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 Post subject: Re: Unexpected telltale results
PostPosted: Sat Aug 24, 2013 10:24 am 
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Interesting !

About the telltales in sails, I will also agree that good sailors can "feel" the sail (flow) really really well.
But even if they can, telltales will still be able to give some crucial information that can not be felt the same way.

Because, every sail needs to have the correct amount of downhaul (kicking strap) in order to get the correct twist (AOA changing from the boom up to the top of the mast).
The wind has a very different angle at the lower end (around the boom), compared to around the middle and upper part :naughty:

This angle changes with changing wind strength, and point of sail (also sheet tension), how gusty it is, the stiffness of your mast, and sometimes even water surface conditions.

Meaning, you can change your efficiency and speed and upwind angle by a margin by having the sail twist "perfect" - not too much nor too little, in a very given scenario that changes over the course, thus not fixed.

Having telltales at the lower and middle and upper part of the sail is really crucial here :thumb:


In a kite I find it very interesting to see when the flow separates and how often.
Also, how far out on the lifting surface will you destroy the flow when doing even small bar corrections ?
We all know that the tips of a kite is sometimes fully stalled and the other side fully luffed, when doing a tight kiteloop.
They are not delivering lift though - but as soon as you go towards the center of the kite, you will also have "twist" and maybe a lot of inefficiency even with small control corrections when flying in gentle sinus'es ? Or not - that is where telltales can help visualize.

Some kites tends to have too much or too little AOA on the chord at the center compared to the middles (right and left middle lifting areas) so a lot is "lost" this way.
Here, telltales can reveal when very off :o
"Feel" is NOT possible here - as no matter how good you are, you can only feel the best AOA for a given scenario with a given kite - but as in the sailtwist scenario, you can NOT feel if the AOA is wrong at the center of the kite compared to the middle sections.

Personally I am not much into slots and vents, as it is IMO a considerable performance sacrifice one makes in order to make things "easier" to handle, and not a thing you do to gain anything except that.
Flaps is different, as they can actually make things perform better at high AOA's as in turns, with minimal loss really, and perform better at low AOA's too.


Regarding the "boundary layer" turbulent airflow, yes, correct that it has a significant thickness (compared to laminar flow only a mm or so in thickness).
But I think that if the telltales are not too short, they will still be able to detect when separation occurs - and THIS is the point from where lift is lost and drag increases by a huge amount.

It seems, according to what JS observed till now, that flow is disturbed into separation a lot more than we (I) think it would be :wink:
But a kite is designed to work "adequate" with a lot of control, and really a simple construction, so these areas with semi separation, not a stall nor fluid flow either, are maybe widespread and common ?

8) Peter


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 Post subject: Re: Unexpected telltale results
PostPosted: Sat Aug 24, 2013 10:58 am 
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Thanks for the replies about the slots..makes much more sense now. El Rudo: I would try it, but the only OR kite around here is flown by my buddy..but he has worn it so thin I am afraid it would explode if I touch it...it is literally falling apart :-)


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 Post subject: Re: Unexpected telltale results
PostPosted: Sat Aug 24, 2013 8:42 pm 
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Peter_Frank wrote:
Interesting...

Significant points, Peter... well explained.

A few more points (general, not directed specifically at you, Peter):

1. Regarding sail twist: wind aloft moves faster than wind closer to the water surface, due to friction. That's wind shear. Therefore, boat speed (or kiteboard speed) affects apparent wind direction more near the water and less higher up. Ideal sail twist accounts for that, of course.

2. The same phenomenon happens with kites, when flown at low angles, just above the water. The wingtip near the water normally experiences apparent wind direction much closer to the kiter's direction of travel than the high wingtip, where true wind speed can be double, or even more. This can simultaneously cause a partial luff near the low tip and partial stall near the high tip. Additionally, the higher true (and therefore apparent) wind speed at the high tip can induce a kite to dive somewhat, which has to be compensated for with the bar. Those factors are the reason a kite flying along near the water can require significant steering input ("higher tip" bar pressure) to maintain level flight... ironically inducing the opposite of otherwise ideal twist. This is akin to a turning airplane, it's inside wing experiencing lower airspeed, less lift, and therefore requiring aileron input to avoid dropping a wing... also, like the kite example, unfortunately contrary to achieving good attached airflow.

3. In the past I have tried placing several telltales across the span of a kite (all on the top, within a foot of the trailing edge) to see where stall develops earliest... but recently just a couple a few feet either side of the center strut for a general indication.

Cheers,
James


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 Post subject: Re: Unexpected telltale results
PostPosted: Sat Aug 24, 2013 8:58 pm 
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I think you have been mis-informed about OR and the Cabrinha Race Series at Crissy
(2006-2012)
steve


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 Post subject: Re: Unexpected telltale results
PostPosted: Sat Aug 24, 2013 11:59 pm 
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El Rudo wrote:



On the OR kites, the vents do work.


