New Wing Sail

[John Perry]

Manfred - no I hadn't seen the two threads you mention so thankyou for the links. I have now taken a look at the two threads. The first one includes useful contributions from Tom Speer, amongst others, and some information about wingsail lift coefficients but no firm data for soft sail lift or drag coefficients. The second thread includes a graph showing lift force, but not drag force, for a 7m2 windsurfer sail in a 15 knot wind. From this graph I estimate the maximum lift coefficient to be about 3.0 which seems highly improbable.

I have also looked at the book 'Sail Performance' by C.A.Marchaj, 1996. There are several polar graphs for lift and drag coefficients of soft sails scattered though the book For example, fig. 64 in the book shows results from a wind tunnel test on a 2/5 scale model of a Finn dinghy rig without the hull. Maximum lift coefficient is 1.3 and the best L/D ratio, at a lift coefficient just less than 1.0, is about 5:1. Figure 206 shows results for wind tunnel tests on a model of a masthead sloop rigged yacht, including the hull, this shows a high maximum lift coefficient of about 1.75 and a best lift to drag ratio of about 4:1 at a lift coeficient of about 1.0. Fig. 127 shows some of the measurements made by Eiffel more than 100 yeas ago - wind tunnel tests of a sheet metal model of a bermudian una rig gave a maximum lift coefficient about 1.25 and a best lift to drag ratio of about 10:1, again at a lift coefficient of about 1.0.

So, quite a lot of variation there, particularly with regard to drag coefficient, and a lack of detailed information about the models that were wind tunnel tested. How much sail twist did they allow for, if any? Did they allow for any standing rigging? Did they allow for wind shear? Was the mast accurately to scale etc.

I think CFD is the way forward for this kind of study in the future but it would certainly be nice to have at least some dependable wind tunnel or actual 'at sea' data to validate the CFD results - does anyone know where this can be found?

Ilan Voyager, you mention interesting effects on drag of 'vibrating surface of the cloth' - What are the effects I wonder, also, I havent noticed cloth sails that are reasonably well set doing much vibration.

 

[Skyak]

I don't believe their claim of twice the aerodynamic efficiency.
A sail of the same aspect ratio should not be far from its performance.

The vague "efficiency" term leads me to believe that it came from a marketing or PR person. This development was paid for with government money -non technical customer. The positive verbage is their compensation. Would it be permissible for the government to give money to a local company if it only created what already exists?

I suspect the 2X efficiency is referring to nothing more than the 2 element CL max being twice the CL max of a conventional single element main sail. I don't think any politician would inquire about induced drag, even in France.

The performance implications here are negligible. The achievement is in foil management. This is a fully automated (or automate-able) touchless rig. I would go so far as to say I have not seen any better automated rigs, and that the government got their moneys worth in terms of market potential.

This leads me to my top complaint about technology these days -they keep automating the fun things in life but leave the @%$#@^# jobs for humans! I don't want a robot to sail my boat, drive my car, cook my food, do my shopping... I want them to clean my bathroom, pay my taxes, paint my house, bring me money!

 

[Ilan Voyager]

The vague "efficiency" term leads me to believe that it came from a marketing or PR person. This development was paid for with government money -non technical customer. The positive verbage is their compensation. Would it be permissible for the government to give money to a local company if it only created what already exists?

I suspect the 2X efficiency is referring to nothing more than the 2 element CL max being twice the CL max of a conventional single element main sail. I don't think any politician would inquire about induced drag, even in France.

The performance implications here are negligible. The achievement is in foil management. This is a fully automated (or automate-able) touchless rig. I would go so far as to say I have not seen any better automated rigs, and that the government got their moneys worth in terms of market potential.

This leads me to my top complaint about technology these days -they keep automating the fun things in life but leave the @%$#@^# jobs for humans! I don't want a robot to sail my boat, drive my car, cook my food, do my shopping... I want them to clean my bathroom, pay my taxes, paint my house, bring me money!

Efficiency is not a vague term. You have only to precise the domain of efficiency.
For example the health system in the USA is totally inefficient in terms of cost (% of the GDP), coverage of the population and final results (life expectancy) compared to universal health care systems of North and Western Europe, Japan, Malaysia, Singapour, Taiwan, and even Costa Rica.

