The common explanation about how an aerofoil works is something like: the air travels faster over the lee of the sail, which creates low presure and lift is then provided by suction. But surfing the web today it seems this is backwards and that the air travels faster bacause of the low pressure at the lee and lift if provided by a reaction to a downwash; also that any old shape at an angle to the wind will create lift; it's just that an aerofoil is more efficient (which makes sense). Similarly, it seems the 'slot effect' is a "myth". Does anybody have any comments about this. I would like to understand the dynamics of my sails better but I'm getting well confused...

Example sites:-

http://world.std.com/~fhapgood/texts/sails.htm
http://www.aa.washington.edu/faculty/eberhardt/lift.htm
http://www.avweb.com/news/airman/183261-1.html
http://www.av8n.com/how/htm/airfoils.html
http://www.idra.unige.it/~irro/profilo1a_e.html

Frank Bethwaite still talks about suction...

Colin

Take a look at the online book "See How it Flies", chapter 3. It explains things in a simple way.
See How It Flies - John Denker - Chapter 3

The "slot effect" may be a myth, but lots of aircraft seem to use it. Here's one.

Slats (essentially a jib for a wing) are used for generating lift at high angles of attack, which corresponds to down-wind sailing. You see them on lots of short take off designs.

Colin,

Here's a good article from North Sails you have to hand over some personal information but its worth it.

http://www.northsailsod.com/articles/article6.html

Whats the Cat sailing like in the Gower?. I'm from Pembroke originally but now live in Brighton, although I still go home a lot.

Gareth

Excuse the spelling....but one must understand Bernoullis Principle. That states as the velocity of a fluid (air) increases, it's pressure decrease.

Think of runners running around a track, save for a staggerded starting line, the outside runners run a further distance correct? If they start from a straight line, and finish in a tie, then it IS safe to say the outside runner ran FASTER than the inside (more distance, same amount of time)....

Think of our sail now...as the air seperates over the sail, it travels "faster" over the lee (curved) section of the sail than on the inside.

Now apply the principle, as the velocity increases, the pressure decreases, which means we have a low pressure on the outside and a higer (when compared to the outside) pressure on the inside. Mother nature does not like imbalance so the high tries to go to the low, the sail is in the way...

Force to move our boat!!!

We can even accelerate that intital velocity by adding a jib (leading edge slot) to make an even greater pressure differential...

Hope this helps...

I think for Bernoulli's principle to apply, you must be dealing with an incompressible fluid. I seem to remember part of the derivation relied on the continuity equation where the density times the velocity into a closed volume equalled the density times the velocity out of the volume.
Thus some of the fluid is travelling farther in the same time and is generating lower pressure. Where's Wouter or Bill Roberts?
David Ho
TheMightyHobie18 1067

Hi Michael(?),

That's what I thought too until yesterday...!

Regards,
Colin

Hi Gareth,

Thanks for the reply, I'll take a look. I've also discovered the http://www.boatdesign.net forums which seem to have some excellent threads on the subject (as you know!).

Cat sailing on the Gower is growing... At our club (Mumbles) we have about 10 F18s and 20 Darts, a Stealth and an Inter 20, with more and more Darts moving over... What we don't have is a good long distance race. While there is a fair bit of sandy beach around, there is more rock, and folk worry...

Funnily enough I used to live in Brighton; sailed there too, from one of the clubs under the arches on the beach. If you ever fancy a pint or two at Mumbles YC or a sail, feel free to pop in... My mobile number is on my web site.

Regards,
Colin
www.sailwave.com

I think this is not a case of conflicting theories but of two ways of looking at the same principle were some people think only one way must be correct. However in physics often both ways of looking at things may be correct. It is even one of the delights of engineering. Sometimes a problem can not be solved by force equations (or just after many complex calculation) while the problem becomes really simple when handling it from an energy point of view.

Analogue example :

A car accelerator is pushed down which increases the pressure in the cylinder because more fuel is burned in it. The higher temperature causes the pressure to rise. Now this extra force is transmitted to the drive shaft and converted into a moment. This is then put through to the wheels which press puss the car harder along so it accelerates to a new velocity which is determined by the magnitude of the extra force and the time it is applies. (there are other factors but those will make the example too complex)

A skilled engineer can determine the end speed from this.

