<<The leeward side of this sail is only wing shaped right at
the "bulkheads" and collapses ww in between, so the whole efficient
has to be way less. >>
The swing-wing, the way Fercot has modified it, uses curved
fore-and-aft wishbones meeting at a joint. This will give it a
great leeward side in all wind strengths, curving nicely from luff
to leech, even in very low wind, which is really when I'd personally
want the most lift.
The windward side does have the "notch", but this is similar to
the same feature with a wing mast, causing a bubble of turbulence
over which the wind will flow, effectively creating a wing out of a
single skin. In fact, the wing mast depends upon this bubble.
It's the same reason why pickup trucks get better mileage with
the tailgate up, which creates a bubble over which the real wind
flows, than with the tailgate down, which prevents the bubble, and
therefore creates more negative pressure on the truck's back end.
The benefit of the swing-wing would be similar to what Speer
suggests near the end of his paper -- battens that run into the
interior of the wing mast, providing for a smooth leeward side
without getting hyper-attentive to mast rotation.
You'll seldom see that under-rotated leeward notch, or the
over-rotated leeward corner, with the swing-wing. You'll still have
to adjust the sheets as the apparent wind changes, but that goes
with every rig. At least you don't have to adjust the sheets and
then go through the second step of adjusting mast rotation.
Also, while the swing-wing is a soft wing, and isn't a perfect
foil from head to foot, it's hard for me to imagine another sail
maintaining better shape in the intermediate sections. At least
with the swing-wing you've got great sections supporting the sail
above and below, without having to pull any strings for
fine-tuning. For a squaretop main, you only have the battens to
help with shape, and will have to rely on keen use of the mainsheet
and outhaul to get a good shape from head to foot.
Finally, the shape of the airfoil on a swing-wing will be more
dependent upon the wishbones than the sail cloth.
On a squaretop main, the sum of all the stresses at each batten
needs to get transferred through the clew. Once the sail stretches
out of shape, the efficiency of the whole rig changes.
On a swing-wing, the stress at each batten goes to its own sheet,
resulting in lower loads on the sail cloth. And even with those
low4er loads, the wishbones would still implement a good shape.
Heavy-duty soft dacron could give you many years of efficient
service, and you'll never have to worry about damage due to folding
it (as compared to mylar or cloth with a high resin impregnation),
or flogging (from a less-supported sail).
---
However, the wing mast would probably be better in two scenarios:
a) high wind, particularly on a beat, where you'd want to flatten
out the wing a bit, and b) whenever you have someone who's willing
to continually micro-adjust the angle of the wing relative to the
sail and the wind, as well as flip the battens over when
tacking/shunting in light wind. The optimal wing mast may indeed be
more efficient than the optimal swing-wing.
If you race, the wing mast might be the answer, and all this
discussion superfluous. I don't debate the efficiency of a good
sail on a wing mast when properly adjusted.
But to me, here's the real question: which rig is faster, safer,
and easier to handle if I don't have a skilled crew member dedicated
to mainsail handling and mast rotation, and/or I'm not willing to
dedicate my total focus to sail efficiency?
In that case, I'd have to say the swing-wing is the more efficient
rig.
Even if I were to race, I'd likely be single-handed, and would
still prefer to pay more attention to the wind and the course than
to my mast.
But again, I'm a lightweight, not a racer, and am focused on that
fastest cruiser I can get. That's not what everyone else wants.
I'll eventually get a flow modeling application to put numbers to
the swing-wing. Or just build miniature rigs and test them with
identical speeds and angles in a wind tunnel. But probably not in
time to actually be useful to this thread of discussion.
- Mike
Gardner Pomper wrote:
Thanks for the paper reference. That is the one that I have been looking at for wing masts, but it is not clear to me how relevant it is to the soft wing sail. The leeward side of this sail is only wing shaped right at the "bulkheads" and collapses ww in between, so the whole efficient has to be way less. From re-reading the article, it does seem that even JavaFoil won't help much in prediction, because it is based on a constant cross-section, since it only works in 2D. With the varying cross-section between the "bulkheads", there could be all sorts of complex behavior, and it might not all be good. I guess we are going to have to wait on more data from the few people who build them.
- Gardner
On Mon, May 30, 2011 at 6:01 AM, Micha Niskin <micha.niskin@gmail.com> wrote:
I believe this paper has the information you want:
This is the model used for David Tyler's swing-wing sails (although he uses a slightly different foil section, I can't imagine that the difference would be significant in these soft wing sails).
On Mon, May 30, 2011 at 12:10 AM, Gardner Pomper <gardner@networknow.org> wrote:
Mike,
I thought that I had seen you post a message that you were going to try out JavaFoil. Did I remember that right? It would seem that is what this discussion needs; some actual numbers comparing the lift of a softwing sail in this configuration to a "normal" sail. From looking at the photos, it would seem it would be worth approximating the shape as a solid wing, with a thickness of half the frame.
And I wish you would stop being so darn reasonable.. I had ruled this sail out a year ago and now you have me seriously considering it again! <grin>
- Gardner