Sail forces and sail plan ?

If you use tufts to make yourself sure that the flow over the sail (and particularly at the leading edge) is attached, then you are basicaly operating at the ideal angle of attack (as explained before) and the flow is well modelled by JavaFoil (for that angle only).

What I'm saying is that JavaFoil is not able to give you a correct Cl-Alpha and Cd-Alpha graph for various angles of attack different from that ideal AoA. The only correct value JavaFoil might give you happens when the sail works with fluid streamlines perfectly tangent to the sail at the leading edge. It is a limitation inherent to Thin Airfoil Thory methods.

An analysis for other AoA's is possible only with a general Navier-Stokes solver, which can arrive to a solution with the leading-edge separation vortex or bubble. No method based on Thin Airfoil Theory can perform that task, if not with gross approximations.

 

well, for those whom don't have Marchaj, attached are a few plots that may help, not in full fancy colour plots...just plain old back and white graphs!

 

daiquiri

Been years since i had to do nav-stokes equations,and by hand too!

If i also recall, the laws of vortices's is that
1) The strength of a vortex line is constant along its length
2) A vortex line must be closed and cannot end in the fluid

But in reality the vortex and lift are not constant over the span of the foil.

This relates to your post above...but I could be wrong, been years since I had to do this stuff in-depth.

 

I trim the sails to the course so I keep the tuffs flying. This was certainly common practice in any sailing boat I have ever been on.

As noted earlier, the only time I did see flow separation was when I was pinching to make a mark. At that stage the keel was also close to stall. Too high and the sail would luff and keel would stall. I lose drive as well as go sideways. The only time it was worth trying was to avoid something less than a 10m tack. Otherwise I sail a little freer to get the boat speed.

So how often would you sail with the sail on its lift limit???? I consider that the point you make is not meaningful to the circumstances of most sailing conditions.

Rick W

 

I have added a few more of the pages from JavaFoil for the sail combination I set up.

You will see I have set the aspect ratio to 4. This is quite a low aspect rig.

You can see that it is possible to get the maximum L/D from the data for the set conditions. The best L/D is almost 8 at 10 degrees while the best Cl is almost 1.7 at 16 degrees. The best sail trim for the boat would be somewhere between these for the sail shape used. This will depend on how the boat behaves under the forces. If the hull can handle the drive then bear away. If the hull is near hull speed then pinch up and gain in VMG.

Rick W

 

Rick W,

Is it possible to get a simple force from Xfoil? For example 500 N at the center of effort in the x direction?

regards

 

I prefer JavaFoil to Xfoil because the data entry and output is easier for me. There are tutorials around for Xfoil if you want to understand how to use it:
http://www.terrabreak.org/groundloop/xfoil.shtml

Rick W

 

By lift limit, do you mean with the leeside flow semi stalled, as indicated by the lee telltales dancing?

If so, it's common among many top sailors in many major classes. Reaching in a Laser Radial or a Tasar, for example, it's fastest to sail almost over-trimmed, with the lee telltales often agitated. Windsurfer ODs are basically always faster with the lee telltales lifting or stalled (that's why we no longer put them on). Some very good big-boat sailors have the lee telltales flicking around when they are powering up in light stuff.

And most classes sail upwind fastest in certain conditions with the windward telltales lifting. It works in J/24s, Tasars, Flying 15s, Etchells, Lasers etc. The loss of lift created by sailing slightly above the "ideal" angle of attack is less than the loss of lift created by reducing the angle of attack, and the gain in height is more than the gain in speed created by sailing with all windward telltales streaming.

And of course in a breeze in something like a J/24, Laser, Tasar or JOG racer, you're sometimes sailing with the front 1/3 or so of the jib inside out and the windward telltales going crazy, but the boat is still rockin' along fast and high.

 

No. In that case the sail is on the luffing limit.

Rick W

 

It is all wrong what you said, and this practice you call "common" might be common only among people who don't fully understand the relation between sail drive force, VMG and the lift force diagram.

C 249 has it perfectly right. If you want to optimize sail drive force for VMG, you need to adjust the sail in such a manner that it works at angles of attack which can be very large, depending on your course.

You are a very analytical person, so take Ad Hoc's diagram in the post#17 and think over it's meaning. You will certainly understand where is the error in your "common practice".

I'm talking about things which any competitive sailor knows very well.

 

You cannot use JavaFoil to obtain cl-alpha diagrams you have enclosed. You are using that software outside of boundaries where it is validated.
It is ok for a qualitative visualization of a potential flow around sail geometry, but the numerical results should not be relied on because the physical models used by JavaFoil are not intended for use with thin airfoils at anything but very small angles of attack.

 

That's the problem of "over-user friendly" software programs. Almost no one reads the manual properly, because it is so easy to just click and learn (so they think), and even less understand the limitations of the algorithms.

 

So provide something better.

Rick W

 

You are telling me that if you want to pinch up on a mark you will steer a course for maximum drive!!!! Have you ever raced a sailing boat???

Rick W

 

You do realise the foil I have shown is cambered???

I have shown 10% cambered foils. For the configuration of the foils I have used JavaFoil is telling me that the best operating range will be between 8 to 16 degrees to the apparent wind. In this range the Cl will go from 1.06 up to 1.69 at RE# 100,000. This is for a rig with aspect of 4.

As you have access to the very best knowledge base on this how about you provide more accurate numbers so we can all see how poor JavaFoil really is for this analysis.

I will add that I did not spend a lot of time trimming the sails relative to each other to see if I could get a better result. I just set the slot at what looked about right. Also I have not checked if the jib is backing at 8 degrees.

Rick W