Wing sail for an A Class Cat

We found that a longish telltale attached ahead of the trailing edge, but long enough to extend past the trailing edge, could be seen to lift as the trailing edge separation began, and then would flip forward out of sight as the stall progressed. It was a good way to judge max lift.

The same thing works with a soft sail, too. However, it's best to put the telltales half way in between the battens. Tying them to the batten ends is pretty useless in my experience. But attached several inches ahead of the leech works like a charm. If they don't get hung up on the stiching.

Most landsailors have gone with just a single short vane mounted on the leading edge centerline, with the vane as close to the LE as possible. It flicks from one side to the other according to the stagnation point, as you point out.

Phil Rothrock employs a really clever wind vane. It's mounted on a small mast a foot or so above the body of the yacht, way forward. A "V" shaped wire with the ends bent up provides a reference like a conventional windex setup. The clever bit is the wire V rotates about the same axis as the vane, slaved to the wing rotation by a pair of cables running inside the body. So the wire ends indicate the proper angle of attack of the wing when lined up with the wind vane.

The vane's location also puts it right in the normal field of view, so the pilot doesn't even have to look away from the direction of travel to monitor the wing trim. A nice feature when you're crossing tacks and avoiding obstacles at 80 mph.

 

Windex Location

Those were interesting comments about windex and telltale locations.

Putting the windex at the masthead is not particularly practical. It is hard to see and is not in the field of view. I have seen catamarans and dinghies with the windex mounted at deck level on the bow. The idea of standing the windex off the leading edge of the mast seems practical. It would need to be at or above the forestay.

How far forward of the mast would be the most practical? I have a rotating mast (but not a wing section). The windex would always be on the windward side of the boat where it would be most visible. Do you have any additional experience with this mounting? Any thoughts?

 

um... clear plastic model aircraft covering film anyone? may be tough enough for the job, depends where you sail it.

Tim B.

 

Bear in mind that the wind indicators at the leading edge have nothing to do with indicating the apparent wind direction. The leading edge indicators are in an area that is dominated by the flow about the surface, so they have more in common with telltales than they do with masthead vanes.

The difference is, the leading edge vanes are intended to track the movement of the stagnation point, as Steve Clark points out. The stagnation point is a good indicator of the lift on the wing section. The same lift could be obtained by changing the angle of attack or by deflecting the flap. Increasing either will move the stagnation point toward the windward side.

Phil Rothrocks wind indicator was intended to show the direction of the apparent wind and the angle of attack of the wing, so his was similar in function to a masthead indicator.

How much error is acceptable? These figures show the flowfield around an airfoil at different angles of attack:http://www.diam.unige.it/~irro/profilo1_e.html. A wind vane that read the true apparent wind angle would be horizontal in all these figures. The angle of the streamlines from horizontal indicates the position error at that location. Roughly speaking, the error will be inversely proportional to the distance to the quarter chord of the rig.

I've never tried mounting a vane well ahead of the leading edge, but many people sail by telltales on the shrouds, which have a similar problem.

 

It is so easy to confuse me.

Could a windex at the front of the mast above the forestay be used to improve performance? The wind direction 2/3 of the way up the mast is likely more "typical" than the wind direction 400mm above the mast. And even if the direction shown by the windex at the proposed location was not True Apparent Wind, could it be easily interpreted to adjust sail set? Would it's indication be intuitive? Would the information be easy to apply?

Masthead instruments seem to be vulverable, especially in the Vendee Globe (and at the boat ramp).

 

The problem is the airflow near the mast is highly deflected by the sail rig itself. And the amount of deflection depends on the sail trim. The reason one puts a windex at the top of a mast (and ahead if possible) is to get it as far from this deflected air as possible.

Try it yourself - tape a wire to the mast with a piece of yarn attached to it. Sail in a straight line and see the difference between luffing and trimming in hard.

 

Rigid Wing Sails

I was looking for some material on the web and ran across this interesting experiment with a thick, double surface airfoil. Brian
_________________________________________________
RIGID WING SAILS

by John Holtrop

First published in EPOXYWORKS, Spring, 1993



The high aerodynamic efficiency of a rigid wing sail is a well known fact. Rigid wing sails have been used to defend the AMERICA'S CUP and are seen on many other high performance catamaran designs. Aerodynamically, a rigid wing sail is identical to an airplane wing. Both cases reward increased lift and penalize increased drag. Airplane wings are simple to design, however, since they fly with a fairly constant angle of attack. This means that an airplane wing's shape, or camber, can be optimized for a normal set of flight conditions.

