Why so stable?

Correct. Achieving the absolute maximum lift was not as much of a consideration for this rig compared to the C-class because the area was unlimited. If more lift was needed, it could be achieved with less drag by adding sail area instead of pushing the wing to the maximum. Section maximum L/D was more important than achieving higher maximum lift.

By eliminating the C-class #2 element, we simplified the structure and saved weight. We also saved the time it would take to analyze and optimize a 3-element section. We had to freeze the outer mold line and get on with construction in order to deliver the wing on schedule. The two-element section gave us what we needed.

That said, the maximum lift coefficient was still predicted to be much higher than for the soft sail rig.

 

Unawing?

Is this implying that there is/should be a gap between the sail and wing mast of a unisail, so that the two can act like a wing and separate flap? If so, it seems like it might be possible to get significantly higher lift coefficients out of a soft sail. I can imagine you might need to design the mast car to put a virtual pivot some distance ahead of the trailing edge of the mast.

Something like?

 

Possibly. The problem is getting the right gap geometry. It's not just that there is a gap between the sail and mast, it has to be properly positioned relative to the mast.

The luff needs to be somewhat to windward of the mast trailing edge and, ideally, a little forward of the mast trailing edge. This is done in the rigid wings by putting the pivot about 10% of the mast chord forward of the trailing edge. As the flap is deflected, it moves to windward, forming the right gap geometry. On the C-class, the flap has a horn that contacts the #2 element trailing edge and deflects the #2 element as the flap is further deflected so the gap maintains a fixed width as the flap (#3 element) is deflected further.

A soft sail is probably going to bow to leeward in between the battens, and this will affect the slot geometry. In addition, the forward pivot location of the battens will interfere with being able to raise and lower the sail. These problems could be alleviated by using a rigid leading edge to the flap. This would look like a very narrow rigid-wing flap with luff groove in its trailing edge to accept the sail.

Twist control of the sail will mean sheeting loads will be taken along the leech, as with a conventional rig. These vertical loads will need to be reacted by the pivot links to the rigid leading edge of the sail. However, I believe the loads can predominately be taken at the head. There will be internal vertical loads taken by the rigid luff element to provide the luff tension of the sail. So a triangular/sloped link at the head should be able to transmit the sheet loads across the rigid luff element to the mast. It might be desirable to provide for vertical loads at the bottom end of the rigid luff element as well as at the head. All the links in between would be flexible or have clearance so that they do not take vertical loads, resulting in all of the vertical loads (up or down) on the rigid luff element being taken a the ends.

The mainsail halyard would probably still be routed inside the mast, but would need fairleads or blocks to take it to the rigid luff element and provide proper lateral support of the halyard as the luff element was deflected relative to the mast.