Wing Sails

Hi there, I'm wondering if anyone can explain to me briefly the advantages and disadvantages of using a rigid wing sail. I'm wondering if they offer greater efficiency, and also, how they are furled (or not) in bad weather. Thanks.

 

Hi VB,

I'm not an expert on these, but I think you've got the main points already

Rigid wing sails are (if correctly designed) more efficient than soft sails, as long as the conditions match those they were designed for. To achieve an advantage requires a lot of care at the design stage, and preferably, plenty of cash for high tech materials.
Wing sails that perform well in a variety of wind speeds and apparent wind angles may end up having a lot of moving parts, with two or three adjustable slots in the wing.

And the big down side, as you alluded to, is that no one (that I know of) has yet come up with a convenient way to reef or furl them.
Additional problems that are particularly telling for offshore work include increased fixed windage and increased fixed weight aloft.

There are a few reefing/furling options that have been built, but these tend to be complex and/or heavy:
It is possible to reef some of the more complex wing sails without reducing area by adjusting the trim of slots in the sail.
Alternatively several wings can be installed, and then progressively lowered to the deck as wind increases.
Another option is to pivot the wing laterally around its center, and then to reef (or furl) you progressively lower the wing closer to horizontal, thus the sail is "furled" when the wing looks like a aircraft wing, and produces only vertical lift.

As a consequence of these factors their use at the moment is almost exclusively confined to day racing at the cutting edge of performance, such as the C class cats of the little americas cup, and to experimental designers who value the elegance and robustness of the concept more than the sailing.
At the risk of over-generalising:
In all other conditions soft sails are a better option as they can be lighter, cheaper, easier to access, change and repair, are easier to remove in extreme weather, and can be more easily trimmed to changing weather conditions.

 

Your points about rigid wing sails are correct. When higher efficiency is discussed it usually means the lift/drag ratio (l/d) which is considerably higher in a rigid wing. However this applies when the wing is operated at a low angle of attack or alpha, whereas sails typically operate at higher alphas.

L/d is crucial in an aircraft where the “wind” is coming from virtually dead ahead. L/d is not always of much significance in a typical sailboat which operates over a much wider range of aerodynamic conditions.

Nonetheless, a rigid wing can score big time over a sail when sailing close to the wind, which is why they are often used in very fast craft which, due to their speed, create an apparent wind that is shifted well toward the bow.

There are a few other factors. Although a rigid wing cannot be reefed or furled it can be designed to feather quietly, unlike a sail which will thrash around violently if let fly in a breeze. There are also several designs of “soft wing” that can be reefed and furled; they are not as efficient as a rigid wing since the profile cannot be as precisely controlled, so less critical, lower efficiency profiles are used.

 

All of the rigid wings I see are in the shape of aircraft wings, that is, a very low width and extreme length. Could a wing sail more similar to a gaff rig (triangular) be made? Perhaps one of this shape could be designed to collapse more easily, accordion style? (My gf adds, exactly like a large collapsible fan)

 

No reason why not, but I don't see why it would be easier to reef or furl. I assume you are speaking of a fabric sail here not a rigid sail.

However, with the caveat that I am not an aeronautical engineer, as I understand it broad low aspect ratio wings have higher drag and lower efficiency than high aspect ratio wings. Gliders where efficiency is crucial have extremely long and narrow wings, as do competition land sailers and ice yachts. For the same reason performance sailboats have tall narrow rigs. The main reason for having a wingsail in the first place is to obtain higher efficiency.

 

Very good points, thanks. My main interest in wing sails is the idea that they might be easier to control mechanically without having direct access to the sails in a closed cabin, vs being out on the deck. I was thinking along the lines of a traditional Chinese junk sail in a triangular shape, something closely resembling a fan in shape and function, but slightly curved like a wing. I figure a triangular shape could collapse down flat more easily than a longer wing shape.

