Ram-air sailwing

As I hinted in another thread I’ve been toying with the idea of using a ram-air inflatable foil as a sailwing. I have a speedfoil similar to these that I had built years ago, http://www.cobrakites.com/spdfoil.html before I started building and designing bigger complex bridled canopies. They can reach incredible speeds in the air. Not onle due to low parasitic drag for lack of a bridle but because of their use of negative camber in the last ~20% of the wing profile. The negative lift created by the TE changes the AoA of the wing at greater speeds reducing lift and also more importantly the amount of profile drag. This makes it both pitch stable and self tuning. There is also a factor of AoA control from the bendy spar in the LE brought on by lift and line tension but that’s particular to it’s use as a kite and I won’t go into boring detail. Similar control can be achieved through bottom skin tension for our use.

The biggest issue would be tacking and gybing. You’d need to change the camber and swap around the LE vent to the high pressure point on the LE or by some nefarious rigging flip/rotate the whole wing around the centre point on the LE. The latter is probably the simplest though could make high speed gybes into a small exciting airbourne detour as the wing goes through a horisontal phase.

I was planning to start with a simple symmetrical wing profile with the LE supported by two or three light carbon spars to control the bottom skin tension for setting AoA. The third spar only to help retain the LE geometry and support a mast structure. The LE vent doesn’t have to extend the whole LE. As we learned from kitesurfing kites and later designing better foils you only have to vent a small part of the LE efficiently and inflate the rest of the structure by simple valved vents in the ribs. You can now build a simple rotating semi cylindrical valve in the LE to help keep the structure inflated that only spans 5-10% of the LE and doesn’t weigh too much. A perfect stationary point to have the valve shutter is where the three shrouds support the mast I think. The whole lot would be controlled by rotating mast base through a boom and with it the mast structure in the LE allowing it to autotack like a normal rig.

It sounds like a fascinating concept, although I have a little trouble visualizing how it would be able to tack into a new camber & still have an efficient asymetrical airfoil section. I’m sure you have some good ideas about how to make it work.[:D]

I have to admit you won’t necessarily achieve higher performance in all cases. However this scale does allow us to R&D some interesting rigs. Also takes up a lot less space when deflated which could be handy.

Running a couple of ideas:

  1. symmetrical foil - might do this just for the fun of it. Many old traction kites had symmetrical foil sections and still provided heaps of grunt. Little bit sensitive at lower AoA but the simplest setup of the lot.

  2. inflated wingmast with sail - similar to your setup except for the wingmast being ram-air
    I like this idea since you can stabilise the simmetrical foil section or even adjust foil thickness through outhaul. However small thin ram-air foils don’t hold their shape very well so you’d have to scale it up till it works.

3.a - Flip wing - seen some proas do this, swing the whole wing around the midpoint on the LE
3.b - Flip tacker - a triangular frame supports the LE and either flips in an arc with the tack or in some designs flips the whole chassis with it. Both require some mechanics and some deft servo work. A setup like this would allow the use of a self tuning foil section that would deliver rather nice topspeeds though. I’ve made a few drawings of a setup like this I’d like to try as well. Maybe in winter when I have a little time on my hands.

  1. Asymmetrical foil with internal camber structure. Complicated and usually too heavy, pretty much the Holy Grail of wingsails.