However a smart engine (or a lazy one) just asks the amount of extra spend fuel and the overall weight of the car and equates the amount of spend fuel times the know efficiency ratio of the engine to the engine build up in the extra car speed. He solves this equations and knows the end velocity. No need for complex calculation taking into account the gears and different radiusses of shafts and wheels.

Does this mean that a car is not accellerate by force by rather by spend energy ? Of course not. THe energy view is nothing more than a higher abstracted way of looking at physics at work and it is intimately tied to the force view.

The same can be said by this discovery of how a wing works.

By changing the view (moving with the wing, not moving with the wing) the describtion of what happens changes and sometimes this can change alot. This however does not mean that the describtion that is linked to the OTHER point of view/reference is incorrect.

The rest of the article disproof things that the author himself postulates and that most engineers will regard as an accurate describtion of common (eductated) believe anyway.

One claims that their is no suction on the leeward side at all but it is rather a lower pressure than the ambient pressure that is present there.

This is a naive discovery as negative absolute pressures (HIS interpretation of suction) does not exist in the universe
.

Zero pressure is the absolute void. You can never encounter sub zero pressures. Therefor suction is nothing more than a common name given to all pressures lower than a reference pressure (often the ambient pressure). His discovery is therefor a discovery of his own misinterpretation rather than a new insight into the way lift is produced.

One other article attacks bernouilli. This is unfounded. Although I share his view that Bernouilli is not the he sole explaination of the suction on the leeward side I do refer to Bethwaite for the paramount importance of bernouilli is the operation of efficient wings shapes. Bernouilli is very important in explaining the low drag characteristic of shaped wingsections. It is the reason why proper leading edges are so important.

With regard to upwash and downwash ; yes that is the right describtion from a stationairy reference point while a wing passed. It explains where the energy of the engines of a plane is partly dispensated too. We sailors allready knew of these downwashes as we call it dirty air. Ever found that you can't point as high when overtaking another boat to luff in close proximity. It is the downwash or rather windwardwash that gives you the appearance of getting a header.

You can compare this view to the energetic view of the car. In an analogue way.

HOWEVER, this "new" view (which is not new at all as it is taught in universities for some time now) does not say that their is no suction on the lee side and a overpressure on the luff side just as the "old" theory. In fact it does arrive at the same pressure situation as the "old" theory, it therefor does not conflict or contradict with the old theory. SO it is not one or the other but rather both simultaniously and intimately linked and it is dependent on your reference point (stationairy or moving with the wing) which one is dominant.

With regard to Bethwaite, He has written a book that is intended to be understandable to less educated readers. He something makes concessions for this reason. HOWEVER his describtions are correct in basis and he has been lightyears ahead of all most of us. He was one of the sailors who rediscovered that low speed wingshapes behave differently than high speed wingsections and that was a major happening. All his describtions are made from a "moving with the wing reference point". Here the individual particle theory has no merit even though it is there.

DHO, Pressure, density, volume are linked. A rise in pressure is accompanied by a rise in density or volume for a given quantity of fluidum. They are all part of the same set of equations and will never contradict eachother. With regard to your equation, they way it must be applies and the magnitude in which it helps you is very dependent on how you draw your system boundery (enclosed volume)

Of course Bernouillis law is close linked to these pressure, density, volume laws/equations. Also in the energic sense. Meaning, without out side force (a piston) a fluidum can only accellerate when it's pressure is lowered. Energy stored in the presssure is transformed into speed and visa versa. Therefor a speeding up of the airflow is ALWAYS accompanied by a drop in pressure. The articles are right in the respect that not all of the suction is caused by a accelerated flow. Part of it is also caused by angling to flow into a more downward/windward direction.

To my the partical stationairy viewpoint theory is nothing more than a higher abstracted different view than the moving with wing reference viewpoint. Both describe the same principle and explain the same pressure differences.

I hope this makes sense.