Yes this tiny third world country has a universal health care which costs 10% of the GDP (USA 19%), covers 99 % of the population (USA les than 90% with several serious limitations in the extent of the coverage), and the ticos have now a greater life expectancy than the Americans. That's efficiency in social care. In this case it's to spend less for covering all the population and getting better results. Add the ethical and moral satisfactions as bonuses. I like politicians whose main job is taking care of their citizens.

Coming back to the wing sail the 2 factor efficiency is only on the ratios of surface. You need twice the surface of soft sail to get the same power compared to the wing. To get this result you have to get a better Cz (lift=power) and a lower Cx (drag). Besides there are more factors leading to a better general efficiency for powering a boat; lighter, smaller and better orientation of the obtained power with the hull. That means that in the diagram of forces more power is made to move the boat, and less force in wasted in heeling, with acceptable drag. Add also that that the usable angle with the apparent wind is larger. Make the diagrams of a 360 degrees high lift variable camber wing...

This wing is primary designed for work ships. Cargos and fishing. So to be efficient in the economical domain it has to be automatic as nobody in shipping and fishing will pay for a special crew dedicated to the sails. It's an important factor among the obtained savings which will amortize the initial investment in a decent lapse of time for a shipping company. There are many other requisites for a successful wind powered auxiliary system.

That explains the subventions given to VPLP. The money is used for a reefable high lift wing, a very important factor of security, for ships. The Europeans are concerned by ecology and climatic change. The total subvention is far less than a single Tomahawk missile. It's a very serious project, not a new gadget or toy for rich boys. The yachting applications are collateral and serve only for testing.

Automaticity has advantages. I love my Daewoo automatic washing machine. I wouldn't like to fill it with water buckets, stay besides the time needed for washing and stop it. Pump out the water, fill it again. Stay for the rinse etc. I fill it, close it, set the program kept in the EPROM and the bell rings when washing is finished.

I'm sure you love automatic phones and cellulars, and internet connections you have not to tweak each time you try to connect.

And you are free to make your own choices about automatic and robots in your life.

 

[redreuben]

Ilan Voyager:You can add Australia to the list of haves. Great example

 

[John Perry]

Its my understanding that in the context of aerofoils the term 'efficiency' is usually taken to be synonymous with 'lift to drag ratio'. Perhaps someone will correct me if this is not so.

 

[bjn]

I suspect the 2X efficiency is referring to nothing more than the 2 element CL max being twice the CL max of a conventional single element main sail. I don't think any politician would inquire about induced drag, even in France.

The statement would make sense in that case. CL=2 compared to CL=1, maybe. If you compare a normal (low aspect ratio) bermudan rig with a high aspect ratio two-element wingsail. And it may also be true that the peak lift to drag ratio is twice in this comparison.

But if you compare a rig with a rotating wingmast and fully battened sail, to the two element wing sail. Not much difference.

 

[John Perry]

But if you compare a rig with a rotating wingmast and fully battened sail, to the two element wing sail. Not much difference.

Do you have any dependable measurement data for 'a rig with a rotating wingmast and fully battened sail'? - if you have I would be most interested since I could do with it for my numerical modelling work on hydrofoils.

If I were to try to aquire such data myself my first thought would be to record it by mooring a full size craft with force sensors in the mooring lines. Probably not a new idea but I am not aware of it having actually been successfully implemented. Not quite as easy to do as one might initially imagine though.

You would need a patch of water without water currents, in the UK that could mean a lake, but the location should be in open country clear of on-shore wind obstructions such as trees.

I think you would need at least three mooring points for a stable craft position and heading.

As well as force sensors such as loadcells, you would need angle measuring devices to measure the alighnment of the tethers both vertically and horizontally relaitive to a craft frame of reference, or alternatively multi-directional force measurement at each attachment point.

You would need wind speed and direction sensors located well clear of the craft, possibly on a second craft(s) or bouy(s). Ideally you would include measurement of wind shear.

For each set of readings you would need to collect all the data over a period of time, at least minutes I would think, so as to average out the effect of variability of the natural wind and possibly also the effect of any wave action on the craft. Almost certainly this would mean a computer controlled data aquisition system which is actually the easy part of the job these days.

