The biggest difference between the trim of a conventional main sail and the trim of the SRW is the region of the sail you are trimming to optimise. With a mainsail the power is developed from the leech of the sail, while with a wing, most of the power comes from the front of the sail. This is because on a conventional mainsail the flow is separated in the region behind the mast and flow is not re-established until some distance along the sail. Consequently the pressure in the forward region of the mainsail is not as low and hence is not generating as much lift. So when trimming a mainsail we need to concentrate on keeping the aft part of the sail working as this will create the most lift. With the SRW, the flow is not separated at the mast and hence much higher flows and lower pressures exist. Hence, we want concentrate on the flow near the front of the wing and can even accept separation over the aft 1/3 or more of the wing in the pursuit of lift.
It is quite fortunate that we are concentrating on trimming the front of the wing as, for the most part, it is difficult to see the leeward tufts from the normal sailing position. So how can we use the luff tufts to trim the wing?
Upwind in moderate breezes boat stability tends to be the limiting factor in the trim. So we are typically looking for less drag at the sacrifice of lift. In other words we are aiming to have a higher lift to drag ratio. We know from modelling and testing that the SRW creates maximum lift to drag at about a 6-7 degree angle of attack. At about 4 degrees angle of attack (i.e. sheeted out a little more) the tufts just behind the mast on the windward side begin to lift. This is a good indication that lift is dropping and drag is rising. So you are either too high or sheeted out too much. Bearing away a couple of degrees, or sheeting in a couple of degrees will move you back towards maximum lift to drag ratio.
On the K8, moving the traveller car from one bolt to the next is about 2.25 deg change in angle of attack. So if you trim out until the tufts just behind the mast on the windward side begin to lift, and then trim back in one bolt (100mm) on the traveller you will be pretty close to maximum lift to drag trim.
When sailing off the breeze we are less stability limited and are typically looking for more lift. As we sheet in more (or bear away more) the leech tufts will begin to flick to leeward. This is because the flow towards the leech of the wing is separating. A large amount of separation can be tolerated. Drag is increasing along with lift. Drag is not detrimental downwind so in this scenario (given we are not overpowered) we are looking for maximum lift, irrespective of the drag. So how do we determine when we are trimmed for maximum lift? Fortunately, again, the windward luff tufts are a good indicator. As we get towards maximum lift, these tufts will start to flick forward. This is because the stagnation point (the point where the wind can go either around the leeward side or the windward side) is moving aft as the angle of attack and the lift increase. At the point of maximum lift the stagnation point will have moved aft of the mast and the tufts will flick forward. By this point the leech tufts will be flicking to leeward constantly as they will be in separated flow.
Beyond maximum lift, the leeward tufts are the best indicator. As you bear away, more and more of the leeward side of the wing will become separated until ultimately the wing stalls (and acts like a normal downwind mainsail). Less cambered sections will stall sooner than more cambered sections but you will be able to sheet out 5 to 10+ degrees between maximum lift and stall.
Obviously this is somewhat dependant on the wing set up but it is a good starting guide. The diagram demonstrates the above concepts for one wing set up. However, nothing will replace time on the water.
Just like with a conventional main sail and jib combination (jib/main), the wing sail and the jib (jib/wing) work to complement one another. However the balance of benefit is a little different.
With a jib/main combination, the jib has an important job in helping to re-attach the separated flow behind the mast. To quote Arvel Gentry “The primary effect of the jib is to slow down the flow on the lee side of the mainsail, reduce the pressure gradients and, therefore, prevent separation on the mainsail.”
With a jib/wing combination, there is much less need to slow the air between the jib and the wing as the flow does not need to negotiate the separation region behind the mast. Closing the slot too much serves to reduce the velocity over both the jib and the wing and hence reduces the lift generated from both. Consequently, the jib is typically set to a winder angle (and with less overlap). When set correctly, the flow from the leech of the jib is exiting into the high velocity flow near the leading edge of the wing. This has a significant positive effect on the jib. The increased velocities towards the leech of the jib will help the flow stay attached longer.
