sail aerodynamics

[CT249]

I'll be interested to see results when they line up against a fleet of comparable boats. Why they haven't done so (as far as I can see from results searches) when there is a sportsboat fleet in their home town is beyond me.

Double surface soft sails are not new, after all. The sector of sailing which partial-length double surfaces are most commonly used (windsurfing) often rejects them due to handling and complexity issues, even for high performance sailing. Of course that doesn't mean they are not great sails (personally I love big double-surface luff windsurfer sails) but the downside should be noted.

 

[sandhammaren05]

I am a bit confused with the explanations of how the sail actually pushes the boat forward. Do anyone have a good scientific explanation? Not just "The lower pressure on the leeward side sucks the sail forward".
/ErikW

It's an airfoil. The lift is due to circulation just like a wing or propeller blade. The net lift acts at the center of pressure, so you have to use the rudder to aim the boat where you want. Study wing or hydrofoil theory and you have it.

 

[MikeGBR]

I was very interested to stumble across this thread and have spent the last couple of days reading it from start to finish. Thanks to all who contributed to the discussion and shared their expertise and knowledge.

One item that particularly caught my attention was Tom Speer’s reference (post #60) to A.M.O. Smith’s observation, in his paper on high lift aero dynamics: 'if you place a bluff body like a circular cylinder behind and below the trailing edge, you get an increase in lift on the surface due to the deflection of the flow at the trailing edge'.

I wasn’t aware of A.H.O. Smith’s paper but had come across studies of the flow around headlands on the east coast of Australia, which found that off-lying islands eliminated back eddies.

I have been wondering for some time whether it would be of any significant benefit to put rods (possibly split back-stays) behind a mainsail to allow the main to be sheeted harder, without stalling, to create more lift? Equally, whether the type and positioning of shrouds could have significant influence on close sheeted, non-overlapping headsails? If so, what would be the smallest diameter of the rod to be effective and how far should it be positioned from the trailing edge for optimum performance?

 

[sandhammaren05]

I was very interested to stumble across this thread and have spent the last couple of days reading it from start to finish. Thanks to all who contributed to the discussion and shared their expertise and knowledge.

One item that particularly caught my attention was Tom Speer’s reference (post #60) to A.H.O. Smith’s observation, in his paper on high lift aero dynamics: 'if you place a bluff body like a circular cylinder behind and below the trailing edge, you get an increase in lift on the surface due to the deflection of the flow at the trailing edge'.

I wasn’t aware of A.H.O. Smith’s paper but had come across studies of the flow around headlands on the east coast of Australia, which found that off-lying islands eliminated back eddies.

I have been wondering for some time whether it would be of any significant benefit to put rods (possibly split back-stays) behind a mainsail to allow the main to be sheeted harder, without stalling, to create more lift? Equally, whether the type and positioning of shrouds could have significant influence on close sheeted, non-overlapping headsails? If so, what would be the smallest diameter of the rod to be effective and how far should it be positioned from the trailing edge for optimum performance?

You're talking about the same effect as the flap on a wing, or the cup on a prop. Consider two flat plates at the same angle of attack, one with the flap the other without. The flap can increase the lift by about 30% depending on flap angle The flap or cup must be right at the trailing edge unless you want to mess up the Kutta condition.

 

[tspeer]

...I have been wondering for some time whether it would be of any significant benefit to put rods (possibly split back-stays) behind a mainsail to allow the main to be sheeted harder, without stalling, to create more lift? Equally, whether the type and positioning of shrouds could have significant influence on close sheeted, non-overlapping headsails? If so, what would be the smallest diameter of the rod to be effective and how far should it be positioned from the trailing edge for optimum performance?

Getting more lift in a sail rig is easy - just add more sail area. Reducing the drag, while maintaining the same lift and height of the center of effort, is hard.

Smith's illustration of a very large cylinder behind and to windward of the trailing edge was in no way intended to be used as an actual device. He was showing that the cross-flow produced by the cylinder changed the boundary conditions at the trailing edge and this altered the lift on the entire wing. The cylinder took the place of a slotted flap, but it was the most inefficient slotted flap possible, and was a shape that would produce no lift by itself. His point was that a real slotted flap produces a similar cross-flow, and this cross-flow accounts for the effect of the flap on the wing.