Even if done on an amateur basis, such a project would probably cost a few thousand pounds/dollars in bits and pieces of equipment but would probably be far cheaper than wind tunnel testing on a full size craft or even a large scale model.

 

[bjn]

I don't have any measurement data. My statements are just based on simulations with xfoil and similar. If the real world sails will deform significantly from the forces, if my understanding of aerodynamics is wrong, or if the simulation results doesn't match reality, I am wrong.

But as I see it, sails or wings are just bending air. If the airflow is smooth, there is almost no drag. The drag is then only induced in the ends. Which makes aspect ratio the significant factor.

Would be very interesting to see the results of real world measurements.

 

[Skyak]

The statement would make sense in that case. CL=2 compared to CL=1, maybe. If you compare a normal (low aspect ratio) bermudan rig with a high aspect ratio two-element wingsail. And it may also be true that the peak lift to drag ratio is twice in this comparison.

But if you compare a rig with a rotating wingmast and fully battened sail, to the two element wing sail. Not much difference.

I presume that they are talking about a comparison to a state of the art conventional mainsail with the same profile area. Their wing has twice the aspect ratio because it splits the area into 2 long foils. At high lift, the wing drag will be predominantly induced drag, and since it has the same height as the conventional sail the induced drag will be the same proportion of lift -as you implied earlier in the thread.

 

[Skyak]

Its my understanding that in the context of aerofoils the term 'efficiency' is usually taken to be synonymous with 'lift to drag ratio'. Perhaps someone will correct me if this is not so.

John, that may be true for airplanes, but using L/D as a measure of efficiency makes far less sense for sails because of the variable direction and strength of the wind. Synonymous it is not, so I would always say L/D when I mean L/D.

Efficiency=thing you want/thing you need to provide

Take downwind -That VPLP wing will be inferior to the conventional sail because it is split down the middle. Not 2X better -actually inferior.

I could certainly be wrong thinking it is CLmax/area -I am guessing based on who is talking, and who they are talking to. Ask VPLP.

 

[CT249]

If the lift to drag ratio of a solid wing is twice that of a good conventional sail, why have solid wings failed to have superior performance in so many real on-the-water tests for so long? For over 50 years people have been putting solid wings onto boats, and they have proved themselves to be faster around the course in about two cases - C Class cats and AC classes. Even in the A Class and 18 Square Metre cats, the solid wings did not prove to be faster.

Furthermore, Tom Speer - a person who has designed both wing sails and aircraft wings - has said here "If a softsail rig is given the same height as a comparable rigid wing rig and designed to avoid separated zones, then there's no fundamental reason why it would be less efficient."

To quote Tom again; "There's nothing magical about a rigid wingsail. The soft sail rig on USA 17 was faster than the wingsail in some conditions. Whether a wing or soft rig will be faster depends on the class constraints and the sailing conditions."

The fact that a sail is rectangular is not a benefit if righting moment is constrained (as it usually is) because it results in too much heeling moment for the amount of drive that is produced.

VPLP are great designers but with respect, it's doubtful whether they are the most successful in history - Farr and S&S are likely to have won more events and against larger fleets. Most of the world doesn't sail the classes where VPLP dominate. Perhaps it's a bit like being the best Gaelic Football player or Aussie Rules player; impressive but not really comparable to being the world's best football (soccer) player.

 

[Ilan Voyager]

John, that may be true for airplanes, but using L/D as a measure of efficiency makes far less sense for sails because of the variable direction and strength of the wind. Synonymous it is not, so I would always say L/D when I mean L/D.

Efficiency=thing you want/thing you need to provide

Take downwind -That VPLP wing will be inferior to the conventional sail because it is split down the middle. Not 2X better -actually inferior.

I could certainly be wrong thinking it is CLmax/area -I am guessing based on who is talking, and who they are talking to. Ask VPLP.