In both cases, the aft sail creates an upwash effect, which effectively allows the jib to be sailed higher into the wind than if no aft sail was present. The increased efficiency of the wing and the effect that it has on the jib mean that this effect is amplified. So, even though the sheet angles are higher for the jib in the jib/wing combination, the combination of the jib flow exiting into the high velocity flow of the leading edge of the wing and the increase upwash of the wing make the jib a powerful ally to the wing sail.
 Arvel Gentry, A Review of Modern Sail Theory, Proceedings of the Eleventh AIAA Symposium on the Aero/Hydronautics of Sailing, September 12, 1981, Seattle,Washington
A common misconception with soft wing sails is that just having an aerofoil section is enough. A significant part of the benefit of a wing sail comes from eliminating the mast from the airstream. With a wing sail the pressure on the leeward side is lowest and air velocities are greatest around the leading edge. A lot of the power of a wing comes from this effect. If a mast is present it will have the effect of causing a large increase in pressure behind the mast where the flow is separated. This reduces the velocity of the air dramatically and it is velocity that you never get completely back. So the mast will reduce the effectiveness of the sail or wing that is behind it. Having an aerofoil behind a conventional mast is unlikely to be effective. The flow is likely to be separated well back onto the aerofoil section as is depicted by this sketch. If the mast is present the advantage of the leading edge of the wing is lost.
This is why the Advanced Wing Systems Semi Rigid Wing incorporates the mast into the leading edge of the wing. Our mast design provides a smooth transition from mast to sail and maintains attached flow as can be seen from the photograph. Luff tell tails on the leeward side of the sail can show whether the flow is separated. A wing sail design should maintain attachment over this section of the wing for its range of operating angles. Careful design of the leading edge is required to ensure that this happens.
Up to date renders of the K8 Sports Boat provided by GYD (gyachtdesign.com). This is exactly as it will be set up for the one design class, with the Advanced Wing Systems wing sail. All the details are finalised, down to the rigging systems and fittings.
DOCK TALK- Karma Yacht Sales Blog: Thursday, January 9, 2014. WING Sails- Are Not just for America's Cup Boat's Anymore! ... For More Information Ask Karma Yacht Sales by email or call 1-877-KARMA-Y-S (527-6297).
Y&Y AWARDS: WINNERS ANNOUNCED By Toby Heppell // 15th January 2016 After thousands of votes cast by readers of Yachts & Yachting magazine, the 2016 Yachts & Yachting Awards were presented last night, Thursday 14 January at the London Boat Show.
Greg Johnston's insight:
Advanced Wing Systems K8 Sports – Yachts and Yachting Performance Sports Boat of the Year.
Since the K8 hit the water on August 29 last year, interest in the boat and in the SRW has grown steadily. In November the K8 Sports was nominated for the Yachts and Yachting performance sports boat of the year for 2016. I know many of you voted for us. Well, we can now inform you that the K8 was triumphant, narrowly beating the highly acclaimed C&C30 to the top spot. This award is really recognition of many years of hard work and effort. Bringing a concept like the SRW to fruition requires a sustained effort over a long period. We are proud to win this award and hope that it heralds the introduction of wing sails into mainstream yachting. Special thanks have to go out to Bret Perry who fell in love with the SRW from the moment he sailed it, and whom has worked tirelessly in helping us bring this project to the attention of the yachting community, and to Nicolas Goldenberg from G Yacht Design, who had the courage and foresight to get involved in this project by contributing his design expertise into the K8.
Everyone that sails the K8 loves it! People are surprised by how easily the performance comes. This speaks loudly of the great job that Nico did in matching the hull to the characteristics of the SRW. The aim was to produce an easy to sail, yet high performance sports boat. I can confidently say that this is what we have achieved
We have all heard of the desire for an elliptical plan form in the mainsail. But what does that really mean? Why is it important? Does this mean that the best shape for the leach is the shape of an ellipse?
Without going into the maths, the load on the sail or wing needs to be elliptical to reduce the amount of drag generated from vortices from the tip of the wing. This does not mean that the sail profile needs to be that of an ellipse, but that the span wise load on the wing needs to be elliptical (as shown by figure 1).
The force generated by the wing is proportional to lift coefficient X area X velocity squared. So at any velocity, to get an elliptical load distribution it is the lift co-efficient multiplied by the area that needs to follow the elliptical relationship.
