Coanda craft: why not?

Discussion in 'Boat Design' started by joceline, Feb 2, 2011.

  1. joceline
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    joceline Junior Member

    Why isn't anyone using the coanda effect to generate lift (like a hovercraft), and to then push forward into ground effect or to hydrofoil?

    The advantage would be that in order to generate lift, you already have "wings" of sorts. In a flying hovercraft (type "Universal Hovercraft"), you use a skirt to concentrate air, and then you have to add wings.
     

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  2. jehardiman
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    jehardiman Senior Member

    Search the net for Ekranoplan or WIG, plenty of the out there, some for personal use in kit form.
     
  3. gonzo
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    gonzo Senior Member

    I would say that the first time one of those flaps hits a wave the vessel will be out of control. In flat water racing conditions there may be some advantages if the rules allow them.
     
  4. joceline
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    joceline Junior Member

    Dude... Neither Ekranoplan nor WIG have anything to do with the Coanda Effect.
     
  5. jehardiman
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    jehardiman Senior Member

    I would suggest you go back and look at the designs, especally the large Soviet Ekranoplans and thier lift engine locations.
     
  6. daiquiri
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    daiquiri Engineering and Design

    I would see the Coanda effect useful as a mean to allow the craft to take-off and land in tight spaces (perhaps even vertically).
    But I believe it wouldn't be as efficient way of generating lift at cruising speeds as common wings are. That's because you need to blow either a mass of air or engine's exhaust gases over the wing's surface in order to generate lift.
    - If you choose to use mass of air accelerated by a propeller, then you are diminishing the efficiency of the propeller because it's wake is not free but is obstructed by the wing surface.
    - If you choose to use compressed air, than you waste a significant amount of power due to heat losses associated to air compression.
    - If you use the exhaust gases from, (for example) a gas turbine or a jet engine, then you create an additional back-pressure at the engine exhaust, which translates into a power loss. Besides, the jet would have to be tilted downwards, thus diminishing the precious horizontal thrust component.
    In any case, and in addition to the above, an accelerated mass of air (or exhaust gass) travelling over a surface is subject to friction drag, which again is a power loss.
    So, apart the possibility of STOL / VTOL operation (which could be useful or required for some missions), you don't gain much more when compared to a conventional (non-coanda) airplane or a WIG, imho.
    My 2 cents worth.

    Cheers
     
  7. hoytedow
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    hoytedow Carbon Based Life Form

    How is the coanda effect different from the magnus force?
     
  8. daiquiri
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    daiquiri Engineering and Design

    In few words:

    The Magnus effect:
    - lift is generated by a cylindrical body rotating inside a moving fluid
    - the power is applied to the body
    - the lift is generated only if the fluid is in motion respect to the cylindrical body, and the freestream velocity needs to have a component perpendicular to cylinder axis.

    The Coanda effect:
    - lift is generated by a forced blowing of a fluid over a convexly curved surface
    - the power is applied to the fluid.
    - the lift is produced even if the surrounding fluid is still.

    Hope it's clear enough. :)
     
  9. hoytedow
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    hoytedow Carbon Based Life Form

    Thank you. It is not your fault I don't differentiate them well.
     
  10. Zurael
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    Zurael New Member

    Coandă effect aircraft are unstable and inefficient in forward cruise flight. That is why aircraft which take advantage of the effect were cancelled, such as the VZ-9 Avrocar.

    The Coandă effect is defined as the tendency of a fluid jet to attach to a surface.
    The Magnus effect is defined as the force generated perpendicular to the forward motion of a spinning object in a fluid.
     
  11. hoytedow
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    hoytedow Carbon Based Life Form

    Thanks, again.
     
  12. joceline
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    joceline Junior Member

    This is only true for aircraft which are entirely based on the coanda effect. What I am suggesting is to use coanda merely for lift.

    There is a STOL airplane that uses coanda for lift, and that is highly successful. It's the An-72 and all its later variants. Many of them are in use today.

    In my concept, you would not permanently use the coanda engine; you would use it simply to increase the transition to ground effect.
     
  13. gonzo
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    gonzo Senior Member

    What do you do about wave action?
     
  14. cthippo
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    cthippo Senior Member

    I'm thinking aloud here, so bear with me...

    Since Coanda effect is tied to the upper surface of the wing as opposed to the lower as in ground effect, the wing could be mounted fairly high on the vessel, minimizing wave issues.

    The wing would have to be fairly far forward because the center of lift (the back of the wing) would have to be at or forward of the CG of the vessel or else it will want to nose over.

    I can see using Coanda effect to supplement lift in a WIG design, but I'm not sure what it would bring to the table by itself.
     

  15. jehardiman
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    jehardiman Senior Member

    This is exactly how it is used. As I pointed out earlier, the Coanda effect is used by many Ekranoplans and marine WIGs. By placing the engines/props ahead of and above the lift wings, a lift force is generated at low speed/high thrust. This lift force decreases the displacement of the in-water hull, reducing drag and wetted surface to the point that sufficient forward velocity can be picked up to get the lift wing working in ram/ground effect.

    To fit an engine just for the Coanda effect would be counter productive as daiquiri alludes to. For efficient high speed marine applications, the whole idea is to reduce hydrodynamic wetted surface and air drag to a minimum. Adding weight and volume just increaes drag or reduces payload, so unless there is an over-riding requirement for high speed in extreamly shallow water there is no benefit. And for high speeds and heavy loads in shallow water, other options generally better fit operational needs, like some hovercraft/LCAC

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