Some Big Paradoxes..

...about aeroplane wings and sailing rigs...

..and about sails with different foil sections.

From time to time aeroplane aerodynamics is brought into the discussion about sailing rigs.
Aeroplane wings and sails work in the same way, kind of, but since their parameters sit in the opposite end of most scales, they are more dissimilar than similar.

Common basic rules.
Most wings and (close-hauled) sails make use of attached airflow at the upper (or lee) side of them to make them efficient. With that airflow attached to the sail, the wind on both sides will leave that sail parallel with its trailing edge. If the angle of attack is increased too much, the airflow on the leeside of the sail will separate itself from it (indicated by the collapsed telltales). On an aeroplane, that means that the wing will no longer be able to carry the aeroplane. In a sailboat’s sail, it only means that the boat will slow down a good deal.

Performance-wise the biggest difference between wings and sails sits in the lift/drag ratio (L/D). The L/D of and aeroplane’s wing can be 10, 20, 30, and up to 60:1 on the hottest sailplanes.

For most single-ply sails, the L/D will rarely exceed 4:1.
For this reason, it makes little sense to fret too much about niceties like leading edge shape and vortex at the tip of the sail. In particular, armchair discussions not backed with practical observation on board, have limited value except in producing interesting ideas that can inspire to practical testing.
Have a look at the diagram showing three (or 3½) airfoils on junkrigs.

Case 2 shows the two very different foil shapes in a cambered panel sail in the lower section of a Johanna style sail. On one tack, the curve is nice and even. On the other tack, with the mast on the leeside, the sail foil is badly distorted by that mast (plus a bit by the Hong Kong parrels). Even so, the sail performs well on both tacks, and it is easy to make the telltales at the leech fly. To me the use of telltales at the leech is essential to show if the airflow at the leeside of the sail is attached or not. If I over-sheet the sail, those telltales collapse behind the sail.

Without this practical experience, I would have been prone to saying that the sail could not work properly on port tack, but it does (..remember, with the L/D in the 3.5-4:1 area...). Test-sailing the first cambered panel sail in 1994 gave a happy surprise there.

Case 1. The flat sail.
This was how the first sail for Malena looked like (1990). Although it worked well enough downwind, it turned my Malena into a lame duck when close-hauled  -  and this despite her 32sqm sail which gave her a SA/disp. of over 25.

I have drawn in a collapsed telltale indicating a stall. Actually, I didn’t fly telltales on that sail, so I only guess here, based on the over-sheeted cambered panel sail.

The Case 3 sail has ‘air pockets’, as found on some Chinese vessels.
I have no experience with this, so can only speculate. It will be up to the observers and users of that traditional Chinese sail to tell how it works. Again, I wish they put telltales at the leech to let them see how the airstream leaves the sail there. If it proves to produce an attached airflow on the sail’s leeside, this could be an interesting way of making big junk-sails.

Camber induced by twist.
I haven’t tried this, but it surely is an interesting subject.

Arne

(Full size diagram in Arne's sketches, section 7-40)

Anonymous wrote:

The shape of the pocket in photo no. 2 (Bohai sail) is, in my opinion, acceptable as it does not disturb the air flow too much. However, I am intrigued by the pockets on the sail in photo no. 1 (Fujian style sail), because this is not the first photo in which I see pockets in a junk sail formed seemingly against the laws of aerodynamics (in my opinion - they disrupt the flow of the air stream). But after all, similar pockets are used in parachuting, so maybe it has a deeper purpose?

Anonymous wrote:

Anonymous wrote:

I was shocked by photo 6! But... if these ropes across the panels were adjustable, maybe sailing downwind would be easier? E.g. Karman vortex reduction? As mysterious as the rhomboid holes in the rudders...

I suspect those diagonal lines are there solely to provide structural reinforcement, countering the downward pull of the sheet. Their effect is to flatten the overall shape of the sail and I think it's unlikely that it intentionally serves any mysterious aerodynamic purpose. 

I wonder if cambered panels or "wind pockets" in Chinese sails simply occurred accidentally because of stretch in the sailcloth. That certainly looks likely in the 3 masted junk shown by Arne.

The diagonal lines both act as reinforcement of the fabric and intentional creation of air pockets. I saw them put in fishing lines inside the reinforcement ropes before sewing onto the fabric. When they sew the rope to the fabric they intentionally create small crease with the needle so that the fabric was squeezed along the length of the rope to create pockets.(see pic)

From the detail (see pic) they hung the sailcloth to the batten on the Bohai junk. I am sure that the pocket is intentional.

I was shocked by photo 6! But... if these ropes across the panels were adjustable, maybe sailing downwind would be easier? E.g. Karman vortex reduction? As mysterious as the rhomboid holes in the rudders...

Later, I am fortunate to see the making of a Fujian style sail, and real junk sails of the Bohai (see attached) and Yunan (see attached) area. My conclusion is that there are "wind pockets" on the junk sail. People from different places have their own ways to create the "draft" necessary under Bernoli Theory. The wind pockets create the lift needed, although they may not be the modern airfoil shaped. In Fujian they sew the quadrilateral shaped pockets (see attached) at the same time when putting in diagonal reinforcements into the sail. In other places they create cylindrical bulges.

The airfoil shape in modern Cambered Junk Sail is just another version of  the "wind pocket".