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.