I seem to recall rules for R/C aircraft, but I don't know if they were AMA rules (Academy of Model Aeronautics), government rules, or generally accepted rules (like Cooper's rules of gun handling). Under one of the three, flying over people would be a serious no-no, and your AMA insurance isn't going to cover you if you hurt somebody.
If those propellers are within 10' of somebody, I'd say your craft is clearly a physical threat. If it's more than 10', I don't see how that t-shirt is going to bring you down.