Preparation : the profile

The caracteristics of a foil cannot be restrained to the properties of a unique component, but the profile is one of the most important. Try to choose one that closely match your requirements. For more information, Daniel Gagnon wrote a quick comparision report between a few profiles. You can find it here. It's very easy with a CAD tool to change the thickness of a profile (doing a scale in only one direction), but it's harder to change the maximum thickness position. You can use Foilmaker to adjust your profiles to your needs, then export them in a suitable format. I wrote a little Perl script that convert a profile file into DXF. You can find it in the download section.

Now, things get serious. Be very patient at this stage, because if you mess something here, you'll probably have to restart everything from the start. Now, import your profile (you could use multiple different profiles, but this is out of the scope of this manual) and orient it in the right view. Don't put it near the (0,0,0) point, we'll use this point later for the center of the foil (but you can move it later). You get something like that :

The profile here is a PW99a with a 15.5% thickness at 23% of the chors. Notice the use of different layers (one for each kind of element). The use of layers is not mandatory, only strongly suggested. On every screen shot in this manual, each layer is drawn in one distinct color (always the same). So, on the screen shots, identically colored objects belongs to the same layer.

Description of the different elements :


In red. Used to position the profile according to the shape of the foil. If your CAD tool allows you to 'snap' on points, create two points on the same layer on each end of the chord, it will be easier later.

Profile upper skin and lower skin

In white. Define the outline of the profile. Usually, I create a curve for the upper skin, and two for the lower skin. Why two ? Because I'll have two kinds of cells : open cells and closed cells. So, for open cells, the vent will be created with a straight line connecting the begining of the upper skin and the begining of the lower skin. Be very careful to the position of the vent. If you can, use a virtual wind tunnel to try to compute the stagnation point.

Maximum thickness of the profile

The vertical white line. I don't use it.

Main bridle points and brake points

In orange. Will be used to create the bridle. Don't put the A point too close from the vent, especially if you want to create D-Ribs. The brake point could be placed at 100% of the chord, or a little bit before, as you want. You could also create more than 3 bridle points.

Cross vents

The white circles. They must be placed between the bridle points. If you put them in front of a bridle point, your profile will tear. For the same reason, try to put them near the bottom of the profile (1 cm or 2 from the lower skin, it's enough).

Pinches marks

The green points. Here, they're placed at 75% of the chord on the lower skin and at 85% on the upper skin.

D-Ribs marks

The blue points. The blue lines are not useful, but it's easier to see the points with them. Only the points will be used later.

The center of pressure (Xcp)

The purple point. Here placed at 30% of the chord. Try to use a virtual wind tunnel to compute it (with the formula : Xcp = 0.25 - Cm/Cl)

Now, the profile is ready to use. Keep a backup in a safe place, so you can reuse it for another foil, and to adjust it according to your observations on the spot (tow point position, vent position, etc...)