I've found a gem!
"Visual Search in Air Combat" (1990)
http://www.dtic.mil/cgi-bin/GetTRDoc...f&AD=ADA241347
Here a really interesting graph (page 6 of the original document):
Of course there are factors like camos, sunlight, haze ect...
I'm going to do some calculations to get the positions of ww2 fighters on that curve.
I would only remember to you that it's not a personal problem: all this discussion its to greatly improve the sim.