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Sails: Floppy Leech Gone?
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scottwerden



Joined: 11 Jul 1999
Posts: 213

PostPosted: Sat May 10, 2014 7:24 pm    Post subject: Reply with quote

Winglets on airplane wings do two things, actually: (1) they prevent a discontinuity at the wing tip due to the pressure differential between top and bottom. The winglet itself is not a lifting foil, so at its tip there is no pressure differential. The pressure discontinuity leads to a vortex which does not affect lift of the wing, but it does cause aerodynamic drag and thus a need to burn more fuel. (2) The winglet also prevents the flow of the air along the top of the wing from trying to "slide" off the tip. Optimum lift comes from air flow that is exactly aligned with the cord of the foil which is in general perpendicular to the leading edge (for a Cessna; swept wings are more complicated). Air flow that slides down the wing negatively affects the lift of the wing, but the winglet prevents that and thus optimizes lift.

So applying this to a sail, from the description by Brimar, there is a vertical pressure gradient due to the twist. This will decrease the vortex drag as he described but will not help the loss in lifting efficiency since air is still "sliding" off the end of the sail (in the case, towards the top of the sail). That is the downside of twist - it reduces vortex drag but also reduces lift.

I don't know what the answer is, but it is an interesting problem. It would be cool to put a sail in a wind tunnel with the smoke flow tracers.
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westender



Joined: 02 Aug 2007
Posts: 625
Location: Portland / Gorge

PostPosted: Sun May 11, 2014 12:34 am    Post subject: Reply with quote

Low tech is to tape pieces of yarn to the sail like the boat guys do to see what the wind's doin.



scottwerden wrote:

It would be cool to put a sail in a wind tunnel with the smoke flow tracers.
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konajoe



Joined: 28 Feb 2010
Posts: 171

PostPosted: Sun May 11, 2014 9:31 am    Post subject: Reply with quote

So, to reduce tip drag, the top of the sail has to have zero draft (no pressure differential) and it has to sit like a weather vane (so it doesn't act like an air brake). So does the top of the leach have to be floppy to achieve this? I don't know.

The very top of the floppy leach sails doesn't twist off in gusts. It twists off under it's own weight just sitting on the ground. So it is twisted off way before it gets even the slightest breath of wind. That section is a drag reducer.
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scottwerden



Joined: 11 Jul 1999
Posts: 213

PostPosted: Sun May 11, 2014 11:08 am    Post subject: Reply with quote

Yeah, I am with you on that. It does seem like the floppy leach is always bleeding off wind, not just gusts.
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QueNeo



Joined: 10 May 2014
Posts: 27

PostPosted: Sun May 11, 2014 12:47 pm    Post subject: Reply with quote

Floppy Leach is gone, eh?

Sure it is.

http://www.seabreeze.com.au/Img/Photos/Windsurfing/7921131.jpg
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MauiMakani



Joined: 07 Aug 1995
Posts: 24

PostPosted: Mon May 12, 2014 3:36 pm    Post subject: Reply with quote

I think that one thing that has changed is the location of the looseness along the leech. At one time, the looseness tended to be over-focused in the head, so that even as the sail twisted off and became taught in the lower portion, the head area still remained loose and floppy. As more progressive twist evolved that designed the twist to evolve smoothly up the leech, we saw less isolated ruffle in the head. On my current sails (Simmer Blacktips, a four batten wavesail), just picking the rig up to carry it to the water causes the weight of the head battens to create a smooth twist-off all along the leech that eliminates the look of any ruffling. The leech is till loose, but you only see ruffling when it is laying on the ground and the batten ends are supported so that they cannot create twist tension. I would suggest that what is gone, is the early days of twist that was too much focused in the head, which very often left that area ruffled and floppy even when the rest of the leech was tight under load.
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