Not in the upwind legs of a race, which are all about low aoa. (and the vented canopy is just extra parasite drag)


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 Post subject: Re: Unexpected telltale results
PostPosted: Sun Aug 25, 2013 3:06 am 
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lobodomar wrote:
El Rudo wrote:



On the OR kites, the vents do work.


Not in the upwind legs of a race, which are all about low aoa. (and the vented canopy is just extra parasite drag)

They only "open" at high AoA.


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 Post subject: Re: Unexpected telltale results
PostPosted: Sun Aug 25, 2013 11:41 am 
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Guys, it is by no means my intention to hijack James post here and make it an Ocean Rodeo advertisement. I only know that Stefaans did very well in 09 by winning the series.
Most of you have been riding long enough to pick the gear that suits you best, I have no intention to mess with that.
In my opinion, after riding vented kites since 2008, I have noticed at least I myself benefit from it.

The reason I'm following this thread is that I like to see riders tune in to the dynamics of their kites. I work as free lance design engineer and have been designing water sports equipment for some while now, including an A-class catamaran and foil and rig elements. I'm surprised by the level of almost scientific interest interest of those racers, compared to the surf crowd, who most of the time seem more worried about how cool they look than how well the equipment performs.


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 Post subject: Re: Unexpected telltale results
PostPosted: Sun Aug 25, 2013 11:55 am 
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Kamikuza wrote:
lobodomar wrote:
El Rudo wrote:



On the OR kites, the vents do work.


Not in the upwind legs of a race, which are all about low aoa. (and the vented canopy is just extra parasite drag)

They only "open" at high AoA.


Exactly. Even in the closed position, the vented canopy (net+outer ripstop) adds extra drag and weight.

BTW I own a 2011 OR razor, so had my share of first hand experience with the venturi system. It's a nice kite, but in my opinion the venturi is a marketing thing, something to stand out from the crowded kite market. Think about airplane slots: they are close to the LE, not to the TE like on OR kites. It's close to the LE that you need to energyze the boundary layer so that it keeps attached to the wing along the chord. Plus most of the lift is produced in the first half of the kite (center of pressure is usually at about 1/4 of the chord).

But of course OR didn't put the vent in the front, I doubt that a naturally vented (instead of mechanically open-close) slot would not screw up the kite's aerodynamics in a such a critical zone. They put it in the back, where it can do neither harm nor good.


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 Post subject: Re: Unexpected telltale results
PostPosted: Sun Aug 25, 2013 12:24 pm 
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lobodomar wrote:
Exactly. Even in the closed position, the vented canopy (net+outer ripstop) adds extra drag and weight.

BTW I own a 2011 OR razor, so had my share of first hand experience with the venturi system. It's a nice kite, but in my opinion the venturi is a marketing thing, something to stand out from the crowded kite market. Think about airplane slots: they are close to the LE, not to the TE like on OR kites. It's close to the LE that you need to energyze the boundary layer so that it keeps attached to the wing along the chord. Plus most of the lift is produced in the first half of the kite (center of pressure is usually at about 1/4 of the chord).

But of course OR didn't put the vent in the front, I doubt that a naturally vented (instead of mechanically open-close) slot would not screw up the kite's aerodynamics in a such a critical zone. They put it in the back, where it can do neither harm nor good.


The initial idea of the OR designers was to put the vents more towards the LE. Tell tale testing showed that they would be more effective towards the TE, to suck the flow back on to the canopy at higher AoA's, like Bille describes above. Airplane wings are designed to perform with AoA's differing only a few degrees. Kites fly with way bigger AoA variations depending on what you do with them.

The extra drag and weight you mention are there in deed, but we're talking a few grams and only very little drag.


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 Post subject: Re: Unexpected telltale results
PostPosted: Sun Aug 25, 2013 12:53 pm 
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lobodomar wrote:
Exactly. Even in the closed position, the vented canopy (net+outer ripstop) adds extra drag and weight.

Think about airplane slots: they are close to the LE, not to the TE like on OR kites. It's close to the LE that you need to energyze the boundary layer so that it keeps attached to the wing along the chord. Plus most of the lift is produced in the first half of the kite (center of pressure is usually at about 1/4 of the chord).

But of course OR didn't put the vent in the front, I doubt that a naturally vented (instead of mechanically open-close) slot would not screw up the kite's aerodynamics in a such a critical zone. They put it in the back, where it can do neither harm nor good.

A quantifiable but (very?) small amount of drag perhaps . . . but does the positive outweigh the negative?

Not exclusively, no. Flaps are slotted too . . .
http://www.cybermodeler.com/special/ima ... _flaps.png

Bf109 had mechanical slats on the LE - they worked. I think it'd be a case of negatives outweighing the positives to do it closer to the LE on kites.

No more of a marketing gimmick than diamond tips, no pulleys, sigma LE etc etc . . . ;)


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