Placing stays near the trailing edge of mainsail will not increase the lift on the mainsail. Many boats already have such an arrangement with running backstays.

 

[MikeGBR]

Thank you for your immediate and helpful replies. Back to the drawing board!

 

[brian eiland]

Flaps on Airfoils, the MastFoil concept

I see the subject of 'flaps' has come up again.

In terms of sailing rigs (afterall we are are a boating forum, not an aircraft one,...very different Reynolds numbers), I would like to reintroduce this very interesting experimentation and implementation being carried out by Chris White

Page 31 of this Sail Aerodynamics discussion...

What do you suppose Chris White has in mind with the aerodynamics of this new rig??

http://www.chriswhitedesigns.com/atlantic_cats/mastfoil/index.shtml

Back to Chris White's new rig, the subject of this lastest posting. Did all of you take notice that his 'mastfoils' were two element foils themselves??
"Why does the foil have an articulating trailing edge flap?
Because a flap adds lots of power to the foil with very little additional weight and complication. Reaching and running the flap is set at a significant angle (approx 40 degrees) to the main foil which increases the overall camber of the foil and can nearly double its power. Sailing upwind only a small amount of flap angle is used but it helps create additional lift with very little drag"

This is an important and distinguishing feature of his design. This imparts a much greater contribution to their effectiveness, both alone, and in support of that headsail located in front of each MastFoil.

This unique design deserves more analysis before being tossed to the trash pile by the 'traditionalist'.

Another subject thread on MastFoil:
http://www.boatdesign.net/forums/multihulls/chris-white-atlantic-47-mastfoil-40670.html

...short video:
https://chriswhitedesigns.smugmug.com/ATLANTIC-CATAMARANS/Atlantic-47/Atlantic-47/i-KCxP879

 

[brian eiland]

V-flap Foil

https://chriswhitedesigns.smugmug.com/ATLANTIC-CATAMARANS/Atlantic-47/Atlantic-47/i-RnZMM5k

...
Is the angle of the v-flap controllable, or does it flap from one side to the other?

Any idea of how this flap is attached to the aft edge of the aerofoil sleeve?

...a reply posted by another gentleman very interested in this technology...

On White's HP i can see two arrangements, one with a big V- flap ( split flap) and one with a small V-flap (flipping split flap)
I think that one with the biggger flap has been the first one.
This setup has to be trimmed much more, 'cause you have three interacting components.
For easyness only upwind
-softsail whose camber is controlled via track at the end of the boom
--MastFoil whose amount of lift is controlled via it's positive angle to the apparent wind
--flap, whose angle to the centerline of the Mastfoil defines again the amount of lift of the MastFoil
A complex system, a lot of telltales are needed.
This setup has a lot of variabilities, the margin of a mistake is big.
With other words , for the average sailer the setup is much too complicated.
That the big flap has been controllable over a wider range ( from 0 to about 40 degrees) shows the control arrangement in vid 40. (Note that the mizzen in the vid features the second, the smaller flap )
https://chriswhitedesigns.smugmug.com/ATLANTIC-CATAMARANS/Atlantic-47/Atlantic-47/i-RnZMM5k
https://chriswhitedesigns.smugmug.com/ATLANTIC-CATAMARANS/Atlantic-47/Atlantic-47/i-V3CHvnK

Smaller flap , also upwind.
--the MastFoil is underrotated a little --about 10 degrees positive to the apparent wind. this angle of attack is more or less universal and easy to obtain.
At this angle the Foil itself ( alone) generates lift with little drag. It is far away from flow separation and very far away from stall.
--at this angle the little flap can be set full . No separation, no stall , but much more lift for the Mastfil with only very little additional drag.
This setup is much easier!
One tacks, one flipps the flap, one rotates the foil about 10degrees to the apparent wind.
Any Sailer can do this.
A keen sailer has the option to fine-tune the Mastfoil on the new leg.
pic showing flip-flap control-unit, note the two holes.
https://chriswhitedesigns.smugmug.com/ATLANTIC-CATAMARANS/Atlantic-47/Atlantic-47/i-T9hbGr2
Tacking, note that the 10degrees are given very late after the third tack , at about 2:10
https://chriswhitedesigns.smugmug.com/ATLANTIC-CATAMARANS/Atlantic-47/Atlantic-47/i-RnZMM5k

One might ask why a flap, why not rotating the foil only for more lift ?
A higher angle of attack means earlier flow separation and much more drag.
using a flap means lower angle of attack of the foil , only little additional drag and much more lift.