Wouter

Wouter: You might have interrest in C.A. Marchaj's explanation of this theme in "Sail Performance: Theory and Practice".

Maybe I should also have written that all short describtions don't do justice to the fully proces.

Indeed, it was a general describtion as many smaller swirls, eddys and backflows are seen and it is a rather dynamic proces with seperating and reattaching flows. And this is something made even more complex in some setups by the introduction of roll-over vortices and what not. Than modern gliderplane something use compressed air to scrape of a stagnant boundery layer.

The end result if that any discussion that doesn't include the better part of the day falls short of completely describing what happens and thus a discrepency between theory and practice can be found now and then.

Best example was of course the fact that theoretically a bee could not fly while it actually did. That is before it was realized that the wings worked in a different due to their microsize (different behaviour of molecules whch didn't reduce in size of course) and the fact that the wings created strong momentary swirls above the wings.

I know of Marchaj's publications, I must admit however that I never fully read through them.

Nils I'm told that another great source is NASA scientist mr. Arvil (?) who determined (in the 60's, and Bethwaite rediscovered this later)
That the venture (or slot) effect didn't exist. The conclusion was that it is not the jib that makes the main more powerful but rather that the main makes the jib more powerfull. End result remains the same however and that is the fact that a jib makes a rig more powerful than what its size would simply suggest.

I forgot to include this in my earlier post.

It seems that the right position of both relative to eachother is the key to performance and not as much the size or shape of the slot.

Like I said earlier we can talk about this all day and still not touch on every detail.

For now the main point I wanted to get across is most developments seems to refine our understanding of what happens rather than alter it. The predicted results using either methode seem to be the same and therefor which theory your comfortable with is not that important in itself.

Regards,

Wouter

Colin,

If you sailed from Brighton I may know you, but I am not very good at putting names to faces, when were you here, what did you sail and who did you sail with.

Brighton Sailing club is still going strong and still operating from the arches, we have had a few problems (see http://www.catsailor.com/forums/sho...w=&sb=5&o=&fpart=2&vc=1) but it all seems quiet at the moment.

I will take you up on the offer of a beer and sail next time I'm down that way. And if you are ever back in Brighton let me know and I will extend the same courtesy.

Cheers

Gareth

Hi Wouter,

> Nils I'm told that another great source is NASA
> scientist mr. Arvil (?) who determined (in the 60's,
> and Bethwaite rediscovered this later)
> That the venture (or slot) effect didn't exist.

Arvel Gentry

http://www.arvelgentry.com

Regards,
Colin
www.sailwave.com

Hi Pete,

I think what the authors are referring to as myth, is not that the jib and main don't interact in a positive way to generate power, rather that the popular explanation of how that interaction happens is not a very good model for what actually happens at a physical level.

Quote: "For instance, many sailors believe that the flow of air speeds up between the main and the jib, creating lift and generating power. This causes them to yell at crew for 'standing in the slot'. In reality, the 'slot effect' is a myth; the two circulation fields cancel each other out. This is instantly obvious in a computer simulation. No sailor who sees this simulation once (and who believes it) will worry about blocking the flow through the slot again."

From http://world.std.com/~fhapgood/texts/sails.htm

There again it's classic for authors of equally successful models to claim that theirs is closer to what actually happens physically...

Regards,
Colin
www.sailwave.com

This is a fascinating thread. Thank you for that last link (fhapgood), because it says what my father (the best sailor I have ever known) always said, that the boat is not sucked forward; it is PUSHED forward.

One great source of understanding airflow beyond reference materials and books as noted is the development of C Class cat design and wings for them . One piece -two piece and 3 piece wing designs ,-zap flaps etc ,bendable twist type rigs -really interesting stuff.
check C class history and design articles.
Applied design and engineering theory and development of wing sails and slot effect.-with a healthy dose of inspiration and instictive insight added ,-a much overlooked aspect to our basic understanding .




Carl

[img]http://us.f1.yahoofs.com/users/a093c6f1/bc/suckblow.jpg?bc5ZUK_AX.a3vVUU[/img]

dang it...

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