Hey Luiz, what you have said is actually the same as I wrote? but with added examples. I never said that hydrofoils would not generate lift on either side under the (extreme) conditions you give, such as aircraft wings which, when the plane is inverted the angle of attack of the foil is increased to such a degree that lift can be generated on their “flatter” side more than the lift on the curved surface and in so doing the aircraft can remain flying etc, but there is a great penalty in drag incurred to do so. Symmetrical foils, as I said generate lift on either side of the foil depending on their angle of attack to their inclination to the direction they are travelling through the water.
Rudders are symmetrical foils and obviously generate lift on one side as well as the other whenever their angle of attack to their direction through the water is altered, and therefore, they then turn the craft in the desired direction.
What I was offering, principally, is the well established, defined fundamental, difference between the main functions of a hydrofoil and a (symmetrical) foil, (which is not considered to be a hydrofoil even though a symmetrical foil, as I said, can be used to generate lift equally in two desired directions)
When a hydrofoil is used on water borne craft, the foil is generally in a fixed attitude to the longitudinal length of the boat and the, profile shape lift that it generates is always “upwards” at (or about) 90 degrees to the waterline of that boat. This is due solely from the asymmetrical shape of that foil
This is the same for most aircraft, and in the instance you give of a plane, the tailplane elevators and the Aerolone on the wings alter the “angle of attack” to the wind direction passing over both the wing and the tail plane. In the case of the asymmetrical shape of the wing, the Aerolone will actually go a long way to altering the “shape” of its exposed “profile” to the wind, of that wing, from that of a fully asymmetrical shape when flying “right side up” to more of a symmetrical shape, to the wind direction when the plane is inverted, and of course most commonly, elevators are symmetrical (not always though) in profile and take over a large percentage of the work in keeping an aeroplane flying when inverted.
The best example of symmetrical foils on a boat generating lift in a multifunctional desired directions (other than the rudders themselves) is a fixed, (in the vertical plane) rudder that has a, non-adjustable “T” foil attached to it below the waterline. When the boat starts to pitch down at the bow with the boat travelling forward, the T foil is angled down in relation to its forward direction. Due to that angle change, lift is generated on the under surface of that symmetrical T foil and it “pulls” the stern down (and conversely the bow upwards). The same applies when the bow pitches upwards. Lift is generated on the upper surface of that symmetrical T foil (for the same reasons) and the stern is lifted upwards (conversely the bow moves downwards) ergo pitch is greatly reduced.
This same principle applies to symmetrical dagger boards that are canted (particularly canted inwards). Due to the “pitch” of the boat, and their continually changing inclination to the forward horizontal direction of that boat, they will generate lift in a desired (at that moment) direction.
So symmetrical foils on a boat can generate lift that is “desired” in both upward and downwards directions (or side to side as in rudders), whereas “hydrofoils” on a boat will generate principal lift only in one direction all the time. That amount of lift (of a hydrofoil) is a variable directly proportional to the “speed” it is travelling. (They obviously generate some lift in the horizontal plane as well i.e. acting to reduce “leeward drift”, but that is generally secondary to their main function)
For boats, the function of a symmetrical foil is more “dynamic” while the asymmetric foils function is mainly “static”
P.S Luiz, these examples are primarily "as they apply to boats" so that the extreme angles of attack as described with planes, (there are other many and varied applications as well), simply should not be applied in this instance.