Regarding the spiral slipstream, I don’t think smaller diameter necessarily gives smaller spiral slip stream? It would if rpm remains constant, the bigger prop stirs more air. But for equivalent power, smaller diameter means smaller amount of air must be affected more vigorously?? -Multiflier

I absolutely agree that accelerating a smaller diameter column by a greater change in velocity can use a comparable amount of power. There remains, however, another factor. If the difference in air velocity between the two situations is considered, we see that there is more rapid loss of energy due to friction from the smaller, faster column. This factor is, I presume, the main reason we do not see direct jets being used in heavy transports, but rather the much larger diameter enclosed fans which transfer the high velocity jet to a much larger volume of air that is then discharged at a more optimum velocity nearer to the aircraft's design cruise speed.

How this factor works out when applied to the spiral slipstream remains to be analyzed, but we know that if the same thrust is generated, the change in velocity of the smaller slipstream must be significantly greater. We do not know if the smaller prop is turning faster resulting in a faster rotating slipstream, or if it is being generated by an increased pitch with increased induced drag and likely, therefore, a more concentrated rotational component resulting. If we assume that the smaller prop is sized and pitched to be equally efficient to the larger one, then the likely result is a more concentrated spiral slipstream. I do think, then, that we would see the more highly concentrated force, moving at increased velocity relative to both the airframe and the surrounding air, degrade at a rate faster than the less concentrated force. This would argue for lesser affect at the removed distance of the vertical stab. No?