About the kinematics , the mechanism of the flip-foil i don't know anything, sorry.
At least one can see lever arms connected to the bigger , tuneable flaps
http://chriswhitedesigns.com/atlantic_cats/mastfoil/update_01-12.shtml
For other solutions please search in Google Pix under " aileron mechanism" .

pogo

 

[brian eiland]

Another good Youtube videos of the MastFoil in action:
https://youtu.be/73n56dxBgYI

https://www.youtube.com/watch?v=PqeTkFjOTc4

 

[Spiv]

This youtube video shows how the mastfoils are rotated and the flap adjusted: https://www.youtube.com/watch?v=ekVzCKlgoxU

 

[brian eiland]

This youtube video shows how the mastfoils are rotated and the flap adjusted: https://www.youtube.com/watch?v=ekVzCKlgoxU

Thanks for that link Spiv. I posted this comment on that link just now...

Appears to me that there are 2 different size flap foils, one much wider than the other? And judging from the sunny conditions where the full height rig is shown, verses the cloudy conditions of the boat show, I venture to guess this video was edited together from several different videos?

I also wonder if the bigger (broader) 'flap' is v-shaped as the one more detailed in this video.

 

[brian eiland]

Flow off that Flap

I wonder what the flow looks like behind that flap,...would love to see a smoke tunnel image/video.

Would it make any sense to have a small v-shaped flap like that behind a more traditional shaped, bare mast,...for drag reduction verses extra lift created??

 

[Spiv]

Short video by Oracle Team on their wingsail: http://sailingcruisingscotlandtv.com/video/6y8n0i79o778

 

[brian eiland]

"Think One" and Flaps

Think one

http://cdn2.hubspot.net/hub/209338/news/98_11_PerfectShape/Main.htm

http://cdn2.hubspot.net/hub/209338/news/Ad_aerodynamics/index2.htm

To understand how the mainsail and the jib work together, it may be useful to think of them as one wing with a slot in the middle. The jib forms the front part of the wing (leading edge in aircraft terminology) and the mainsail forms the back part (trailing edge). The jib is the curved, smooth shaped front part of the wing, the main is the more straight, flat part. When you adjust the main sheet or traveler, you adjust the camber of the whole wing in a very smooth way, around the hinge that the mast forms.

When you keep in mind that pressure always acts perpendicular to the surface, it is easy to see why the jib usually is in charge of most of the forward driving force, while the main causes most only heel. When sailing upwind most of the surface of the jib is oriented in the direction of the motion, while most of the surface of the main is oriented perpendicular to it, the leech often even pulling back. But it would be wrong to think that the jib is more important for that. The mainsail is bending the air in front of the jib, allowing the boat to point higher than with the jib alone, and also helping the jib bear more loading without stalling.

Flaps

The attached PDF gives the relation between an airfoil's lift coefficient and its local incidence angle distribution. This quantifies the stuff I've been saying about what does and does not affect the lift on a sail. The relation comes from thin airfoil theory, but it is still reasonably accurate if the camberline is broken up into a number of smaller pieces. It there's an overlap, then the local incidence of the overlapping sheets should be averaged.

The key part of the relation is the lift/incidence influence function f_a(x), which multiples the local incidence distribution. This f_a(x) is small on the front parts of the airfoil and grows over the rear parts, and becomes infinite (but integrable) at the trailing edge. Basically, any shape changes on the front of the airfoil get mostly quashed by the small f_a values there. Conversely, any shape changes towards the trailing edge get strongly magnified by the large f_a. So to control lift we need to modify the rear airfoil shape, and can mostly ignore the front airfoil shape.

