| Surface piercing foils | Fully submerdged foils |
Foils go through the water-air interface which implies that their submerdged part changes with the altitude of the hydrofoil. |
The submerdged part of the foils remains constant whatever the altitude of the hydrofoil is. |
| Forces that make themselves felt on a foil | |
 The intensity of the lift, which depends on the submerdged surface, changes with the importance of the part of the foil submerdged. Lift makes itself felt normally to the plane of the foil which means inclined versus the air-water interface and weight. The intensity of the contribution to the maintenance at a constant altitude is Fp.cos(a) which means a bigger foil in this configuration than in the fully submerdged case to obtain the same result. |
 Lift makes itself felt normally to the plane of the foil which means colinearly to the weight. Its contribution to the maintenance at a constant altitude is Fp which means a smaller foil that is to say better performances |
| Stabilisation of an hydrofoil | |
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This system remains stable automatically for pitching and roll thanks to the variability of lift linked to the submerdged part of
the foil:
. when the altitude decreases (right foil) the submerdged surface is more significant which induces a more intense lift which creates a vertical movment directed upward. . when the altitude increases (left foil) the submerdged surface is less significant which induces a less intense lift which creates a vertical movment directed downward.  |
Contrary to the surface piercing configuration , forces remain constant which can't give a satisfactory stability . Automatic regulation systems are used in this case. Like the ACS presented below, they control the lift by acting on the foil inclination or on flaps disposed at the trailing edge of the foil when captors detect a change of the altitude.  |
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