Sailboats, on the other hand, must operate under many different conditions. For example, tacking a sailboat forces the airfoil shape of the sail to completely reverse itself, and wind velocity changes require that the sail's camber and surface area be adjustable. In the past, rigid wing sails for sailboats used hinged flaps and complex control mechanisms to achieve this flexibility. These designs were heavy, complicated, expensive, and impossible to reef or stow. Obviously, cloth sails are very practical for normal sailboats. Cloth sails are simple, cheap, and can be stretched into many different shapes with simple control lines. For most sailboats, the increased efficiency of a rigid wing is not worth the bother.

The extra efficiency (increased lift and reduced drag) of a rigid wing sail is most desirable for high speed applications like wind surfers, ice boats, or sand sailors. The biggest problem is how to design a rigid wing sail with minimal weight and complexity penalties. Modern composite materials offer the chance to combine the efficiency of a rigid wing with the simplicity of a cloth sail.

Most people think of composite materials as thick, rigid structural components, like a fiberglass boat hull. In fact, thin composite parts will bend easily without cracking, and are many times stronger, and more stiff, than the best sail cloth. Thicker sections (solid or cored) can be added at specific locations where stiffness is critical. This allows one part of a composite structure to be stiff and rigid, while other areas are flexible. Composite materials can be molded into complex airfoil shapes, and special fibers and core materials (graphite, kevlar, honeycomb, airex, etc.) added to optimize strength, stiffness, and weight.

My wing sail concept combines a thick, rigid airfoil nose, with a flexible center body and thin trailing edge. The wing sail is molded from composite materials in the form of a symmetrical airfoil. If the wing sail is rotated into the wind slightly, aerodynamic forces are developed, which buckle the windward side of the flexible center section "in" and pull the lee side "out". These flexible areas then deform smoothly into a shape resembling a conventional cambered rigid airfoil. The amount of camber is controlled by a combination of out haul tension and the physical stiffness of the materials, just like a normal sail. When tacking, the flexible surfaces snap over to the other side, and the camber shapes is reversed (like cloth sails). The rigid leading edge is stiff enough to handle all bending loads, which eliminates the need for a traditional mast and shrouds.

While the basic concept is simple, the exact shape that the deformed wing sail will take and its efficiency, is difficult to predict without building a prototype. Some excellent research has been done on flexible airfoils, both single and double sided, with a variety of leading edges. Much of the airfoil theory in the following design was gleaned from an article written by Mark D. Maughmer (currently with the Department of Aeronautical and Astronautical Engineering, University of Illinois, Urvana) titled A COMPARISON OF THE AERODYNAMIC CHARACTERISTICS OF EIGHT SAILWING AIRFOIL SECTIONS (# N79-2389, 1972) Mark's paper predicts at least a 50% lift/drag improvement for a thick, double surface airfoil, over a conventional sail/mast combination.

Theories are important, but the real proof is how well a full size operating prototype performs. I selected a wind surfer for the prototype, in order to reduce cost, weight and labor. Three years, and many hours later, the theory was put to test. The basic dimensions I selected were based on a standard 60-70 sq. ft. wind surfer sail. Since the rigid sail was more efficient, I designed it 1/3 smaller, for a total of 40 sq. ft.:

length = 152 in.

max chord (width) = 52 in.

head = 26 in.

foot = 30 in.

maximum thickness = 6 in.

The thickness of an airfoil is usually expressed as a percentage of the chord. For low speed efficiency, 12 - 18 percent is typical. I selected the lower value, 12%, to keep size and weight down. The leading edge shape was taken from a NACA #63 cross section, which has a fairly sharp elliptical nose. I basically guessed at these values. They looked reasonable, but other NACA shapes and thickness may be more efficient. The wishbone was attached to a piece of tubing glassed into a socket molded behind the leading edge, and laced to the clew with a 4/1 out haul. A tapered "C section" spar was bonded inside the wing, from head to foot, at the 1/4 chord line. It terminated with a 12 in. tube, which contained a standard "BIC" gooseneck fitting. This spar, and the rigid leading edge, carry all the bending loads, and form a watertight chamber which provides several hundred pounds of flotation.

