Heres a link for you, if you go down to page 58 you will see a helicopter symbol under meaning of signal, as you scroll down there are more examples and descriptions of signals with the helicopter symbol next to them, these are from the Civil Aviation Authority in the United Kingdom and im not sure if they are correct for the USA but I hope they are of some help
http://www.caa.co.uk/docs/33/CAP637.PDF