ORIGINAL: gearup
Just have always wanted to know how and why rather than just-the-facts. I wonder why the tip vortices off of a prop would act differently than off of a wingtip, if they in fact do. I wonder why the slipstream would retain a rotational component from the prop, when it seems likely that most of the induced prop drag generating the thrust and the rotational component would disipate radially off of the end of the airfoil/prop. (hence q-tips to try to limit the effect). I understand that fluids tend to spiral into areas of low pressure, and move radially away from high pressure, so I wonder why the slipstream, being relatively high pressure would provide a sustaining environment for the rotating component generated by the prop.
Notice taken of the NASA study reference. It would be interesting to look at the testing in detail to determine what controls and variables were examined and observe the range of data derived.
Actually the paper is a NACA study from the 30's and in a 19 page report it skips over the one equation for the spiral in a half a column.
Now you're tripping over the vortex off the tip of the prop verses what the prop does to the air as it passes through it. They act exactly the same.
See this picture
http://www.zap16.com/images/kb04%20B...take%20off.jpg
The props are leaving tip vortices that are formed identically to the wingtip vortices. Here's where the catch kicks in if you look at the spiral,(and this may be what Stick and Rudder saw as the tight spiral around the fuselage) it goes the wrong way to support the slipstream spiral theory. Which is the air inside that outer defined spiral is slowly spiraling around the fuselage in the same direction as the propeller rotation due to the propeller imparting some energy to the airflow.