RIGID WING SAILS
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.