How does Foilmaker compute the bridle ?

It's a very common question, so I've decided to write a few words on the subject. I'm not the programmer of Foilmaker, so I had a play with Foilmaker to try to understand how it works ; and I may be wrong sometimes. I used Foilmaker 1.6.0 final release. The four examples are almost the same, only the parameters for tow point and center of lift are different. The canopy is flat, with very few cells. The leading edge is exageratly curved, so differences between configurations are well visible.

Warning : the notations used on this page do not match the ones used elsewhere on this site (and especially notions of tow point and centre of lift). They are related to Foilmaker notions.

Test cases : 4 foilmaker files available here.

 First case :
   Middle tow point = 25%
   Tip tow point = 25%
   Middle center of lift = 25%
   Tip center of lift = 25%

All the profiles have an angle of attack (AoA) of 0°, wich is ok. The green vertical lines are the projection of the center of lift points (which are same as the tow points) of the center and tip profiles.

With those two points, we can draw a rectangle at a specified distance from the center profile. To calculate this distance, we take the kite's span (here 4997 mm) and apply the percentage given by the 'secondary bridle' parameter (67%). We get 3348 mm.

Foilmaker first compute the point where the secondary bridles meet. In the 'chord direction', this point is the average of the projections of the center of lift of the center and tip profiles (the middle of the rectangle side). The location of this point in 'span direction' is computed by Foilmaker and cannot be modified by the user (there's no parameter for it)

Then, the bridle is created in the following way : the meeting point of all secondaries is connected to the center of lift points of each profile. This gives the secondary bridle. To create the primary bridle, a point on each secondary bridle line (its position depends on the 'primary bridle length' (Y) setting) is connected to the A,B and C points on the corresponding profile.

In conclusion, none of the profile has a real 'tow point' at 25% (except for rectangular foils). By tow point, I mean the projection of the point located at 25% of the chord, perpendicularly to the chord.

 Second case :
   Middle tow point = 25%
   Tip tow point = 50%
   Middle center of lift = 25%
   Tip center of lift = 50%

In this case, the meeting point of the secondary bridles is computed the same way, with a projection of the point located at 50% of the chord for the tip profile. There is no other difference.

The blue rectangle is the one of the previous case, added for comparison purpose.

Here, we have no twist, because for the two reference profiles (center and tip), the setting for the tow point (in percentage) is the same as the center of lift setting (in percentage). The only thing that changes is the location of the meeting point of the secondary bridle (and the bridle as a consequence).

 Third case :
   Middle tow point = 25%
   Tip tow point = 50%
   Middle center of lift = 25%
   Tip center of lift = 25%

The computation of the meeting point of the secondaries is the same, based on the center of lift settings (and not the tow point settings). But, here, we get a 'twisted' foil, which means that the AoA of the tip profile is not 0 (zero) but is a positive value. How much ? Thanks DAG for the formula : Arctan( (towPoint(%) - centerOfLift(%)) / primaryBridleLength(%) ). Here we have arctan(0.25) = 14.03°. For those who prefer a picture :

Of course, this twisting angle gradually changes from center to tip. It depends on the variation of the tow point AND the center of lift. As a consequence, these values must evolve in a uniform manner along the canopy. Be aware that this is NOT necessary the case with Foilmaker. By default, variations of the tow point are 'by position' and center of lift varations are 'by chord', which is very dangerous.

Little brainstorming : should Foilmaker compute the 'twisting angle' from the length of the primary bridle only (as it does now), or should it take the length of the secondary bridle into account ?

Here, we can see clearly that the center of lift is used to create the bridle, and not the tow point (on the chord). We can see also the angle between the center profile and the tip profile (chords are in red). Notice that the center of rotation is the center of lift.

 Fourth case :
   Middle tow point = 25%
   Tip tow point = 25%
   Middle center of lift = 25%
   Tip center of lift = 50%

This is basically the same case as the one before, except that the twisting angle is negative (because towPoint -centerOfLift < 0). The twist is negative.

The secondary bridle is 'virtually' connected to the center of lift point (purple line).

Conclusion

The way Foilmaker create the bridle is not the same that the one described on this site, where the twist depends on each profile's position to keep a constant tow point. I don't know which one is the best (at least I've no real arguments to prove it) so you'll have to build your own opinion. Keep in mind that the most important point to keep in mind is : 'make it fly'. I will continue to create my kites with Rhino, but they won't necessarly fly better than others ...