Hi Marc, There are no impedance differences between GA helicopters and fixed wing aircraft. Helicopters normally use curly cords and a single NATO plug whereas GA headsets normally use the two separate jack plugs. There is, however, a lot of difference between GA and military headsets. It is possible to use military headsets in GA with a special adapter which includes a mic preamp which runs off the "mic audio" voltage supplied from the GA radio. The adapter would also provide impedance matching for the phones circuitry. GA phones are 600 ohm whereas military phones are much lower impedance (around 8 ohm). Hope this is of some help. John (retired avionics LAME)