There are two textures for blurred propeller propeller_80.dds and propeller_80_opacity.dds, the first one is the diffuse component and the second one is the alpha (alpha is in "ati1/bc4" dds format photoshop doesn;t support it use nvidia texture tools)
Here is the material definition from cessna:
{
"name" : "propeller_80",
"diffuse" : "1.0,1.0,1.0,0.0", // the fourth parameter is important, it has to to be zero, it
// is the weight between fresnel and alpha opacity
// transparency 0 means that only alpha is used for opacity
"Ax" : "0.04",
"Ay" : "0.04",
"m" : "1.0",
"tex_diffuse" : "propeller_80.dds",
"tex_normal" : "",
"tex_env" : "",
"tex_opacity" : "propeller_80_opacity.dds"
},