Here is an image of a document I made awhile back on how to control the speed of a wiper motor used in prop building. I've been answering a lot of posts in the prop forum regarding motor speed control so I thought I would post this here. I also have it in PDF for email if you find the image hard to read.
I hope this helps anyone contemplating a wiper motor driven prop. More than happy to answer any questions on the topic. Good luck!