Sails / Wings / Winglets

[joe_moore]

That programme about the Airbus A380 is on the telly at the moment, and they're talking about how the little vertical winglets at the wingtips increase efficiency 7% by stopping the vortex rolling over the end of the foil. Just wondering if anyone has experimented with a similar thing on soft sails. I know modern windsurfing sails often have a very loose top edge which curls right off, not sure if that's the reason but it would form a similar shape if much less dramatic.

With so many roachy and squaretop sails, we're in theory putting a fair bit of sail area in a relatively inefficient place?

Just a novice wondering really.. what's the practical experience in soft sails? 7% is a pretty big increase in efficiency..

[Stuberry]

Just wondering if anyone has experimented with a similar thing on soft sails.

Yes, someone tried something similiar on a Merlin Rocket.

[rich_cunneen]

Interestingly I had a similar thought when I was flying back from Florida, and looked at the wing of the plane, but assumed that it wouldn't work, because otherwise it would have been done.

Kind of like the first picture, but they'd need to be a lot bigger to stop the low pressure "falling  off" the top of the windward side of the sail  (The same effect creates the vortex you see rolling off plane's wingtips).

[joe_moore]

and a less evolved variation on the a320..


The idea is that they stop the wingtip vortex, giving you back the efficiency lost towards the wingtip.

[Stuart Hopson]

I don't think that having winglets on a sail would offer any performance advantage at all. The 7% efficiency gain talked about on the tv program last night is probably true but also relative. The airbus A380 cruises at up to 510 knots and takes of at 156 knots so the relative differece should be so massive that it would be unoticeable. Also just think how much the plane needs to alter its ''leech'' to generate enough lift to take off, with a soft sail it is impossible to do this (there are ways to alter the leech shape, kicker, cunningham, mast bend ect) but not to the same extent as you have on the back of a plane wing due to its mechanical controls.

Theres also the factor that as far as i know theres no definate theory on how air flows over a sail, it is probably very similar to that of a plane, but it would always be affected by the variables within the sail again related to kicker, cunningham, outhaul, mast bend, even the boats heel ect. The engineering required to be able to adjust the trim of any winglets on the move would be pretty tricky and probably impossible on certain areas of the sail. You could garantee that at some points of sail and if at any point the sail was not set up perfectly the winglets would be causing drag due to being incorectly trimmed.

We know how to make a 'fast' or what seems to be 'efficiant' sail through development, and i would be very interested to have a look at sails in a wind tunnel and how different shapes are affected by the all the different variables.

The main reason for having a square top main sail and fat head jibs. Firstly it gives the sail a more efficiant shape sooner as on most sails with a pointed head wont realisticly generate any real flow or lift in the top 15-30cm, where as a square toped sail will generate a good flow or lift much nearer to the top of the sail, meaning that the available sail area is theoretically more efficient.

The chance of there being any performance advantage from winglets on dinghy or skiff sails is very very unlikely. If there was it would still make much less difference than if you made one bad tack or crossed the startline a second late.

Sorry for the rather long reply and any possible spelling mistakes.

I'll try and answer any other sail related questions you have.

[joe_moore]

Time to build a class wind tunnel then 

[Phil Alderson]

The winglets on aircraft are really quite interesting, when you look at the numbers to justify them as a modification it shows that the main benifit is in the cruise, they work best in one flight reigime i.e. cruse during climb and descent they do not help much and in the climb can actually cost fuel as there is extra weight. so for short sectors they are not particularly effective.

They do not really reduce the tip vorteces, they are relocated to the top of the winglet and instead of the recerculating part of the vortex impinging on the horizontal part of the wing reducing lift it impinges on the vertical inboard part of the winglet which is set up with a slight toe in so the effect is a slight thrust from the tip vortex. The increase in lift and the slight thrust offset the drag and the extra weight of the winglet making an efficiency improvement. For best efficiency the planform of the wing and size and shape of the joint and winglet all need to be matched to each other.

The same improvement could be made by making the wing longer but this raises some structural issues and for the A380 space issues so winglets are used.

I think to set them up on soft sails would be incredibly hard, as the kicker is adjusted the direction of flow at the head would change, changing the angle of attack of the winglet possibly by quite large amounts.

[Banshee Ambulance]

is a cuff round the mast (like the 49er) allowed under class rules? If Franks blurb in higher performance sailing is anything to go by it does do something. How would this count under measured area? And would it count as a double luff?

[Will_Lee]

That is allowed, and only one surface of it is measured. However it is all measured, so you are wasting the bit that is touching the mast.

[TSC]

Ok just to clarify a couple of points.

1. The pictures of the Merlin, show devices known as fences. The aim of these is to stop spanwise flow from interacting with the chordwise flow over the trailing edge. In theory if you have a sharp change in leech profile and the bottom of the main is suffering from a trailing edge stall (ie. you've dragged the boom too close to the centreline but retained lots of twist to keep the uppoer portion of the main working) the seperated region will move outboard along the span, the fences stop this. Once you have established breeze and minimal twist (powered up conditions) then these will be a waste of time. I would also question whether the devices in use on the particular merlin were big enough. At such low Re number and mach number the boundary layer growth will be quite large, so these devices will be acting almost entirely in the turbulent boundary layer!!!

2. Winglets are indeed a way of giving an effective span increase with no span penalty, but as has been mentioned, on aircraft they are optimised for the cruise regime. The design is tweaked to minimise low-speed drag penalties. The percentage improvements even at cruise are incredibly small but then aircraft wing design optimisation is at such an advanced stage that we are looking for every minute scrap of performance in order to reduce the seat cost per mile. Interestingly winglets have started appearing on more and more gliders and RC gliders, which operate at closer RE to our sails but still not close enough for us to say that a benefit can be achieved on a sailboat. In terms of alignment, my initial thoughts would be that it would be better to stick such a device on the mast head rather than the sail, but a winglet is assymetric so developing a symmetric fence that achieves anything would be very tough.

I think in the short-term you'll find more gains in getting around a course without fluffing a tack or gybe!