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Creating Lift?

  1. Nov 7, 2009 #1
    Let's say we have a rectangular surface or something. How about a piece of plywood. If we were to blow air say using a fan at the top of the wood and keep the air "calm" at the bottom, would it create lift?

    I know it's not practical but in theory should there be a slight lift?
  2. jcsd
  3. Nov 7, 2009 #2


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    There is an incredible amount of engineering work done on flat plates in flows. Yes. A flat plate will develop lift if you blow air fast enough by it.

  4. Nov 7, 2009 #3
    Try it yourself by blowing on one side of a piece of paper. Hang it off the top edge of a square edged table with some tape. Direct the airflow with a card or a soda straw.
  5. Nov 7, 2009 #4


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    Negative lift or at best no lift. The air just aft of the fan has higher than ambient pressure, and continues to accelerate as it's pressure returns to ambient. Change this so the fan is above and aft of the wood, sucking air instead of blowing it, and the pressure is reduced, which would result in lift.

    I get the impression that you're trying to correlate faster moving air with lower pressure, the Bernoulli principle. The issue is that Bernoulli principle establishes a relationship between pressure and speed assuming that no work is done (total energy is constant). A fluid or gas will accelerate from a higher pressure zone to a lower pressure zone, and the rate of acceleration versus pressure differential allows a relationship between speed and pressure to be approximated by Bernoulli equation (ignoring issues like turbulence). The issue is how the pressure differential was created, and if there was a change in total energy during the creation of that pressure differential. The more work done by some interaction between a solid and fluid or gas, the less "Bernoulli like" the reaction of that affected fluid or gas to that interaction. Away from that interaction boundary, the fluid or gas can be approximated by Bernoulli equation (again, ignoring issues like turbulence).

    This NASA article about propellers explains the point I'm trying to make:

    We can apply Bernoulli's equation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed by the engine (by the propeller) violates an assumption used to derive the equation.

    Last edited: Nov 7, 2009
  6. Nov 8, 2009 #5


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    That's not what the OP is getting at. He wants to flow high speed air over the top surface while leaving the air at the bottom surface unchanged.

    http://www.cortana.com/20061218_Boundary_Layer_of_Flat_Plate_jpg.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  7. Nov 8, 2009 #6
    In any case, the answer is yes. A stream of air directed over a surface generates low pressure on the surface.

    Wikipedia "On aircraft with propellers mounted on the wing, the propwash from the engine will accelerate the airstream over the portion of the wing directly behind the propeller. This results in greater lift behind the propeller than at other spots on the wing."

    The earliest, or one of the earliest attempts to exploit this effect was an airplane where the wing about the propwash was bent in a U-shape. Something like a half-ducted fan. Somewhere on the internet is a old photograph that I can't find at the moment.
  8. Nov 8, 2009 #7


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    This only works on a cambered airfoil due to coanada like effect or what I call void effect, where following the cambered surface corresponds to reduced pressure and downwards acceleration. If the "wing" was a flat board with no angle of attack, then the higher pressure of the prop wash above the "wing" and the ambient pressure below the "wing" would result in a downwards force on the "wing".

    Wiki explains I call void effect in the article on wing:

    In that case a low pressure region is generated on the upper surface of the wing which draws the air above the wing downwards towards what would otherwise be a void after the wing had passed.


    My point here is that it's the pressure differential that produces the lift, not the speed differential. A flow created via mechanical interaction can increase or decrease both the pressure and speed of that flow by peforming work during the interaction. For example the exhaust from a jet engine is both high pressure and high speed.

    A wing is a poor example of Bernoulli effect. A venturi tube is a much better example. Direct the output of the fan through a tapered tube with decreasing diameter, and the friction of the walls, viscosity, and Bernoulli principle will result in a higher speed, lower pressure, smaller diameter air flow.

    Getting back to the OP, in order to create lift, you need to accelerate air downwards in an open environment (normal flight), or create a pressure differential if in a closed environment (piston in a cylinder), or a combination if in a partially closed environment (like a wing in ground effect).
    Last edited: Nov 8, 2009
  9. Nov 8, 2009 #8
    Jeff, the prop wash is low pressure air. I suppose you could invoke the Bernolli effect, but notice that the airstream downstream of the propeller is low pressure as evidenced by the constricted airsteam. A 5 minute table-top experiment with a soda straw, that I discribed above, should be convincing without going into theory.
    Last edited: Nov 8, 2009
  10. Nov 8, 2009 #9


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    The constricted airstream is an idealized case based on constant mass flow, and occurs regardless of pressure (compression effect reduces this by a tiny amount). If the mass flow is constant (this ignores viscosity effects on the air surrounding the prop wash), then the cross sectional area decreases linearly with an increase in speed, (ignoring the minor compression effect), regardless of the pressure. As noted in the wiki article mentioned above, the prop wash (aft of the propeller) pressure is higher than ambient.

    A spinning propeller sets up a pressure lower than free stream in front of the propeller and higher than free stream behind the propeller.

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