sail aerodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by Guest, Mar 21, 2002.

  1. markdrela
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    markdrela Senior Member

    Try this:
    http://www.allstar.fiu.edu/aero/airflylvl3.htm

    BTW, a lot of the explanations in that article aren't quite as airtight as they might seem.
    For example, all the "momentum" arguments ignore one crucial fact: a lifting wing does not change the vertical momentum of the entire airmass. There is indeed downward motion between the tip vortices, but there's also upward motion outboard of the tip vortices. The net vertical momentum behind the wing is zero.

    The article also invokes the Coanda effect to explain why fluid sticks to the upper surface of an airfoil. This is sketchy at best, since the Coanda effect has to do with enhanced turbulent mixing of an excess-speed jet adjacent to a curved surface. A conventional airfoil does not have an excess-speed jet on it.
     
  2. Doug Lord

    Doug Lord Guest

    Thanks, Mark!
     
  3. Petros
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    Petros Senior Member

    Allow me to try and make this very simple (I worked in aerodynamics for many years, and have studied it even longer).

    When ever you accelerate any mass you get a reaction. Newton discovered the well known relationship F=ma. That is the force (or lift in this case) is equal to the mass times the acceleration. And the first law of motion, for every action there is a reaction. Accelerate the mass of the air over the curved surface of the sail, and you get thrust (or lift).

    Anytime you curve a mass you get an acceleration. A simple example would be a weight at the end of a string, when you swing it around in a circle you get tension on the string. The weight is "curved" into a circle the radius of the string, and the reaction "F" is outward on the string. You can also see this with a garden hose on the lawn: lay it in a curve over the lawn, and turn on the water. As the mass of the water curves around the hose, it will react outward, pushing the hose over the grass away from the curved path of the hose (effectively the mass of the water wants to go strait).

    Similarly, when the mass of the moving air (i.e. the wind) is curved or accelerated off of its strait path by the shape of the sails, you get a reaction that pushes the boat through the water.

    There are lot of complicating details of course on how to do this efficiently (getting the most amount of thrust from a given amount of wind), but the principles are the same for all fluids. You have to minimize drag, turbulence and keep the flow attached over the sail surface. But the thrust off the sails comes from curving or accelerating the air mass, or F=ma.

    This BTW is also how the keel and rudder works, a propeller (a curved surface going around in a circle through a fluid), aircraft wings, a paddle blade on an oar, etc.
     
  4. brian eiland
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    brian eiland Senior Member

    Under the subheading, “Wing Efficiency”, the explanation begins with these sentences:

    “At cruise, a non-negligible amount of the drag of a modern wing is induced drag. Parasitic drag, which dominates at cruise, of a Boeing 747 wing is only equivalent to that of a 1/2-inch cable of the same length.”

    My question, is there a grammatical mistake here? It appears to me they are saying both the parasitic drag and the induced drag are significant factors at cruise speed? Should the “non-negligible” wording really be ‘negligible’??

    On another subject, Powered Flight verses Sailing;
    ...would someone venture to express in similar terms these same aero observations in this paper from the viewpoint of sailing rather than powered flight….in sailing we are extracting energy from the airflow verses power flight were we are injecting energy to fly??
     
  5. markdrela
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    markdrela Senior Member

    For an airplane which is well-optimized for cruise, the induced and parasitic drags are comparable in magnitude.
     
  6. markdrela
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    markdrela Senior Member

    Yes. But where the sailing power comes from and where it goes depends on the observer.

    1) To a stationary fish, or to the sailor:
    The sail reduces the magnitude of the apparent wind behind the sail, via its tip and root vortices. So the sail extracts Kinetic Energy from the moving air mass. Some of this energy goes into the keel vortex wake motion, some goes into wave motion, and the rest heats the air and water.