The aspect ratio is not the most important. The thing resides in the slot and the 2 rather thick profiles. Do not forget the camber or fullness impossible to get on a soft sail without stalling...the two parts of the wing and the slot have to be treated as one very cambered and rather thick wing with ventilation of the extrados of the flap. They work together like a plane wing landing in hyper lift mode with flaps fully extended. Look at the camber and angle of attack of the wings of a 747 landing...On older planes compare with a Lysander or a Fieseler Storch wing.
About downwind, make the drawings and the tensors of lift and drag of a wing of high camber with ventilation slot able to attack without stalling an air flow somewhere 55 to 80 degrees and able of rotating over 360 degrees...You'll see very nice and surprising things.
The difficulty is to have the mechanism for making work the wing on the 2 sides, each side being the mirror of the other and as important reefable. Hence the subventions.

 

[Skyak]

The aspect ratio is not the most important. The thing resides in the slot and the 2 rather thick profiles. Do not forget the camber or fullness impossible to get on a soft sail without stalling...the two parts of the wing and the slot have to be treated as one very cambered and rather thick wing with ventilation of the extrados of the flap. They work together like a plane wing landing in hyper lift mode with flaps fully extended. Look at the camber and angle of attack of the wings of a 747 landing...On older planes compare with a Lysander or a Fieseler Storch wing.
About downwind, make the drawings and the tensors of lift and drag of a wing of high camber with ventilation slot able to attack without stalling an air flow somewhere 55 to 80 degrees and able of rotating over 360 degrees...You'll see very nice and surprising things.
The difficulty is to have the mechanism for making work the wing on the 2 sides, each side being the mirror of the other and as important reefable. Hence the subventions.

"The thing" is span. It becomes AR because CL and CD are dividing by area. It is as certain as F=ma which the proof is based on. It doesn't matter what sort of wing is in the cloud, if you know the velocity, and you know the lift and the span, then you know the induced drag, which is the majority of drag for all good high lift wings. In fact, if you know it is a good wing, you also know the lift distribution over the span. It does not mater how you achieved the lift (though as a rule, the more you mess with it, the greater the mass defect) you can not escape the induced drag setting the minimum drag/lift of the span. Span determines the mass of air that you are accelerating with your wing, velocity and angle of attack determine the acceleration.

The two element sail or wing, can bend the air stream in it's span up to twice as much, but the induced drag will be no less than the same twice as much. So in order to be twice the L/D of a conventional sail -that conventional sail would have to be crap -or you would have to be comparing to a stalled conventional sail (also dishonest).

Your suggestion that if I do the math for down wind sailing... I find condescending. At best, you simply don't understand the problem. The VPLP wing can not match or better the performance of the conventional sail by trimming forward of beam until flow attaches.

Up wind, the span-wise lift distribution goes to zero at the ends of the wing. Just inboard of the edge the pressure can be no more than a small amount -any higher and the air would accelerate toward the lower pressure edge -thus you can not have large pressure without a large distance from the edge of a wing -a large span. Any increase in drag of the wing detracts from performance turning the force of the wing away from the desired direction.

Down wind is different in that you don't have fresh air sweeping by. The better the sail, the lower the wind velocity (higher the boat speed), and the lower the drag, the more the lift vector is pointing away from the desired thrust direction (drag is good). The highest performance (greatest force at deepest angle) comes from a sail with the greatest area the greatest distance from any free edge ->aspect ratio=1.

To get any benefit from the multi element wing you need to bear away from the down wind and try to make up for the weakness by sweeping more air. This is only faster if the boat is very low drag with a very tall rig. In short, the only way the high AR wing can be faster down wind is to be on such a low drag boat that there is no down wind apparent. This is clear in the race results cited by CT. To clue in on just how low the drag must be, look at the race results. In the little AC, if the hulls are in the water, the conventional sail beats the wing down wind. Foiling, the wings win. I think there are plenty of cases in beach cat races where old lower aspect rig cats had an advantage down wind. Poke CT and he will list them longer than you can listen.

 

[Erwan]

Why not move your topic in the Aero & Hydro forum ?

There is already a topic about wingsail technology,

It is likely to trigger comments from top level architects

Too bad VPLP did not make one for an A-Cat.

 

[Konstanty]

Don't worry about drag induced in the ends of sail. There is end plate of the wingsail. I'm worry about any wrinkles on the space-time in the front part of sails in the windward side. At the inner space should be hypertension from the point of stagnation top wind speed.