In reality, the lift coefficient is going to vary along the span. The lift co-efficient is also going to vary with the local angle of attack. Consequently, the shape of the sail or wing is going to be at an optimum drag for something other than an elliptical shape.
The lift co-efficients along the span can are influenced by the twist set in the sail or wing, and by the shape of the local section of the sail or wing.
A sail or wing with less area in the top requires a (relatively) higher lift co-efficient in the upper sections than one with greater area. With a triangular shape plan form, this becomes problematic as the lift coefficients need to be higher than lower in the sail. Essentially, a high lift section is required and low twist is required. In fact the lift coefficient required in the head is in the order of 10 times that at the foot. See figure 2. This is simply not possible and hence a triangular sail is a very high drag configuration.
By contrast a sail or wing with a square head requires a lift co-efficient in the head which is substantially less than the foot, depending on the head size. See figure 3.
With the SRW we use a patented control system to control the thickness and camber of the upper sections of the wing. The wing can also be easily twisted. This allows the lift co-efficients in the upper section of the wing to be controlled to reduce the induced drag created by the tip vortices.
This is a simple overview as the ideal setting is influenced by many other things, like stability, the presence of other sails, wind speed, etc. However, it is good to have this understanding as a starting point in both design and trim.
The Advanced Wing Systems K8 Sports boat has been nominated for the sports boat of the year in the presitgious Yachts and Yachting Awards for 2016. Please like and share and make sure you follow the links and vote for us!
Talented young Argentinean designer Nicolas Goldenberg has enjoyed great success with his RG 650 Mini 6.50. He is also not afraid to take on the perhaps more challenging subject of the ‘production’ wing sail and its application
On the afternoon of August 30, 2015, the K8 wing sail sportsboat pushed away from the dock in Valencia on its maiden voyage. Within seconds it was travelling at 6 knots upwind. The day one sea trials went almost without a hitch. At times we saw well over 7 knots upwind in light and sloppy conditions. All very encouraging. The wing setup on the boat really makes it a delight to sail and you could see that the boat liked the extra power. With a little more sorting out the boat will hit its full potential.
The time has nearly arrived to mate the Advanced Wing Systems Semi Rigid Wing to the new K8. So much looking forward to having a boat specifically designed to take advantage of the performance on the wing. Just a little bit excited!
First revealed to the public earlier this year, NASA and the U.S. Air Force Research Laboratory have finally begun testing what they hope will be a revolutionary new airplane wing design that replaces moving parts with shape-changing assemblies allowing wings to bend and twist to maneuver a craft through the air.
The concept behind this looks vaguely familiar! Even the same motivations: "...replaced an airplane’s conventional aluminum flaps with advanced, shape-changing assemblies that form seamless bendable and twistable surfaces"
Last week saw the testing of the Advanced Wing Systems (advancedwingsystems.com) wing on the RG650 Classe Mini 6.50. In around 8 knots of breeze the boat was doing over 6.5 knots up wind (hull speed 6.14) and exceeding the target number for the conventional rig. This was the first sail with the wing on the RG650 and there were still a few issues to sort out with the retro-fit. So we are expecting a lot more yet!
The RG650 was designed by GYD in Argentina (gyachtdesign.com) and sold by Katabatic Sailing in Valencia (katabatic-sailing.com). This is the same team that is working with AWS to bring the world's first wing sail sports boat to the market - the K8 (facebook.com/k8-sports).
The preliminary results are very exciting and we may have to adjust our performance targets for the K8 - upwards!
It is not often (for some unknown reason) that we get good photos of the leeward side of sails. But when we do it is a good opportunity to show why the AWS soft wing sail offers an advantage. Here is a photo of a VO65 (see http://www.vsail.info/2014/03/31/dongfeng-vo65-changes-course-heads-back-to-hong-kong/) with an overlay that shows an AWS section just below the sail number. You can see from this image that the leeward surface of the wing sail would effectively eliminate the large unfavourable separation area behind the mast. Another point of note is the increased sideways dimension of the mast, which means a stiffer, lighter mast. See www.advancedwingsystems.com
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