 

Just the contrary is true.
Hard to control, narrow operating range, dangerous as hell in a storm...

There have been experiments - here's one:
http://www.yachtworld.com/boats/200...3-Trimaran-1713367/Southampton/United-Kingdom

 

Post # 6 in this thread describes an attempt to put a “deltaplane” wing on a Tornado and other wingsail experiments.

http://www.boatdesign.net/forums/sailboats/highly-assymetrical-wing-sail-25362.html

PortTacker just to clarify, are you referring to wingsails in general or venomous's idea of a fansail? Fansails were quite common about 100 years ago for sailing canoes.

The boat in the link you give looks to have a wingsail with a control foil although the control foil looks rather large, or it maybe a wingsail ketch. The mainwing looks to have an adjustable profile or slotwing; reminds me of the kinf of thing used on racing land sailers.

 

Rigid wing rigs have these advantages over soft sails:
- The section shape and twist match the design shape
- Slotted flaps can develop a higher maximum lift coefficient
- Larger cross sectional moment of inertia for greater stiffness and strength
- Less drag at low angles of attack due to avoiding windward side separation

Rigid wing rigs have these disadvantages over soft sails:
- Inability to reduce area as the wind increases
- Heavier (generally)
- Difficult to transport and handle
- Must be "flown" 100% of the time, making mooring problematic
- More expensive (many more parts, labor to construct)

The higher maximum lift coefficient of slotted flaps has been a key factor in their success in C class catamarans, with their sail area limit that precludes the use of spinnakers off the wind.

AFAIK, the most widespread use of rigid wing rigs has been in landsailing. They have dominated the C class cats, but there are comparatively few of those compared to the larger number and greater variety of landyacht wings. The fastest of the rigid wing landyachts run neck-and-neck with the fastest of the wingmast/soft-sail landyachts, so one can't generalize the superiority of one over the other.

In bad weather, rigid wings are taken down and stored indoors. Landyachts at a regatta are typically "moored" by using dollies under the rear wheels that have their axles aimed at the front wheel. This allows the yacht to pivot freely about the front wheel, like a boat on a mooring, but the toed-out dolly wheels resist backward motion. It's not uncommon to see tracks in the morning that indicate the yacht has swung through 360 degrees during the night.

In principle, a rigid wing yacht could live on a conventional mooring. However, when a gust or wind shift hits the wing, it will develop lift and drive, causing it to sail forward and possibly capsize. Aerodynamic control of the wing using a tail to make it aerodynamically balanced allows the wing to react much faster than the hull can move, which will alleviate gust loads. However, the wing must be able to swing through 360 deg (and for an unlimited number of rotations) if the hull is to remained fixed. Aerodynamic control has been successfully used offshore in the form of the Walker Wingsail, although I don't know how extensively they have sailed offshore.

The experience of sailing a rigid wing has some big differences from sailing soft sails. A wing doesn't "talk" to you the way a soft sail does because it doesn't luff. Instead, it backwinds, which can slow you down quickly or even pitchpole backwards bow over stern. When tacking, if the wing is not rotated quickly enough to follow the hull, the angle of attack will become negative and the wing will backwind as the hull turns. If the wing is rotated too quickly during the tack, the wing will backwind. A wing that has positive control (manually rotated instead of being free to turn) takes some practice to tack properly. An aerodynamically stable wing that is free to turn is easier to tack because it can be allowed to feather itself, like a luffing sail. It can also be sheeted conventionally, pulling in on the wing and allowing aerodynamic moments to rotate it outboard.

Especially for high-speed craft like landyachts, flutter is a definite possibility. This is a self-sustained oscillation due to coupling between rotation of the wing about its pivot axis and the side-to-side translation as the wing rotates laterally about its mast step due to flexibility or slack in the rigging. Positive control is sometimes used to minimize flutter, but moving the center of gravity forward, preferably on or ahead of the pivot axis, is a more effective remedy.