    2) To a hovering bird or an observer in a balloon stationary with respect to the air:
    The keel reduces the magnitude of the water speed behind the keel, via its tip and root vortices. So the keel extracts Kinetic Energy from the moving water mass. Some of this energy goes into the sail vortex wake motion, some goes into wave motion, and the rest heats the air and water.
     
  7. Petros
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    Petros Senior Member

    This is a non-trivial issue to keep strait. You extract energy from moving air, which must be moving relative to your craft. A sailboat can not move in still air, but a powered airplane can.

    Consider a sail plane, in still air it can only fall and take energy out of the fall to move forward and overcome drag. They can only stay up if there is upward moving air from the heat of the sun, or from ridge riding where the moving air is forced upward by a mountain (taking energy out of the forward movement of the mass of air). But the powered aircraft overcomes the drag of forward movement by adding power the system.

    If the water surface and the air were moving together at the same speed, a sailboat can not make any headway relative to the water surface. So it depends on you being able to have relative movement between the two to extract energy out of it to make the boat move relative to the water.

    This is actually a good issue to always keep in mind. I have gotten into pointless on-line "discussions" with a NA who should have known better. He clearly did not have this understanding of the relationship clear. And apparently his ego did not allow him come around to understanding this relationship, no matter how patiently I tried to explain it to him. He wanted to argue rather than to learn.
     
  8. Petros
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    Petros Senior Member

    No grammatical mistake, just obfuscation. This is collage professor speak trying to say: "you have two large components to drag, parasitic drag and induced drag on a wing that generates lift. Both are significant. On the 747 parasitic drag is larger at cruise speed".

    Note this may not be true of other designs or even at different speeds. At lower speeds for example you can have induced drag being much larger than the parasitic drag with the same aircraft.

    The faster you go, parasitic drag goes up exponentially, but you need less angle of attack (lower Cl) with the higher airflow over the wings, so the induced drag goes down (the weight, and the total lift requirement, of the aircraft is presumably the same at both speeds). At low speeds you need a high angle of attack (or higher Cl) to keep the aircraft in the air, so induced drag then dominates. The "induced drag" is the "cost" of generating lift.
     
  9. gggGuest
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    gggGuest ...

    Whilst not in any way disagreeing with the content I think its better to say that you extract energy from the relative motion of wind and water. A lot of muddled thinking comes from not realising that the sailing craft can only operate when there is a difference in velocity between these two media, and the result of its passing, as well as heat, is that this velocity difference is reduced.
     
  10. tspeer
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    tspeer Senior Member

    What you say is true, however, you're confounding several different things, here. The sailplane has two forms of energy - potential energy due to its height, and kinetic energy due to its speed. When a sailplane does a loop, it is exchanging kinetic and potential energy as it rises and falls. However, it is also continuously losing some of its energy to drag - its specific excess power is always negative, although varying in magnitude. In a steady glide, it is bleeding potential energy into work done on, and heating of, the atmosphere.

    The powered airplane also has both kinetic and potential energy. However, its excess power can be positive, zero (unaccelerated flight), or negative. It has the chemical energy of the fuel that adds to the potential and kinetic energy.

    A power boat is much like the powered airplane, except that it only has one form of energy - kinetic - because it travels on an equipotential surface. Plus the chemical energy of its fuel. A submarine has both potential energy (by virtue of its variable buoyancy and ability to rise or sink) and kinetic energy.

    A sailboat also only has kinetic energy, which is used, for example, when shooting for a mark, or landing at a dock with the sails lowered. However, unlike the sailplane, powered plane, or power boat, its thrust is dependent on its direction of travel. You are right in that it depends on the difference in velocity at the boundary between the two fluids, and cannot achieve any thrust if there's no velocity difference.

    As Mark Drela points out, which medium is considered the donor and which the recipient is very much a matter of one's point of view. This is especially true when one considers wind-powered craft that can sail directly downwind faster than the wind. An example is a land-based craft with a propeller in the air geared to wheels. (Yes, such craft have actually been built and shown to work.) When traveling slower than the wind, the momentum of the air leaving the propeller disk is lower than the air entering it, but when traveling faster than the wind, the momentum of the air leaving the disk is higher than the air entering it. So one might reasonably say when traveling slower than the wind the craft is extracting energy from the air and transferring it to the ground, but when traveling faster than the wind, it is transferring energy from the ground to the air. But both air and ground get hotter in either case!

