Don't forget, when the airplane is stationary the air is still moving past the propeller disk with good velocity. The vortices from the prop tip also move rearward with it making the spiral pattern, just as the wingtip vortices move downward after the wing has past. If the twist of the slipstream was actually as tight a spiral as the pictures suggest, control, and indeed level flight would likely be impossible. The actual twist of the slipstream is quite gentle by comparison.