By the way, for those interested, there are more NACA reports to the ducts:

Ed posted this one: http://naca.larc.nasa.gov/reports/1951/naca-tn-2323/

these are some more, but I didn't read into them yet:

http://naca.larc.nasa.gov/reports/1945/naca-acr-5i20/naca-acr-5i20.pdf
http://naca.larc.nasa.gov/digidoc/report/rm/13/NACA-RM-A50F13.PDF
http://naca.larc.nasa.gov/digidoc/report/tr/43/NACA-TR-743.PDF

...the last one is not specifically about NACA ducts but about "Ducts for
cooling radiators within an airplane wing"


Thomas Jakits