Thanks. In an earlier post I said I was skeptical that there was much spiraling going on. My reason for questioning that like you describe. I used to fly the DC-6. The engine nacelles, especially the longer inboard ones were a fairly regular cylindrical shape until they met the wing. There was always a plentiful supply of engine oil running everywhere, and there were plenty of streamline streaks of dirty oil on the nacelles which pretty clearly indicated where the air was flowing (you could see how it flowed around the intake and oil cooler scoops, for example) but one thing that you couldn't see (and I studied it pretty closely) was *any* indication that the air flowing down the nacelle was spiraling. That's where I started to develop my skepticism about the "spiraling slipstream" theory.