This means that the shape of the rear half of the airfoil on average has 4.5 times more influence on the lift than the front half.

The means that the shape of the rearmost 1/6'th of the airfoil controls half of the total lift.

So now I am 'Thinking One' and looking at this new alternative aftmast concept I posted over here:
Alternative Aft-mast Rig

Looks like it might be fair to say that it presents two nice headsails to drive forward with, and a controllable flap by way of that aft mainsail/wingmast arrangement? (I might even be tempted to term that mainsail a mizzen, it is mounted so far aft).

I even wonder if that aft mainsail/wingmast might be replaced with a more brief MastFoil arrangement operating as a flap device. :idea:

In either case I think I would prefer this more brief arrangement on the after portion of my sailing rig than an over-tightly sheeted traditional mainsail trying to act as the flap. I've experienced over sheeted mains, and mains pulled up pass the centerline, and sails with leech control lines, and in most cases I did not see big improvements with my windward sailing,...in fact most cases less performance.


PS: I forgot to add a link to the website (blog) of the rig's creator
http://www.vizible.co/p/p.html

 

[David Cooper]

I've been reading through this thread over the last few weeks and I'm now wondering what the current state of computer simulations is. It sounds as if the one used to create the video on page 33 takes hours or days to run and needs a network of machines to share the workload. It also sounds as if it isn't cheap to buy. I'm planning to have a go at writing my own software of this kind, but I've approached the task by working everything out from scratch, starting by thinking about collisions between air molecules rather than simply applying existing formulae. I've come up with a number of tricks for simplifying the task so that there's no need to work with individual collisions, so it's really a matter of working with numbers of molecules and vectors for average movement in boxes, and the size of the boxes can be made smaller for higher resolution results (though only near to the rig).

There are a number of simple things I'd like to know about existing simulations first though. How long do the best 2D simulations typically take to generate end results on a single machine with different resolutions (meaning, how long does it take for the air flow to settle down: it'll take time for the stream of accelerating air to be generated round the outside of the sail, for example - I'm guessing the point of lowest pressure will work its way forward from near the leech towards the luff), and how long would the 3D ones take to do the same thing on a single machine at a useful resolution? How small do the boxes need to be before the 3D programs get results that provide a reasonable representation of the real world? My plan is to start with boxes of 1.28m sides, then subdivide to 64cm, then 32cm, 16, 8, 4, 2, 1cm, 5mm, 2.5mm, and 1.25mm around the most tightly curved components of the rig. The virtual test arena will initially be 12.8 x 12.8 x 12.8 metres in size. I'll be able to set different wind speeds and directions at different heights. One of the things I want to do is work out how the forces are transferred from the sails to the mast, sheets and other rigging and on to the hull (in addition to providing more simple results).

I suspect the biggest problem will be simulating boundary layers correctly. It's easy to introduce drag, but the distance which the effects of it spreads over may depend heavily on the resolution used (the box sizes) next to the sails, so I'll likely have to cheat in some way. I also don't know how much resolution is needed before turbulance appears in the model, but it should be fun just to write the program and see what happens. If it ends up being useful to anyone, that'll be a bonus. If it's too processor intensive for it to be practical to do the whole 3D thing, I might restrict it to 3D slices through the rig, feeding downforce back in at the top at the same distance out from the sail so as to be able to generate figures for lift in directions other than the horizontal.

It's already been a worthwhile exercise just thinking through the way air molecules travel about and collide with each other, because I've finally managed to work out why air accelerates round the outside of a sail. Moving a sail through the air creates empty space behind the back of the sail towards the leech. Air is then going to flow into that space, and it does so purely because air molecules can travel in that direction more easily without suffering so many collisions: they simply move in that direction for longer while travelling at the same speed as before, but the average speed of the air goes up as a result, even though the molecules themselves have not accelerated (although their speed will increase over time due to the collective effect of many collisions). What I have yet to work out is why the place of minimum pressure migrates forward towards the luff, but it must be something to do with a stream of air round the back of the sail being established over time with the acceleration leading to the area near the leech being filled up again and restoring higher pressure there as the air slows back down. That is one of the things I want to explore through simulation.