    As a practical matter, I haven't found it very useful to use energy- or power-balance approaches for sailcraft. The reason is the thrust horsepower available is not fixed, as it is with powered craft, but depends on the speed of the boat. A high-performance sailcraft generates more power as it accelerates because the apparent wind increases.

    I find a force-balance approach is much more useful. For example, even if the drag of the "hull" is zero, the speed of a sailcraft is still finite because the rotation of the apparent wind means there is less and less driving force component available, until the speed is limited by the aerodynamic drag angle.

    The fundamental sailing performance equations can be expressed in terms of lift/drag ratios. These are largely self-compensating and don't vary so much as the speed and direction change, compared to the individual magnitudes of lift, drag, or exchange of energy between the fluids. So the force-balance approach is easier to use for engineering the boat's design.

    But arguments about where energy is extracted and deposited certainly keep things animated at the bar!
     
  11. yipster
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    yipster designer

    i'm trying to keep myself animated installing vista 64 over 32
    do still read this tho and want to trow a motor sailing cat into the equasion as beeing relevant here as well
    yet anyone who has done that vista 64 upgrade in a breeze plz pm me...

    =========================================================

    edit, free updating the oem vista 32 to 64 bit in the microsoft method two is hard to do
    you may also delete the 32 boot partion, vista still runs and now does accept a 64 bit instal

    http://support.microsoft.com/kb/932795/en-us

    Installing a 64-bit version of Windows Vista on computer that is running a 32-bit version of Windows Vista
    If you have purchased an Upgrade license together with a Windows Vista DVD, you must use one of the following methods.
    Method 1
    Purchase a full version of the 64-bit version of Windows Vista.
    Method 2
    1. Remove the 32-bit version of Windows Vista.
    2. Install Windows XP.
    3. Install the 64-bit version of Windows Vista by using an installation method that is listed earlier in this article.
    If you have purchased a full license together with a Windows Vista DVD, follow these steps:1. Back up all the data and settings by using Windows Easy Transfer. Windows Easy Transfer is available on the Windows Vista DVD. However, you must use the version that is on the Windows Vista DVD for your currently installed 32-bit version of Windows Vista.
    2. Insert the 64-bit version of Windows Vista into the system DVD drive, and then restart the computer.
    3. Start Windows Vista Setup from the DVD when you are prompted.

    Note You must start Windows Vista Setup by starting the computer from the Windows Vista 64-bit DVD. The installation package will not run on a 32-bit operating system.
    4. When you are prompted during Windows Vista Setup, remember to select Custom as your installation choice.
    5. When the installation is complete, you can restore the data from its backup location.
     
    Last edited: Jun 30, 2008
  12. brian eiland
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    brian eiland Senior Member

  13. tspeer
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    tspeer Senior Member

    You should buy a copy from AIAA.

    However, if you'd like to see what it'll look like when you do, see the attached file.
     

    Attached Files:

  14. brian eiland
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    brian eiland Senior Member

    Smith's paper

    That's a large file isn't it !!

    Actually I had a zerox copy, but I was looking to utilize it in reference to a posting I wanted to make on this subject thread, and was trying to avoid scanning it into a digital format.

    Thanks
     

  15. brian eiland
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    brian eiland Senior Member

    Aerodynamic Simulation OF Upwiind and Downwind Sails

    I haven't had time to review this paper...just ran across it while looking for something else. But it sure looked like a paper that belonged in this subject thread:

    AERODYNAMIC SIMULATION OF UPWIND AND DOWNWIND SAILS
    http://www.nautica.it/superyacht/517/tecnica/sail.htm
     
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