pimmnz

Of couse, I have done the 'pin and string' test, or I wouldn't have given the results. The airplane was my Atlas, OS.61 and 11x8 prop and it was standing still at the time, I think as soon as it starts to move any displacement of the flow behind the prop will very quickly align with the FSD. You can all try it yourselves...Choose a model with the least fuselage behind the engine...As for offset and cambered fins, heck, even the WW1 Fokkers had them, I bet there are other WW1's...The Me 109 has a cambered fin, even the P51 has fin offset, but all to counter TORQUE. Much easier than the pilot having to hold a deal of rudder at any airspeed other than that that balances the torque. I think you might find that one of the gripes pilots held against the early versions of said '109, was that it really needed a rudder trimmer. Most of the others all had pilot adjustable trimmable rudders. As for 'spiral airflow', you could even inject smoke into the airstream to see if it is wrapped up around the fuselage, and down the other side, in the spiralling airflow. I have tried, with those IMAC models with smoke on, but have failed to detect it yet. If it were a detectable effect, then the exhaust streaks on the noses of most WW2 fighters should be aligned different, one side to 'tother. They don't appear to be, either now, or in photos from 'back in the day'. I have seen 'spiral airflow' but that was from a domestic fan, with really big wind shovels, static, and fairly slow RPM compared with a model airplane prop. And it had nothing behind it. I can confirm RMH's observation that the stream behind the prop appears to contract, but that should be no surprise, the moving air behind the prop will be at a slightly lower pressure (due to its velocity) than the surrounding, and the rest of the air does what it always does, and moves from higher to lower pressure. As for the visible spiral effect from prop blade tip vorticies, thay are simply standing vorticies showing where the blade tip was and in no way indicate the direction of the air behind the prop, else they would point backwards. As for the twin explanation, well, you could use the fan example, then whack a splitter (wing) across the flow, and watch what happens...but then most will already have guessed what does happen. Much more interesting is watching the streamers as you increase the AOA of your 'wing'...But this is all kids stuff, and one would have thought that enquiring minds would have already done much of this...As for the 'Published Explanations', I cannot guess, they may have been written as a simple explanation for pilots or laymen and entered the realms of fact simply by longevity. But then I have been told several times that I am an unprincipaled cynic...
Evan, WB #12.