How does the FAA decide whether an aircraft is deemed a helicopter?
The definition of a helicopter as well as other aircrafts is provided in 14 CFR Part 1 

http://www.ecfr.gov/cgi-bin/text-idx?SID=8403e178a4e1d0f8339151fe2529e600&amp;node=14:1.0.1.1.1&amp;rgn=div5

Manufacturers may seek clarification from the FAA about these definitions if/when there is a requirement .