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The Magnus effect

  1. May 20, 2008 #1

    wolram

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    http://en.wikipedia.org/wiki/Magnus_effect

    Any one have an idea what to use for the gas (globe) of 1 mtr dia for a model of a Magnus effect lifter?
     
  2. jcsd
  3. May 20, 2008 #2

    NateTG

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    Huh? You want a 1 meter size globe (sounds like a weather balloon) for Magnus effect stuff? Why not a cylinder?

    I had very good results doing stuff with a cardboard oat drum spun by a lego motor for a HS science experiment.
     
  4. May 20, 2008 #3

    wolram

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    Eventually i want the lifter to carry a load, the only restriction i have is storage and transport, 1 mtr seemed a good compromise, allthough i have no real idea of how much it will lift.
     
  5. May 20, 2008 #4

    djeitnstine

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    As far as I know the original experiment was done in a wind tunnel with a spinning cylinder.
     
  6. May 20, 2008 #5

    wolram

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    http://www.geocities.com/k_achutarao/MAGNUS/magnus.html

    Newton in 1672(Did this guy have something to say about everything?) noted how a tennis ball's flight was affected by spin. In 1742, Robins showed that a transverse aerodynamic force could be detected on a rotating sphere. (Hence it is also referred to sometimes as the "Robin's Effect"). The first explanation of the lateral deflection of a spinning ball is credited by Lord Rayleigh to Magnus, from which the phenomenon derives its name, the "Magnus Effect". Rayleigh also gave a simple analysis for a "frictionless fluid," which showed that the side force was proportional to the free stream velocity and the rotational speed. This was all before the introduction of the boundary-layer concept by Prandtl in 1904.

    The Robins effect sounds better.
     
  7. May 21, 2008 #6

    NateTG

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    "Lifter" seems like an odd description since it needs non-zero relative airspeed to work. It seems like the simplest design would be some kind of autospinner -- a spinning kite tethered to a tractor prop.
     
  8. May 25, 2008 #7
    Magnus effect

    Spinning balls moving through the air in the real world only generate drag not lift. The Magnus effect is caused by uneven friction drag around the spinning ball. The uneven friction drag is caused by the spinning ball being pushed into the oncoming air not unlike a spinning car tire being pushed into the ground, both tend to turn a circular force into a more linear force. Because the Magnus effect is caused by friction drag, surface preparations that cause more friction drag on the balls surface cause more Magnus effect. The more the ball spins while going through the air the more Magnus effect yet when determining the aerodynamic force that causes the Magnus effect the very large fact that the ball is spinning is totally ignored. Its pretty obvious that they are determining the lift from the relative airflow caused by the balls motion through the air when it fact it is caused by its motion through the air as well as it motion while in the air (rotation). Calling the Magnus effect lift is based on the false premise that the ball is not spinning, and totally ignores the large influence rotation has on the relative airflow that is influencing the ball. Not very scientific but it shows the amount of skew of actual occurrence that is acceptable when writing formula.
     
  9. May 25, 2008 #8
    Welcome to PF and all that! (where are the professional greeters when ya need um?)

    Hu? one premise of the magnus effect is that the ball is spinning.
     
  10. May 25, 2008 #9
    At one time I thought it might be fun to construct a model airplane where the wings were replaced with fuzzy paint rollers. With the art of electric model flight well developed and the lower RPM of electric motors, it should be feasable.
     
  11. May 26, 2008 #10
    First thank you for your gracious welcome.

    The fact that the Magnus effect is caused by the spinning of the ball as it moves through the air is never ignored by texts or me. The spinning is ignored when determining the aerodynamic force that causes the Magnus effect. If a non-spinning ball moving through the air were to generate a force perpendicular to its flight path this would indeed be lift. Lift is most accurately defined as being perpendicular to the relative airflow that caused it and if the ball is not spinning its relative airflow is caused by its flight path through the air. There are three ways to generate relative airflow, you can move an object through the air, you can move air over an object or you can move an object while in the air (rotation). The relative airflow that is influencing the ball while it is producing the Magnus effect is caused by it motion through the air as well as its motion while in the air (spin). The large influence the spinning has on the balls relative airflow is totally ignored because aerodynamic force is determined solely on just the balls motion through the air. The relative airflow caused by the balls motion through the air alone will never produce the Magnus effect yet this is exactly what the aerodynamic force that causes the Magnus effect is determined by. Hope this clears it up some.
     
  12. May 26, 2008 #11
    Good grief Roy. Lift is defined as perpendicular to relative airflow. Drag is defined as oppsite the relatative airflow--the friction you talk about. Abit, friction is not lift.
    The rotating ball or cylinder preforms the same function as an airfoil; generating a captive vortex in an airsteam. So--i don't know what your argument is. Of course the Magnus effect is caused by unballanced drag over top and bottom surfaces; that's the whole point. It is this drag that is responsible for generating the superimposes vortex on the airsteam. I suspect we're both arguing the same point, perhaps.
     
    Last edited: May 26, 2008
  13. May 26, 2008 #12
    Unfortunately lift has many definitions but it is most accurately defined as being perpendicular to the relative airflow that caused it. Drag is always in the direction of the relative airflow that caused it not opposite it as you said. If drag was opposite the relative airflow that caused it sailboats could sail directly into the wind. The friction that I talked about always had the word drag after it, this should be dead giveaway that this friction is not lift. The rotating ball or cylinder does perform the same function as an airfoil, the difference (big difference) is one is spinning one is not, one produces its function using drag and the other used lift. A paddle wheel and a propeller on boats do the same thing the difference is one produces is using drag and the other used lift. It is the same as a squirrel cage fan and a conventional house fan. Drag like lift is infinite in direction, they can do the same things, and I have plenty of examples will be glad to share.
     
  14. May 27, 2008 #13
    "If drag was opposite the relative airflow that caused it sailboats could sail directly into the wind. "

    right, my error. As for the rest, get a book. Im done.
     
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