[idonno]

If you have a mod that contains plane folder, find these two folders B-29(multi) and B-29(USA) inside these folder find propellor.mat files open those with notepad
and find this line:
Colorscale 1.0 1.0 1.0 1.0
change the last value to for example 0.6. lower value will give more transparency.

*If you don't have such mod you can create these paths like:
MODS\my_b29\3do\plane\B-29(multi) and
MODS\my_b29\3do\plane\B-29(USA) you can extract propellor. mat file by sfs extractor.

I have B29 and B29_Silver_Plate folders in the MODS folder. The B29_Silver_Plate has the two other folders you mentioned along with the propellor.mat files. I edited them and it worked fine for the Silver Plate version, but the B29 folder has no propellor.mat file in it.