I went to start my engine today, threw on the electric starter, and the prop came loose! took off the spinner, wiped off the nut and spinner, and tightened everything again, and it came loose again! I've had this problem with both planes i have owned. Whenever i put a spinner on, the prop likes to come off. I've noticed the whole area around the prop gets fuel or grease on it. I'm guessing this is the problem. I washed everyhting off with rubbing alcohol, and am now waiting for it to dry up. I've never seen other people (Even beginners at my field) have this problem, yet it happens to me all the time. Any suggestions?