thin rounded leading edges, they F'n SHARP!

Cavitation/ventillation reasons?

In the aircraft world everything subsonic region is a rounded leading edge, because it has less abrupt stall characteristics. and the laminar flow is less likely to be disturbed by small changes to the angle of attack causing drag. Watching kite board racing, the board is all over the place in the chop and there is no way these things a rocking a steady AofA.

a sharp edge can create low drag but this is typically only at AofA of zero
The fins on a race board are symmetric and generate lift from the drift angle, no way these things are operating at zero AofA

Sharp edges has advantages in supersonic flow by preventing bow shock and wave drag in front of the aircraft in supersonic speeds. So is often what is found on fighter planes and supersonic aircraft but NOT on subsonic craft.

any windsurfers care to elaborate on the evolution of sharp LE fins?

or is this just cuz windsurfers and kite racers are a bunch of monkeys?
http://paws.kettering.edu/~jhuggins/humor/banana.html


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 7:24 am 
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never saw a racing fin with sharp leading edge... do you mean trailing edge??? they are sharp, I could slice bread with mines...


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 8:18 am 
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Yes, the trend in kiteracing is for sharper leading edge of the fins, I think that the goal is to achieve more speed and more control, by sharpening the leading edge you get less lift but more control at high speed.

Having 3 fins kiterace boards do not have problems of spinout, so less problems than ws at the variation of the angle of attack and may be the higher density of the water instead of air, give the same advantage, with a sharp leading edge, like the supersonic flight ?.

But if the fin is too sharp it becames dangerous, we have problems yet with the trailing edge that cut like a knife, but usually you just cut yourself, please don't exagerate also with the leading edge, if you hit someone you'll kill him...

I think that IKA should put a limit to the sharpness of the fins and check them before each race.


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 9:04 am 
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Sharp trailing edges give rise to low drag at small Reynolds numbers (below 100, see page 117 of http://pnn.dk/phd/outreach/20120420_PNN ... efense.pdf)
The Reynolds number of a race fin is probably around 500,000 (10 m/s, 5 cm), so I agree this is not the explanation.

Commercial aircraft operate at orders of magnitude larger Reynolds numbers and they only have to generate lift in one direction. I think the bidirectional lift is the key...
For this reason I would expect the foils on foil boards to have rounded leading edge (http://www.kiteboardingpr.com/multimedi ... 1f6c06.jpg).


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 9:10 am 
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I know on non-race windsurf fins, they went to sharper (still not sharp) front edges for higher top speed.

The spinout is more sudden and complete and you have to bear off further to recover the laminar flow, but the best fins are still very useable in that you can still sense when they are about to let go and ease off on the push on the fin - you just have to be more sensitive than with the more rounded front edge fins.


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 9:40 am 
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i kind of agree with this. one of teh reasons is a big cut can mean 2 weeks out of the water - a big bruise - keep riding! know of 4 people with fin cuts from raceboards needing stitches, a couple a lot of blood - nothing life threatening though! Have nearly landed hard onto upside down raceboard roll tacking and wave chop tipped board over.


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 10:34 am 
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Silver_surfer wrote:
Yes, the trend in kiteracing is for sharper leading edge of the fins, I think that the goal is to achieve more speed and more control, by sharpening the leading edge you get less lift but more control at high speed.

Having 3 fins kiterace boards do not have problems of spinout, so less problems than ws at the variation of the angle of attack and may be the higher density of the water instead of air, give the same advantage, with a sharp leading edge, like the supersonic flight ?.

But if the fin is too sharp it becames dangerous, we have problems yet with the trailing edge that cut like a knife, but usually you just cut yourself, please don't exagerate also with the leading edge, if you hit someone you'll kill him...

I think that IKA should put a limit to the sharpness of the fins and check them before each race.


this makes sense to me. especially considering the other inhabitants in the water. We should not risk getting our limbs cut off.

oh well.


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 11:05 am 
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hmm i can understand that.. have had super random divers far out in 30 knots+ with no buoy/flags up your way..


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 5:59 pm 
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Quote:
higher density of the water instead of air,

from what I have read, I think the Reynolds Numbers for water at 15-30knots are pretty much in the neighborhood of the Reynolds Numbers for air in trans-sonic to low supersonic range, due to the density water:air ~/= 800:1

Too sharp a fin is very foolish though, regardless.


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 Post subject: Re: sharp leading edge on racing fins
PostPosted: Wed Oct 24, 2012 6:09 pm 
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BWD wrote:
from what I have read, I think the Reynolds Numbers for water at 15-30knots are pretty much in the neighborhood of the Reynolds Numbers for air in trans-sonic to low supersonic range, due to the density water:air ~/= 800:1

This is very wrong, since the viscosity also increases with a factor 50.

Reynolds number = <velocity> x <length scale> x <density> / <viscosity>
Airbus A380: Re = 200 m/s x 10 m x 1.3 kg/m^3 / (20 mPa s) = 130 millions !
Race fin: Re = 10 m/s x 5 cm x 1000 kg/m^3 / (1 mPa s) = 500,000

Unless you think of factor of thousand difference is the same "neighborhood"...


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