Early ideas on our first wingsail
As the new hull proceeds under the skilled hands of Erl, it's time for me to begin moving on the sail for our second milestone. Here's a little breakdown of my thought so far. Comments and feedbacks (even negative ones!) are more than welcome, of course.
A wingsail instead of a traditional sail
There are many reasons behind our choices. Some of them are admittedly speculative:
- A wingsail is more robust than a traditional sail.
- A wingsail is more efficient (better lift-to-drag ratio) than a traditional one with the same surface.
- A wingsail can be positioned regardless of the direction and strength of the wind (A traditional sail requires the opposite forces of a line (pulling it towards the center of the boat) and of the wind (pushing it away from the centre of the vessel).
- The position of the mast in a wingsail is arbitrary, this grants flexibility but also the chance to make the mast pass through the aerodynamic centre, resulting in less strain on the servos operating the wingsail.
We are still at an early prototyping phase of the project, so we want to priviledge flexibility and cost-effectiveness over performance and optimisation. In essence, we would like to experiment with various sail designs without spending dozens of hours and thousands of kronor (hundreds of euros) over each of them. Also - to date - the durability (robustness) of the wingsail is not an issue, as we will only perform short tests in calm waters.
We decided to have a go with foam wings, borrowing from the RC model airplanes world.
Wings of RC airplanes are often just a core of foam layered with some other protective material that increases the wing's strength and increases the smoothness of the surface. This is possible because the wing is normally rigidly attached to the fuselage.
Because of the specific needs to rotate the wing respective to the hull, we are planning to have a core frame in the wing that we will be able to reuse for various wingsails. The frame will look something like a ǂ with the horizontal beams passing through the main vertical mast. We are unsure on the idea of covering the wingsail with any material. I suppose that could be done only for wings that we would like to test extensively, as an extra protection.
From a building perspective, we are planning to cut two symmetrical halves of the wingsail (cutting along the plane where all the wing chords lie) and to glue them together. Each of the halves should have ᴗ shaped tracks for the frame rods, so that once glued, it should be possible to slide the rods within the sail.
Designing the actual wing
I suppose most of the decisions will be taken conducting real life tests, yet we have many questions like for example:
- What should the "aspect ratio" of the wing be? Should the wing be short and broad or tall and thin? The second design is theoretically more performing (it will catch the wind better, it will have lower Raynolds number, less wake turbolence, etc...) but also inherently more fragile and requiring a heavier keel.
- What are the reference values around which we should optimise the design? For example: what is the average wind speed over the X sea during the Y period of the year (substitute X and Y with location and time of our various milestones)? What will it be the predominant direction of the wind (and therefore what will the typical AoA of the wingsail)?
- would it be more convenient to have one or two or n sails?
For the actual geometry of the sails, there are numerous resources available on the internet. So far the best I could get hold of is XFLR5, a free (as in freedom) software that allows to design and perform various kind of analysis on a wing. Other FLOSS tools that grab my attention were Xfoil and the NASA's FoilSim (admittedly quite limited, but also the easiest to use).
For the moment we will stick to the easyest-to-implement idea we have at hand, i.e. we will operate the mast with a servo, connected to it through a string. Again, this is just to have something to test with as soon as possible... future iteration will have more accurate and reliable mechanisms.
In a far future...