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Boot Strapped Lift

  1. Dec 1, 2004 #1
    A school mate of mine was joking when he said you can lift your wind tunnel up if you mount the airfoil to the sides with the high Angle of attack and fast enough airflow. Is this true? If you have say a rectangular glass wind tunnel (fish aquarium shape) and put a large size wing and fast airflow, will the whole thing lift off the table? Or does the wing push down on the bottom of the air and wind tunnel? Say the wind tunnel weighs 1000 lbs for all parts and you found a airfoil that can make lift of 1100 lbs, will the whole thing lift itself by the bootstraps so to speak?
    Thanks :surprised
     
  2. jcsd
  3. Dec 1, 2004 #2
    As a good friend of mine always says...

    "You can't lift yourself up to the ceiling by pulling on your collar."

    However, this analogy does not apply here.

    Think about it this way. All you're doing is making flow pass over a wing to generate lift. Consider three cases:

    1) A plane is in open air with two wings extending from the cockpit and is propelled by a motor and propeller.

    2) This same plane is surrounded by a box extending from its wingtips up and down and connecting above and below the cockpit, but is still driven by the motor and propeller.

    3) The plane in 2) has the motor put in front of the box and the propeller at the front open edge of the box.

    These three cases are the same. Airflow is being passed over a wing to generate lift on the vehicle. It doesn't matter from where the airflow is generated.

    Now, why don't we see ducted planes flying around. First, the box (or test section) is mass that doesn't contribute to the lift. Second, the box would stifle airflow near it making the tips of the wings less efficient. And third... there is no third, the first two make it so inefficient as to be impractical. However, it is theoretically possible that with a light enough test section and a powerful enough (and light enough) motor that such an airship would be possible.

    Cheers...
     
  4. Dec 23, 2004 #3

    Q_Goest

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    Control Volume

    Consider this: Put a control volume around the wind tunnel, with the boundry on the OUTSIDE of the tunnel. Now do a force balance on it. At the inlet and outlet, I would assume there would be only horizontal forces. For the sides of the box, as well as the top and bottom of the box, there would be no forces. Now, the net force on the box can be seen to be zero. The box would not lift up.

    So although it would seem the box around an aircraft should still fly, I doubt this is true given the above. The problem I see is the forces and momentum change of the air going over the wing are counteracted on the inside of the box.

    I'm 99% convinced this is a reasonable arguement, though what bothers me would be a counter suggestion of "what about a large, weightless box?" ie: if large enough, how could the wing have any affect on the box many miles away? Good question!
     
  5. Dec 23, 2004 #4

    russ_watters

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    Just to clarify: The way the question is worded sounds like we shouldn't assume the inlet and outlet flow is horizontal. The airfoil inside a tunnel can affect the inlet and outlet flow and if it does, the entire tunnel will have a net lift. However, you are right in that real tunnels do have horizonal flow - they have flow vanes to keep the air straight and even until it gets to the test section.
     
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