Are you intrigued by the foil? We explain how it works and how we design them.

More and more novices or board sports enthusiasts are getting into it! Foil has many advantages. In order to understand the different aspects and the functioning of a foil, we are going through this article to define the terms used to draw a foil. We will then discuss the notions of balance and finally study the different factors that influence the design of a foil. You want to discover our foils products, it’s here.

In order to understand the foil mechanism, let's define a few terms

The scope

A foil is defined by its wingspan. The shape seen from the front, the rounded shape downwards, is anhedral, upwards it would be euhedral. The span of a front wing has an influence on the performance and functioning of a foil. We are also talking about section, and chord, which is a section of the foil.

3D drawing to explain the term foil operation

Elongation or aspect ratio

We also often talk about aspect ratio(AR), when we talk about wings or foils. It’s a relationship between wingspan and surface area.

AR = b² / S

To give an example, a glider, which is a super efficient aircraft, has an Aspect Ratio of 33. Another motor plane, which is a Cherokee, has an Aspect Ratio or Elongation of 5.6.

This elongation has an impact on the speed performance in terms of drag and lift and therefore on the functioning of the foil.

flying cherokee

The lift of a foil

L = ½ x ρ x v² x S x CL

Let’s start with this mathematical formula. This is the basis for understanding how a foil works. We have the surface, which is an important parameter, the speed, ρ is the density of the water in our case. In the air, for aircraft, it’s the density of the air. We have a coefficient CL which is often due to the shape of the section and other parameters. This formula, let’s say magic, a bit basic, allows to calculate the lift of a wing or a foil.

The lift is therefore dependent on speed squared, area and two other coefficients.

The wings are in fluid, and the faster you go, the more lift. But also the more you increase the angle, the so-called angle of attack, the more lift is created. On the upper part of the wing, we have a depression that creates, and on the lower part an overpressure. This difference between the two creates lift. So the more you increase the angle of attack, this is the angle that the wing or section takes, the more the lift increases, the more you have, in fact, a difference between pressure and depression and this is what creates the force. And as the strength increases, so does the drag.

drawing with surfer with the different forces to understand the functioning of the foil

Comment fonctionne un foil ?

Nous avons donc l’aile avant, le stabilisateur à l'arrière, le fuselage et le mat. Comment s’équilibre tout ça, comment ça marche ?

Pour soulever notre poids avec ce que nous avons vu précédemment il faut que nous ayons une certaine vitesse, une certaine surface et un certain angle dans notre foil. Nous avons une force propulsive qui suivant le support que nous utilisons, voile ou une pagaie, nous donne la vitesse. Si on atteint une vitesse suffisante, on décolle.

Cependant il faut ensuite réussir à équilibrer. C’est pour cela que sur un foil, nous avons une aile et un stabilisateur. Si nous n'avions pas de stabilisateur nous aurions un mouvement où la planche aurait tendance à tomber, disons avoir le nez qui tombe dans l’eau. L’aile avant a vraiment le rôle de portance et le stabilisateur a le rôle inverse de l'aile avant. C'est tout le fonctionnement d'un foil.

Un foil, c’est comme un avion, il y a une aile et un stabilisateur à l’arrière. Mais dans le cas d'un avion, le contrôle mécanique est joué sur des parties mobiles qui vont permettre de garder l’équilibre de vol. Dans le cas du WINDFOIL, SUP FOIL, ou WING FOIL, nous n’avons pas de parties mobiles. Les seuls paramètres sur lesquels, nous pouvons jouer c'est la force propulsive et la position de notre poids.

Quand nous n'avançons pas très vite, il faut donc donner plus d'angle d'attaque pour générer le même poids. Et plus nous allons aller vite, moins nous avons besoin d’angle d’attaque pour générer ce poids-là.

D’un point de vue pratique, on mettra du pied arrière pour aider à prendre de l'angle d'attaque, et du coup pour soulever le nez de la planche. Plus on ira vite, plus on va avoir besoin d’écraser le nez de la planche.

Comment régler son foil ?

Chaque marque a ses propres spécificités de réglages. Cela peut se faire via le rake, les cales,... Tous les foils AFS sont livrés plug and play. Il suffit de monter le foil sur sa planche et d'aller rider. En fonction de votre évolution, vous allez pouvoir optimiser vos réglages grâce à des cales. Ces cales auront pour effet de régler les problèmes de nose up ou nose down de votre planche mais surtout de pouvoir 'kicker' ou 'bracker' votre stabilisateur. Nous vous expliquons cela en détail dans cet article.

Invoices for the design of a foil for a smooth operation

Stability criterion

When you draw a foil, you think about the takeoff speed. The factors in the design that mainly determine this take-off speed are the size of the wings and the drag of the foil. Another important criterion is the longitudinal stability which depends on the position of the surfer or windsurfer, the position of the wing and the stabilizer, the wing surface, and a last parameter, the length of the fuselage. So we have to play with these parameters and that is what gives us our stability criterion.

If we want to make a very stable foil, we will have a longer fuselage with a stronger stabilizer, for example.

Maximum speed criterion

For the design factor of maximum speed, we will go for minimum drag and a smaller wing and stabilizer surface. In this case, we gain in maximum speed but we will lose take-off speed. That means our takeoff speed will be delayed.

The bigger the wing, the more we’re going to cap in high speeds. We still have that famous balance between take-off and top speed. The drawing of a foil is a cursor that is placed, depending on each person, morphology and practice. That’s the complexity of how a foil works =).

Fuselage length

The length of the fuselage implies notions of balance. Backing the stabilizer away from the front wing will bring stability. We also have more powerful foils. In fact, you create moment at the back and therefore you have to have foils where you have to press a lot more and which can saturate in speed. However the disadvantage is also that as it is much more powerful, it is more difficult to hold after certain paces.

This mainly makes the difference between WINDFOIL and WING FOIL, SUP FOIL, SURF FOIL fuselages. In the first case, we’re going to look for stability and therefore a longer fuselage. In the second case, we’ll go for maneuverability.

Wiglet

Wiglets are “ears” that are pulled up to the wingtip. Indeed, vortices can form at the wing tips, causing vortex disturbances. They are the result of the overpressure and depression which meet at the wingtips. Adding these wiglets helps to reduce these whirlpools.

Foil practices

The foil in the world of gliding arrived via windsurfing with the windfoil. AFS was one of the pioneer brands in this field, bringing out the first foils for the general public to fly a windsurfing board and to make people discover new sensations.

For some time now, the foil has been democratized in board sports. We are starting to see more and more surf foils in the water. One of the pioneers in the field is Laird Hamilton who tackles the biggest waves in the world . Before tackling this type of wave (or never), there is a learning phase – we give you all the good advice.

Like many of the great sailors in the Vendée Globe, more and more people are also taking up wing foiling, which is a practice that is developing rapidly. After a passive in kitesurfing, windsurfing or completely new in the field, this sport seduces by its simplicity of implementation and its accessibility. Often complementary because of the same type of foil and board, don’t hesitate to try sup foil too!

Finally, a simple way to take those first steps into the world of foiling is wake foiling. Towed behind a boat, you can experience your first flights.

Would you like to go into more detail about how a foil works? Do not hesitate to watch this webinar with Marc Menec, R&D engineer and Antonin Poirier, product engineer at Foil&Co.