How Does a Hydrofoil Work?
But how does a hydrofoil work?
Most would describe a hydrofoil as an airplane underwater. A foil in water basically follows the same laws as a sail in the wind. Because water has a higher density than air, even much smaller profiles create tremendous forces.
What does lift mean?
Every wing has a leading edge (point A) and a trailing edge (point B). On the upper surface of the wing, the distance between the point A and the point B is longer than on the bottom side of the wing. Because there is a larger distance across the upper surface, in order for fluid to reach the trailing edge at the same time for both the top and bottom parts of the wing, fluid traveling along the upper surface has to travel faster than fluid traveling along the bottom side of the wing.
In the 18th century Daniel Bernoulli discovered that if fluid speed increases, its pressure decreases (Bernoulli-Principle).
Since the fluid travels faster on the top side of the hydrofoil, following Bernoulli, it would result in low pressure at the top of the wing and high pressure at the bottom of the wing, which would result in lift force.
Unfortunately, this theory does not explain, why airplanes can fly upside down or why a flat wing generates lift.
Another theory that explains lift, is based on Newton´s third law, which states that for every action there is an equal and opposite reaction. This particle kinetics theory states that incoming molecules are deflected downwards by the fluid and consequently the foil feels a force upward.
Although, this theory does not match reality, as it fails to account that for the interaction of molecules, with each other and fails to explain pressure differences on both sides of the wing
The third theory relies on the so-called Venturi effect, which states, constrict the fluids flow it will move faster. Based on Bernoulli´s principle, we know that if fluid travels faster, it produces low pressure and due to the pressure difference between the top and the bottom it produces lift.
As the theory, this theory does not explain how planes can fly upside down or how a flat wing can generate lift.
Nevertheless, the most scientists agree on the following explanation. Lift is a reaction force experienced by the wing due to it turning the flow downwards. Any object, such as the hydrofoil, must have some force pushing the fluid out of the way, f.e. momentum or engines If more of the fluid is pushed downwards than upwards by that object such as the hydrofoil then the difference is called lift.
Hydrofoil or airplane wing, the lift comes from the turning of the flow downwards as the wing introduces curvature of the flow around it, high and low pressure areas, develop which leads to higher average velocity of the fluid on the top side of the wing due to Bernoulli´s principle according to research hydrofoils are the most efficient when the angle of attack is three to four degrees with a lift to drag ratio of about 20-25 to one. If the angle of attack is higher than 15 degrees stall can occur depending in the wing design. Overall compared to airplane wings, hydrofoils use a smaller angle of attack due to the waters increased density and viscosity.