I've seen two ways of simulating spinning props.
One used clear disks with the blurred prop with yellow tips painted onto it.
You can have a look at any one of a million in-flight photopraphs to get a pattern to paint onto the disk.
The other was an article I saw some time ago - it was about a photo-etched bit of metal that was kinda in the shape of the blurry moving prop. There were many different sizes (diameters) and I think there were several different patterns depicting two, three, and four bladed props. They didn't look too bad, actually.
Now, the nasty part - I have absolutely no idea when or where I saw these photoetched spinning props - I'm really sorry about that.
Hopefully, someone else out there saw the same article and has a better memory than I do and can tell you where they saw them.
If you want, email me and I can give you more details about what they looked like - I can make a fairly clear drawing of what I remember.
Cheers,
LeeTree