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sharp leading edge on racing fins

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BWD
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Re: sharp leading edge on racing fins

Postby BWD » Wed Oct 24, 2012 7:18 pm

Well, what I read.... might be right or not...
Also there might be a "few" considerations missing I think from your calculation.
Quantitating fluid dynamics is a little past where I like to go in internet talk.
Looking at your rule-of-thumb type numbers, there seems to be a thousand missing when I plug into my calculator, fwiw, probably putting the examples in the same order of magitude. Especially when you remember seawater is more like 1020-1029kg/m^3 ;)... BTW note which R no. number is higher, and that the chord of race fins is bigger than 5cm (tho I cant speak to the appropriate scale and flow condition).... etc.
Airbus is a weird choice anyway, you could have gone for something faster for argument's sake I guess?

Johnny Rotten
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Re: sharp leading edge on racing fins

Postby Johnny Rotten » Thu Oct 25, 2012 1:47 am

BWD wrote:Well, what I read.... might be right or not...
Also there might be a "few" considerations missing I think from your calculation.
Quantitating fluid dynamics is a little past where I like to go in internet talk.
Looking at your rule-of-thumb type numbers, there seems to be a thousand missing when I plug into my calculator, fwiw, probably putting the examples in the same order of magitude. Especially when you remember seawater is more like 1020-1029kg/m^3 ;)... BTW note which R no. number is higher, and that the chord of race fins is bigger than 5cm (tho I cant speak to the appropriate scale and flow condition).... etc.
Airbus is a weird choice anyway, you could have gone for something faster for argument's sake I guess?



BWD kristian is in the ball park, chord length is typically MAC (mean aerodynamic chord) which for a fin tapering linearly down to nothing and a 10 cm root chord (most racing fins) it would be somewhere around 5 cm. (since they don't taper to "nothing" probably closer to 7cm realistically)

depending how you swing you're numbers reynolds number of about 500,000 to 1,000,000 (1,000,000 = 30 knots, 0.07m chord length )

Agree weird choice for comparison the air bus (really high reynold number due to it's size and speed) but his point is valid we're much closer to small low speed planes like a Cessna which is in the ball park of RE ~ 2,000,000.

This may be the true answer to my question, If you have a low reynolds number, the viscosity effects are relatively higher on a kiteboard than an aircraft, meaning under our conditions the fluid is "stickier" to the wing, So you can probably get away with shit at low reynolds numbers to reduce drag (like a sharp leading edge) that would otherwise detach and screw up the flow on an aircraft that operates with higher RE.

Can anyone with a good brain confirm if I'm on the right track here?

kristianE wrote: I think the bidirectional lift is the key...

I disagree with this, the bi-directional lift is easily accomplished with a symmetric foil mounted at 0 degrees of toe in (Almost all race boards) and this capability would be unaffected by a sharp or rounded leading edge.

Great discussion guys......

KristianE
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Re: sharp leading edge on racing fins

Postby KristianE » Thu Oct 25, 2012 9:16 am

Maybe it would be usefull to look at a related field (same Re) with a strong foundation in engineering (the level of engineering in kiteboarding is very low). Maybe a propeller producer?

I would not be surprised, if a rounded leading edge is superior, so that the premise of your questions is invalid!


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