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Effects of Wings in a closed environment

  1. Jul 17, 2014 #1
    I was wondering if somebody could let me know if wings inside of a closed environment (tube of sorts) with air flowing over them would generate lift, or if the air flowing over them would be directed downwards onto the bottom of the tube, canceling any lift generated by the wings?

    The first wing you see is an auto gyro rotor at an angle, the second is a conventional airfoil. The arrow represents the direction of the airflow in the enclosed tube. The marks you see are simply question marks, oops.
     

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  3. Jul 17, 2014 #2

    davenn

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    Hi airex
    welcome to PF :smile:

    Have you heard of wind tunnels for testing the aerodynamics of scale models of aircraft during the design stage ?

    if so, therein lies your answer

    cheers
    Dave
     
  4. Jul 17, 2014 #3

    rcgldr

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    Take the case of a model aircraft flying in a horizontal circle inside a sealed box, another example of a closed system. As long as there is no net vertical component of acceleration of mass within the box, then the weight of the system is the total of the weight of the box, the air inside the box, and the model.

    Without the model, the air exerts it's weight inside the box via a pressure differential, lower at the top, higher at the bottom, so that the net downforce exerted by the air onto the box equals the weight of the air inside the box.

    With the flying model, the wings impart a downwards impulse onto the air, increasing the pressure differential within the box so that the net downforce exerted by the air onto the box equals the weight of the air plus the weight of the model.

    In the case of the tube with internal wings, any force exerted by the wings onto the supports is opposed by a force exerted by the supports exerted onto the wings. The force exerted by the supports onto the tube will eventually be opposed by the force exerted by the air onto the tube (assuming that the impulse doesn't carry beyond the exit end of the tube).

    The closed environment of a tube prevents a net downwash of air. To reduce this effect in a wind tunnel a relatively tall wind tunnel can be used. If the wind tunnel is relatively large compared to the model or wing being tested, the closed environment effects preventing local vertical wash components near the wing are reduced.
     
    Last edited: Jul 17, 2014
  5. Jul 18, 2014 #4

    CWatters

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    As above.

    If the tube is very long (compared to the chord of the wing) then the wing will not produce a net force that lifts the tube relative to the ground.

    If the tube is short then you have to think about what happens to the airflow as it exits the tube. Is the airflow vectored downwards?
     
  6. Jul 18, 2014 #5
    rcgldr gave you your answer.

    I would add that, although the system as a whole will not generate lift, the airfoils will actually generate lift for themselves that is greater than usual. This is because of "ground effect". Because your air foils are so large in comparison with the tube, it would be a very strong effect. In a wind tunnel test, the model is usually several times smaller than the diameter of the tunnel.
     
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