The relevant telltales are on the lee side, where they are not visible through an opaque rigid wing. Phil Rothrock solves this problem by having a wind vane forward on the body of his landyacht, with the arms that form the references slaved mechanically to the wing so they rotate with the wing. This allows him to gauge the wing angle of attack as he's looking forward to see where he's going. Another form of telltale that would work with a rigid wing is the close-mounted wind vane at the leading edge, like is used by many wingmast sailors to locate the stagnation point.

Going back to your original question about efficiency, If a softsail rig is given the same height as a comparable rigid wing rig and designed to avoid separated zones, then there's no fundamental reason why it would be less efficient. Those are two big "ifs" though. The flip side is also true. If you design a rigid wing to have a low aspect ratio, it may not give you any performance edge over a soft sail.

The ability to adapt the soft rig to the sailing conditions can give it the edge over the rigid wing rig - adaptability through wing twist was the key to Cogito's success. The ability to add sail area or to reduce it through reefing makes the soft rig the better choice for most application.

 

This wing I designed and built myself. It's 1,5 meters, on a 1 meter x 1 meter landyacht.
http://www.box.net/shared/sgtbx438nl

The shroud attachment bracket forces twist of the upper sail, the top of the wing is a single piece, the rest of the foil forms articulate and also are free to pivot on the central internal spar. At the bottom there's an arm which, because the sheet attaches to it, forces the wing to articulate, but also has stops to limit the articulation. The entire thing weighs ounces, it's actually lighter than the conventional main and jib with booms on a graphite mast. It's flipped many times, no damage, it's stronger than I thought it would be!

It worked pretty well, the thing was wicked fast. Not much good in lower wind speeds. Once moving - accellerates quickly! But it often capsizes in gusts so quickly I cannot react, even though I using a very fast servo and a single arm for fast action. It generates enormous power compared to a comparable sized soft sail, first outing it broke wire shrouds, and it was easy for it slide the rear wheels sideways if you aren't careful. I raked it "less aft" and it quit doing that.

But if the gang gets back into landsailors again (as opposed to sailboats - a few of us built landsailors just for fun) I'll be building a new rig. I think a wingmast with a narrow soft sail behind it might give me more versatility (read: control) and good speed.
BTW, I never got up the nerve to try it on the trimaran! (also 1m x 1m)

 

I took a close look at a repro WW1 aircraft some time ago; the wing construction was fabric over ribs with a spar and a leading edge molding as is common for models, but the trailing edge was a cable under tension, which explained to me why so many early planes had scalloped TEs. I wonder if this construction could be adapted to a wingsail that could be furled.

 

Check out http://bigcatcatamarans.com to see my version of this, and to see relevant links.

 

soft wing sail

I've been fascinated with the Idea of a clean airfoil as a soft wingsail for several decades. I just reworked it for the umpteenth time. The hoisting and furling is a little involved, but doable. I suppose the next step is a good sized working model.
The whole thing of a workable full sized sail seems as much art as enginering, even today. I appreciate that.

 

not advocating rigged or freestanding, wingsails, the ^ frame or any peticular sail
i am fascinated by wings and sails and so made a little study on them
was crazy enough to ask an inflatable boat exhibitioner at a boatshow if it would be possible
changing the form to an inflatable wingsail, than noticed alinghi expirimenting with inflatable battens
looked at the crab claw and delta wing, now imagining a > mast, maybe its something
havent done (m)any testwork but am looking into all aspects knowing simple is best
much is possible and name Ilan Gonen as one who did make a reefable wingsail
in my gallery some sketches, links, pdf's

 

Self Trimming Wingsails are very user friendly and need not be expensive or heavy

I have been experimenting and testing self-trimming wingsails for a number of years. Some of the foregoing comments are misguided and just conjecture without actual experience.

I have collected together details of my research on my website which is http://www.sailwings.net pleased to see any comments you may have.