A plug was built up from wood strips, and used as a male mold for the rigid elliptical nose and tapered spar. Plywood sides were attached to the base of the plug to form a tear drop cross section. The plug was filled, sanded, sealed, and coated with a fiberglass release film. Two layers of 6 oz. fiberglass were laid over the entire plug, with several extra layers in the nose region. The cured part was pulled off, and foam strips glassed into the inside for increased stiffness. A "C" section main spar was molded to fit the plug base, fitted to the nose, and then glassed into its final position. Two layers of 1" wide, unidirectional graphite fiber were added to the spar caps to increase its bending stiffness.

The single surface trailing edge, was molded on a flat plywood form, from two layers of 7 oz. KEVLAR cloth. The unsupported trailing edge was a little too flexible, so three foam battens (1/4 x 1 x 18) were added to one side, about 24 inches apart. The openings at the head and foot of the double surface section were filled with foam plugs, rounded, and glassed watertight. The finished structure was stress tested by supporting it at both ends, and then standing on the center of the spar. My 190 lb. weight deflected the wing about 1/2 inch, causing some creaking, but it held. With a fresh coat of paint and fitted with a wishbone and gooseneck, the finished sail weighed 36 pounds. This is about 10-15 lb. heavier than a conventional rig.

Now for the moment of truth! Is this theory stuff really true? Three of us trucked my BIC and the wing sail over to Lake Isabella in Kern county. Known for good wind, Isabella is a very popular lake for wind surfing. When we arrived, the winds were medium, about 15 kts, with occasional gusts to 20. Everyone was using big sails. We tested the wing sail for the next four hours, and while we didn't blow anyone off the lake, the sail got a lot of attention, and I learned a great deal about the design.

The positive features we found were:

(1) The wing was very sensitive to changes in "angle of attack". It could be feathered easily, then generate lift with just the slightest pull on the wishbone. At speed, the wing seemed to have a narrow "sweet spot", and the rider could dump power very quickly. This made gusts easy to handle, and was predicted in Mark's article, where his data showed the L/D curve for a double sided sail to be quite narrow. The single sided conventional sail has a flatter L/D curve, and is not as sensitive to small changes in the angle of attack.

(2) The wing was much stiffer than a standard rig. There was no "give" during gusts or when pumping. Twist was not excessive, and the trailing edge (leech) was stable and did not flutter.

(3) The double surface section responded to air loads very nicely. The deformed shape looked like a "real" airfoil, and could be controlled with out haul tension. Due to the stiffness of the wing, the out haul had to be quite loose to develop the proper amount of camber for this wind condition. This made the wishbone feel sloppy, but did not cause control problems.

(4) Speed was only slightly less than other similar wind surfers. During gusts the boat accelerated well, and matched speed with the other boats. The general feeling was that our boat was performing very well for such a small sail and light wind. 25 knots, or a 60 sq. ft. sail would have been ideal.

(5) The rig floated high in the water and was easy to up haul. No water starts were attempted, but the wing lifted well, and beach starts were easy.

Negative features were:

(1) The sail area was to small for the wind conditions, and we had no easy way to increases it. With the advantage of hindsight, I would design the upper 3 feet to be removable. A larger than normal sail might be OK with a wing sail, since it luffs cleanly and very quickly. This would help under strong wind conditions.

(2) With the out haul loose for more power, the wishbone felt sloppy. A "slip joint" out haul would hold the wishbone firmly, and still allow for camber adjustment. A 1-1 purchase is adequate, since the wing sail is so rigid.

(3) When the wing was dropped, the rigid foot area impacted the side of the board. The fiberglass would not give, like cloth, so the foot of the sail ripped about 12 in. This let water into the center section, but the damage did not get any worse, or effect performance. The foot section needs to be heavier, and padded like most goosenecks.

(4) 36 lb. is a little too heavy. Using a foam core leading edge and thinner laminates could save 5 lb., maybe more. On a larger boat, the extra weight may be offset since halyards, shrouds, turn buckles, and chain plates are not needed.

