Erinhdisc wrote: ↑
Sun Sep 27, 2020 6:54 pm
It seems there is some dispute over the differential flow theory. Angle of attack and displacement are larger factors.
I almost clicked on that video in another list on youtube last night.
It does do a nice quick summary of the history and thoughts on hydrofoils.
Airfoils are designed differently for supersonic.
In any case in both water and air, when people talk about low pressure, that pressure is totally reliant on atmospheric pressure from what gravity exerts on the air between any airborn wing and space. In water, you add the water pressure from the foil depth level to the air pressure at water level to get the total average working pressure. You can therefore theorise that a foil will be more cavitation resistant at lower altitudes and deeper under water.
When we slide a kiteboard sideways like a surface wing, we have mostly the effect of the inertia of water resisting moving down and in that water resisting that downward pressure for a moment, we get lift on our board.
I think for a submerged wing, the pressure above the wing has subtracted from it the force we impart by redirecting that water on top downwards also. In effect, approximately doubling our water mass displacement lift by working on both top and bottom surfaces at the same time. It's the combined atmospheric and water pressures AND gravity that cause the water not to simply separate and stay separated. This does align somewhat with what she said in the video near the end though I feel like she missed something in her explanation. However those two combined forces can't be telling the whole story unless hydrofoil manufacturers have it very wrong utilising similar wing shapes to air wings.