Don't clobber me if I'm wrong, because I'm no Aerodynamicist, but I think there are certain wings with a flat enough bottom and high enough upper camber that they could concievably develop lift even at a slightly negative angle of attack...(am I full of s**t?). Clearly the P-51 wing isn't one of them, and those pictures do seem to show the wing is pointed downward into the relative wind. That's why I think Buzz might be on to something....
From what I understand ,unfortunately not. The AOA is measured from the angle of the wing chord to relative wind as you know. The angle of the chord is drawn from the tip of the leading edge to the tip of the trailing edge. The chord angle will change with the deployment of slats and flaps.
An extremely efficient wing designed for pure lift will sustain enough lifting force for level flight at a given regime in the lowest posible "Positive" AOA. So theoretically, you could get to positive 0.001 degrees and be producing lift in level flight. But as soon as the chord dips negative AOA, then you will be producing potentialy lower pressure under and higher pressure over the wing which results in a loss of altitude.
Now with a non conventional fixed wing aircraft it is possible to have level flight with a slightly negative AOA as long as there is another source of lift.. ie. Lifting body, vectored thrust, etc.
Now why would a designer incorporate a very high upper camber wing vs little or no lower camber (High lift wing)? By doing so, they are reducing the need for high AOA CHANGES THROUGHOUT THE FLIGHT ENVELOPE TO MAINTAIN A GIVEN LIFT IN RETURN FOR HIGHER INDUCED DRAG. We will see a good inverse representation of this when A2A releases their Starfighter. It has such razor thin airfoils, that critical angle of attack will be reached in a much narrower flight envelope than a more conventional wing with more aggresive camber. This limitation comes with the function of much less induced drag throughout the same flight envelope. Trying to recall, but I believe that up to 20-30% of the lift generated by the starfighter was due to airframe lift.
Edit... I just browsed through some old aerodynamics material to confirm my statements. I found that it is possible to create positive lift with a very aggresive asymetrical airfoil in a negative AOA situation. Cessna made a very high lift wing where the "Zero Lift Line" was measured at -4 degrees AOA when measured at the chord. Now remember, thats the zero lift line, meaning thats the point where the airfoil creates no lift. We are talking about maintaining level flight, so even though a wing may produce positive lift in a negative AOA situation, to sustain level flight, the amount of positive lift must be equal to the opposing forces. (Gravity / the weight of the aircraft).
Please, if I am wrong, somebody correct me, as this is what I have learned over my past years.