Did you know that the characteristic thump thump thump of a helicopter is caused the the leading edge of a propeller blade encountering the trailing wake vortex of the blade before it? I learned this nifty fact while serving on Jury Duty a few years ago. This year the Jury Duty summons was for the week after Christmas. A slow week. In the event I got called in, I was going to bring Forensics for Dummies to help pass the time.