This is NACA 0000 aerofoil:
This is just a straight line - roughly what a paper plane's aerofoil section looks like (I chose this aerofoil for its simplicity). If we put this aerofoil in a wind tunnel, then this is my approximation of what the air-flow around it might look like:
The air-flow isn't ideal; there is flow seperation already at the leading edge. But still, the aerofoil will produce a net positive lift.
This is explained as follows: through a convergent duct, the air-flow velocity (and thus dynamic pressure) increases. This is accompanied by a reduction in static pressure, such that the total pressure remains constant (the opposite is true for a divergent duct).
Is this true? and so will the aerofoil produce a downforce instead of lift in the narrow wind tunnel?