I don't have access to the book right now--its in a box, but, its basically a trim analysis if I'm reading the graph correctly. I've got some methodology on how to do it, all the formulas, etc. as it relates to the pitching moments of the wing. I guess if we can get the proper data from some WWII aircraft, we could plug it in.
The shift from a negative deflection to a positive deflection as speed increases shows the change in pitching moment of the wing over that speed range. Is this from an actual aircraft or merely an example curve for illustration purposes?