How do sails work by Paul Bogataj

[lunatic]

Good text on sail dynamics,quite readable even for the aerodynamically challanged at http://www.pultneyvilleyachtclub.org...ils%20work.pdf

[rwatson]

Had a quick look, and it raised some other questions for me, so I did some more research.

I noticed in a couple of other sites, a correction of an error commonly disseminated by school textbooks, that I had taken as fact


The diagram on the left hand side, illustrating airflow, is shown to be innacurate. More detail can be found at sites like
http://www.eskimo.com/~billb/wing/airgif2.html

My original puzzlment was the diagram on the first page of -

http://www.pultneyvilleyachtclub.org...ils%20work.pdf



It took a while to realise that the words "low pressure" didn't mean the whole area where the arrows were. Obviously you cant have low pressure in front of the leading edge.

They should have had another indicator to show where the low pressure occurs. Did this mislead anyone else, or is it just me ?

[TeddyDiver]

Originally Posted by rwatson

Obviously you cant have low pressure in front of the leading edge.

Yes you can.. Got to go Mach 1 before the effects of an airfoil (or any object moving throw fluid) are not "ahead" of you...

[rwatson]

Have you got some discussions I can refer too?

I didnt think it is possible to generate low pressure in front of the leading wing, simply because the air is in compression till it is forced over the top.

I am not talking about the "effect" either (where you have a vector of lift as shown by the diagram under discussion) I am talking about an actual low pressure area as measured by a barometer for example.

eg http://library.thinkquest.org/C01116...is_created.htm

This diagram is a lot more informative I think. The 'red' area is low pressure, and it doesnt occur forard of the leading edge.

[Zilver]

Originally Posted by rwatson

This diagram is a lot more informative I think. The 'red' area is low pressure, and it doesnt occur forard of the leading edge.

But in this example the resultant force is angled (too much ?) backward. A sailboat could never sail upwind/close hauled if this picture was correct.

Hans

[rwatson]

The lift is *always* at right angles to the direction of travel of an aerofoil. This lift vector has to combine with the resistance of the keel or centreboard to allow tacking to windward.

There is no 'forward lift', although the force vectors combine to produce forward thrust. Basically, without a centreboard or keel, it would be impossible to tack purely on lift from the curve of the sails.

That is why when reaching, the mainsail is let out close to right angles to the hull - then the 'lift' is then more to the direction of travel.
Obviously there is also considerable 'thrust' generated on the other side of the sail from the sheer pressure of the wind, which is still a large component of the total thrust.

I remember a very lengthy and terse series of letters in boating magazine along these very lines, where a qualified engineer had to get the concept over to a few readers. But if you think carefully about it, a wing travelling at speed can be deflected at right angles to its travel, but it could never create its own low pressure area ahead of its direction of travel. As the diagram shows, the air directly ahead is in compression, not a partial vacuum that would be required to create 'forward lift'

[TeddyDiver]

Originally Posted by rwatson

Have you got some discussions I can refer too?

Allmost any book related to aerodynamics is better for basic understanding than discussions in the internet..

[lunatic]

Both diagrams seem exaggated but I assume air in upwash has already increased speed with decreased pressure long before it, if it ever does, hits the leading edge.

[rwatson]

Originally Posted by lunatic

Both diagrams seem exaggated but I assume air in upwash has already increased speed with decreased pressure long before it, if it ever does, hits the leading edge.

It seems very unlikely to me. I have never seen any indication in anything I have read, nor any diagram, that anything but compressed air occupies the space immediately before the leading edge.

Even if there were upwash, that would create compression as it hits the solid wing, not low pressure.

[tspeer]

Originally Posted by rwatson

It seems very unlikely to me. I have never seen any indication in anything I have read, nor any diagram, that anything but compressed air occupies the space immediately before the leading edge....

Actually, the air is not compressed to any significant degree at sailing speeds. The pressure ahead of the leading edge is higher because the air is traveling slower. It has exchanged kinetic energy (speed) for potential energy (pressure). At the stagnation point, all the kinetic energy has been stored as potential energy, and the local pressure equals the total pressure.

Total pressure is a (simplified) measure of the energy in the flow. It consists of the local pressure plus 1/2 density * speed^2. Outside of regions where energy is going into spinning and heating the air (such as the boundary layer and its wake), the total pressure is constant.

[markdrela]

Originally Posted by rwatson

This diagram is a lot more informative I think. The 'red' area is low pressure, and it doesnt occur forard of the leading edge.

If the red and blue colors in that diagram are supposed to indicate pressure, then that diagram is completely wrong. A subsonic airfoil flow does not have such a massive fore/aft pressure assymetry.

Bogotaj's figure is correct, but only indicates the surface pressure as surmised. Where the arrows point out, the surface pressure is below ambient. Where the arrows point in, the surface pressure is above ambient.

[clmanges]

Here's another very good resource that explains airfoil behavior:

http://www.av8n.com/how/htm/airfoils.html

[Guest625101138]

Originally Posted by rwatson

It seems very unlikely to me. I have never seen any indication in anything I have read, nor any diagram, that anything but compressed air occupies the space immediately before the leading edge.

Even if there were upwash, that would create compression as it hits the solid wing, not low pressure.

You should take a look at JavaFoil. It is quite easy to use and will give reliable and easily comprehended results very quickly.

I have attached an image showing pressure profile over a foil produced in a few minutes using the JavaFoil applet.

Rick W.

[lunatic]

Would a drooping D section leading edge develope low pressure sooner on more forward facing surface and if so, any examples in sailboats.

[rwatson]

Originally Posted by clmanges

Here's another very good resource that explains airfoil behavior:

http://www.av8n.com/how/htm/airfoils.html

Yes, I came across that one as well in my research, but I didnt read down as far to this diagram previously.

The most interesting diagram is this one to me


Its info is very similar to the one Rick did, and shows (S) suction appearing above and in front of the leading edge, but not very much of it.
It appears that the real work is done closer to the wing and near the top of the curve, which is what I had thought.

It makes sense when I think about it some more, if there is a lot of low pressure above the wing, it would have to be starting a drop in pressure a fair bit in front of the wing as you couldnt expect a suddent pressure gradient change.

Mind you, it also shows suction under the wing (very much less and about the same as in front of the leading edge by the look of it). I guess this would vary with angle of attack etc as well.

I very enlightning exercise all round.