Fossati's Aero-Hydrodyamics

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Jun 27, 2024.

  1. CarlosK2
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    CarlosK2 Senior Member

    Last edited: Jul 17, 2024
  2. CarlosK2
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    CarlosK2 Senior Member

    Screenshot_2023-07-08-13-50-10-56_e2d5b3f32b79de1d45acd1fad96fbb0f.jpg

    No one in their right mind would give permission to fly such a device.
     
  3. CarlosK2
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    CarlosK2 Senior Member

    Screenshot_2024-07-17-13-32-34-03.jpg

    Modern Yacht

    Either the rudder is too small, or the Wing (keel) is set incorrectly, or the Center of Gravity is set incorrectly, or the Fuselage (hull) produces too much force.
     
  4. CarlosK2
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    CarlosK2 Senior Member

    IMG20240717134526.jpg

    The sailboat balances upwind because the rudder blade is to leeward

    R: Rudder force
    Keel: Centerboard force
    S: Sail force

    But the sailboat is hydrodynamically unbalanced, which can be verified by going down a big wave without sails.

    ---

    Aeroplane and Yacht

    Upwind

    Wing = Sail

    Downwind

    Wing = Keel
     
  5. CarlosK2
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    CarlosK2 Senior Member

    Screenshot_2024-07-17-14-08-59-73.jpg

    Why unbalanced and not suitable for carrying passengers

    Because of (1) inertia of the past, (2) ignorance, and ...

    (3) to maneuver with agile gazelle speed and win a summer regatta.
     
  6. CarlosK2
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    CarlosK2 Senior Member

    Screenshot_2024-07-17-15-41-12-72_e2d5b3f32b79de1d45acd1fad96fbb0f.jpg

    There are only four things:

    (1) the yaw moment of the hull

    (2) the lateral force of the centerboard/Keel and position of that force

    (3) the lateral force of the rudder blade, and

    (4) the effect of the centerboard/keel on the rudder.

    What happens

    Well

    (a) The hull yaw moment is underestimated or simply no one has bothered to calculate it.

    (b) The hydrodynamic center of the centerboard is forward of the center of gravity.

    (c) When heeling a wide stern sinks the bow.

    (d) A center of gravity forward of the center of Flotation which facilitates the push of the wave coming from the stern.

    (e) A very sharp bow that sinks very easily and on top of that creates a very vertical leading edge that produces a lot of lateral force.
     
  7. CarlosK2
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    CarlosK2 Senior Member

    IMG20240717160202.jpg

    Question

    In the year 2025 we could reach the technological level of 1925

    Maybe

    We cut out the plan of the wetted surface of the hull on cardboard, Balance it on the edge of a sharp knife and find the center of the sailboat:

    Center of Flotation

    Center of Flotation

    (Google site:boatdesign.net Center of Flotation)

    Center of Flotation

    We place the Center of Buoyancy a little behind The Center of Flotation, and the Yacht Center of Gravity a little behind Center of Buoyancy

    HydroDynamic center of the centerboard/Keel near the vertical of the Center of Gravity of the Yacht

    A bow that does not produce too much High lateral pressure, and a stern that does not produce too much Low lateral pressure

    Two rudder blades

    Discuss the color of the hull
     
  8. CarlosK2
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    CarlosK2 Senior Member

    Screenshot_2024-07-17-16-32-04-28.jpg

    - No, I don't want to, I don't want to move the Mast aft

    - Well, then a narrow stern
     
  9. DCockey
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    DCockey Participant

    That arrow is located at the "stagnation point" which is the location where the velocity is zero relative to the airfoil. At that location the pressure is given by Bernoulli's equation. It can also be measured with a stagnation pressure probe.
     
  10. Sailor Al
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    Sailor Al Senior Member

    But he did, and that, plus the reviews by practitioners (e.g. ORC) make it a "respectable" book.
    I would like to keep this thread focused on understanding the material presented in Appendix 1 of Fossati.
     
  11. Sailor Al
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    Sailor Al Senior Member

    What I don't understand, though, is the arrow, pointing away from the wing, seems to indicate a negative over-pressure
    upload_2024-7-18_13-15-8.png
    Unfortunately neither Fossati's nor Marchaj's ( example upload_2024-7-18_13-44-56.png from which I think this figure is copied) have a pressure tap at that location.
    Wouldn't a pressure probe, or pitot tube, at that location, facing directly into the airstream, report a positive over-pressure, not a negative one?
     
    Last edited: Jul 18, 2024
  12. mc_rash
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    mc_rash Senior Member

    Marchaj numbered only 3 test points so the reader can compare the same results in different visualizations, the arrow forward can be assumend as a test point. Marchaj's foil is angled, nobody says that point is at the stagnation point..

    Stagnation point is where flow velocity is zero at the foil surface pointing in direction of the flow stream, but it doesn't need to be the point where the free stream flow is perpendicular to the foils surface.

    I do not see a reason why the arrow pointing forward shouldn't be there.

    Fossati apperantly even copied the numberd test points 1 and 2..
     

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  13. CarlosK2
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    CarlosK2 Senior Member

    IMG_20240718_113228.jpg

    We throw a flat stone against a lake, the stone hits the water, and a High pressure produces a Force.

    Some gentlemen in white coats arrive and have the mania of analyzing this Force in two components:

    Lift and Drag

    This is how an F-104 flies

    Now where everyone seems to get tangled up is with Sails and Wings of the first world war, which in addition to high pressure create Low pressure.

    Why no one asks for explanations for High pressure and there are thousands of heated discussions about Low pressure.

    Apparently our ape-like brain understands pushing and bumping very well and understands sucking and pulling on a rope with difficulty.

    IMG20240718114339.jpg
     
  14. CarlosK2
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    CarlosK2 Senior Member

    What we achieve by creating Low pressure

    Two things

    To better orient the Force for our purposes, for example the sails of a sailboat

    To get enough force with very very low Velocity

    (It could be said that High pressure is a matter of brutes and Low pressure is a matter of artists)
     

  15. CarlosK2
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    CarlosK2 Senior Member

    Wings, Sails and Parachutes

    Beating upwind @ 25-35 degrees a sailboat (StaySail+ MainSail) builds 1 Wing with 2 Elements

    1 Wing for low speed like a 1914 airplane landing

    Reaching sails are sails

    Downwind the sails are parachutes
     
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