Twin Wing Landyacht
Initial Feasiblility
The landyacht concept I'm working on may have been tried before on the water, but I've never seen it applied on land. I've noticed that the boatspeed/windspeed ratios of landyachts drop off dramatically as the wind picks up. I've personally timed (with a RADAR gun) landyachts going 4 - 5 times the speed of the wind in light airs, but that ratio drops off to the extend that in winds of, say 25 - 30 mph the best yachts will only be doing 80 mph or so - a ratio of around 2.5 to 3. It seems to me that the key to breaking the mythical 100mph barrier is not to go out in 50mph and do 2 x the wind speed, but to be able to carry higher ratios, say 4:1, out to 25 mph. If this can be done, then it may be possible to shoot for speeds of 200 km/hr or 120 mph in 30 mph winds. Just as with the skiffs, stability and sail carrying power seems to be the key to high speed.
In high winds, we loosen our stays to lean the rig 10 to 15 degrees to leeward. This lowers the effective center of effort to relieve heeling, and it generates a downward force component that helps to prevent skidding. The problem is that the heeling relief is limited by the fact that the mast is stepped on the centerline of the body. If it were stepped as far out as the windward wheel, a given amount of tilt would be twice as effective. It would be possible to build an asymmetric yacht like this for record breaking. The rig could either be cantilevered or supported by a strut to leeward. But I want a yacht that can still race.
So I decided to split the sail area in half and have two rigid wing panels joined at the top in an "A" frame. Table 1 shows some typcial dimensions, etc. from a very crude sizing analysis I did some time ago. The rest of the data I'm including are also from this analysis. I'm carrying a single wing landyacht through the whole effort as well, in order to see where the twin's strengths and weaknesses are.
Figure 1 is a freebody diagram of this concept. The moment produced by the leeward panel is the same as an upright rig would be. But the heeling force of the windward panel has a much lower effective center of effort. There's some loss in efficiency because the panels are not aligned at right angles to the wind, and because their force vectors are not horizontal.
The sailing strategy is to sail with both wings trimmed for maximum lift until the windward wheel starts flying. Then only the leeward wing is sheeted out to keep the wheel on the ground. When the leeward wing is fully feathered, the situation is identical to the asymmetric yacht with the wing to windward. Figure 2 shows the sail carrying power of the twin compared to the single wing yacht. In this figure, once the leeward wing is fully feathered, the pilot starts to sheet out on the windward wing, keeping the leeward wing at zero lift. Unlike the single rig, which only has a low wind range and a high wind range, the twin yacht has an additional medium wind range. The single wing can only reduce heeling by changing the magnitude of the heeling force, while in the medium range, the twin wing is effectively changing not only the magnitude but the moment arm as well.
The need to be able to sail with the leeward wing completely feathered seems to indicate that this concept can only be done with rigid wings. The drag from luffing soft sails would seem to make them a non-starter. This isn't a big restriction, however, as right now the best of the rigid wing landyachts and the best of the wingmast/sail rigs are pretty much equal in speed across a wide wind range. The concept is also completely unsuitable for sailing on water, because there one wouldn't want the down force produced by the twin rig. Far better to simply mount the mast step on a traveler and heel the rig to windward in heavy air. But that's a discussion for a different time.
The induced drag on the rig was estimated using a very simple approach in which each panel was represented by a singe horseshoe vortex. The parasite drag coefficient is in the general range indicated by a wind tunnel test of a single wing landyacht that was performed a San Diego State University. The L/D ratios, which are indicative of the yachts' top speeds, are shown in Figure 3. Figures 2 and 3 indicate that the twin wing concept does have an edge in high winds, but its low wind acceleration, in particular, is distinctly inferior to the single wing rig.
There is another strategy for the high wind range, however. In this alternate strategy, the pilot continues to play the leeward wing, keeping the windward wing trimmed at max lift. This requires going to negative angle of attack on the leeward wing, backwinding it! My intuitive expectation was that the lift vectors may sum linearly but the drag from the backwinded sail would kill the performance. Not so, says the computer.
Figure 4 and Figure 5 show what happens if the pilot is allowed to apply up to 200 lb of backwinding force on the leeward panel (lift on the windward panel is around 350 lb). The medium wind range is greatly extended, and the twin wing yacht has nearly 40% more sail carrying power compared with the single wing yacht. L/D is also improved at high speeds. The twin has approximately the same L/D at 100 mph that the single wing does at 80 mph, which translates into similar boatspeed/windspeed ratios. Not the hoped for 4:1, but not too shabby an improvement, either. Structural loads seem so far to be the main impediment to how much backwinding can be applied, and therefore how much Low speed L/D is still a problem, however.
Since the L/D is much greater than 1, it makes more sense to attack drag than it does to try to make a major improvement in lift. After all, a one pound drag reduction is worth five pounds of added lift. Figure 6 shows that at high speeds, the parasite drag is everything. Lift induced drag is in the noise. So how much cleanup would it take to bring the twin wing's low wind max speed up to the baseline single yacht? Figure 7 shows the answer - about a 12% improvement will do it. But look at the effect in high winds! Of course, the single wing could do a similar cleanup and we'd have the same results as before.
So the main drawback of the twin wing yacht is its low wind acceleration. Chord Reynolds numbers will be less than half that of the single wing yacht, making the problem far worse (lower max lift, higher parasite drag). But there's one other unique aspect of the twin wing yacht - its load paths are radically different from the conventional yacht. Figure 8 shows a side view with the major load paths indicated. The really big loads - like compression on the leeward wing, are taken almost directly into the reaction point on the ground. Other than the wing panels themselves, there are no major bending moments anywhere. The body is literally slung like a hammock by the forestay and backstay. Instead of having to carry the tremendous thrust of the mast in bending, like a conventional yacht, the axle only has to keep the wheels apart. Likewise, the body only has to support the weight of the pilot, and the fore and backstays only have to stabilize the wings against the comparatively small thrust/drag loads. So it should be possible to make the structure of the yacht dramatically lighter than the conventional configuration, and it may be possible to regain the light wind acceleration via a reduction in mass.
The wing panels are a different story, however. They are long, slender columns, heavily loaded in compression (leeward panel), and carrying a bending load - a prescription for buckling. Stiffness will be at a premium, making carbon construction a must.
Well, that's the general outline of the concept. Since this preliminary analysis, I've been running parametric studies with an aerodynamic panel code, and the results are surprising - they contradict some of these numbers, and the twin is looking better and better.
Table 1 - Design Parameters for Feasibilty Study