(5) Visibility behind the wing is poor. Clear windows could be stitched or laced into the sail.



In conclusion, I think these tests show that the basic concept is sound, and practical for some applications. Thick, well shaped, double surface airfoils are more efficient then cloth sails. Composite materials can be designed to deform into a smooth aerodynamic shape under air loads, without complicated flaps, hinges, and control lines. The bending and torsional stiffness in the leading edge is very high, and a free standing rig is a definite possibility for larger sailboats. The design has a lot of potential for ice boats, sand sailors, and very high speed sail boats.

JohnsBoatStuff

 

the java applet at http://www.mh-aerotools.de/airfoils/javafoil.htm has multi element support

 

Wingmast tuning

The comments on using windvanes to tune/trim wingmasts are interesting. If you look at the A-class photos on the sailcenter.se website (and attached), there appears to be a vane on the leading edge of the mast about 4 ft / 1.3 m up from the foot. I assume this is to guide adjustment of the mast rotation

Question is: at what angle should the mast be? Is it always the case that the mast points directly into the apparent wind? Or are there higher lift, lower drag approaches?

 

I think this will explain what's going on when you rotate the mast. I used a large-chord mast for the example because it was easier to see what was going on and because the landsailors that were the target audience for the paper use masts about that size.

Landsailors use a similar windvane on their leading edges. Typically the vane is positioned so the tail will just barely clear the mast. What it indicates is where the stagnation point is on the leading edge. When the mast is under-rotated and the stagnation point is well to the windward side, the tail of the vane will be firmly over to the lee side. If the mast is over-rotated so that the stagnation point is positioned on the lee side of the leading edge, the tail of the vane will be bow to the windward side.

Many landsailors like to trim their masts so the vane is just flicking from side t side, indicating that the stagnation point is right near the leading edge.

When the mast is rotated so that the lee side forms a smooth contour, the flow can be attached all along the lee side. However, there will be a separation bubble behind the mast on the windward side. You can see the extent of this bubble by putting telltales in a row at the same height along the sail.

When you reduce the rotation from the smooth lee-side condition, you reduce te size of the windward separation bubble and reduce the drag it contributes. However, you start to form a new separation bubble that bridges the crease in the contour formed at the mast-sail junction. Especially if you are trying to depower a little bit, the drag from the two smaller separation bubbles can be less than the drag from the one big one on the windward side.

If you rotate to less than lined up with the local flow angle at the leading edge (which will be more lifted than the apparent wind, due to the upwash from the lift on the rig), you are probably not reducing the windward separation bubble very much and you're making the lee side bubble worse.

So the condition indicated by the mast-mounted vane is probably near the minimum good mast rotation angle. If you're looking for acceleration, you might rotate the mast a bit more than that. If you're looking to depower, then a bit less.

 

Thanks - I need to spend some time thinking this through, but it's a good place to start. I'll experiment with mast rotation a little more next time I'm on the race course.

Martin

 

Stagnation point indicator

A few months have passed since my original post, and I've been unable to find a commercial stagnation point sensor... so I made my own (see attached photo).

This is made from a Little Hawk wind vane, 10 minutes with a hacksaw and a little cyanoacrylate (Superglue). Total cost £10 ($18). This just tapes onto the front of the mast as high up as I can reach. The gap between the trailing edge of the vane and the mast is ~1mm.

It seems to be very sensitive; only a degree or so of mast rotation either side of center and the vane flips clearly to one side or the other and stays there. When the rotation is zero degrees, it oscillates from one side to the other.

I don't know yet if this has made me go faster, but at least I know how the mast is rotated relative to the apparent wind!

Martin

 

Martin, very interesting! Just a cautionary note: cyano is not very good in an environment where it gets wet as I found out the hard way early on in rc models...

 

Yep - that's it! Nice looking installation.

 

David,

did you go ahead with the A-class wing, or did you drop the project?

I am very interested in hearing results, and what you eventually did with the wing. Also, are there more information about Cogitos twist control available on-line?
Having just re-read some of Marchajs work, he was not too impressed by wing-sails performance in light winds. But it was an oldish book, where he tought the best look avenue for development was with split flaps. I suppose the world has moved on since he wrote that.


Rolf