Not sure how you would go about including a payload thats basically flexing and changing position with the relevant forces in ether FDM though.
The payload gets its position updated by an equation of motion computed in Nasal.
It feels its intertia and the gravitational pull, based on last and present position we have a velocity vector on which to operate drag, and the distance between load and helicopter is pushed into a rope force equation - when the rope is tight, it exerts a lot of force propertional to the tension, when it's not then there's no rope force. Oh, and we need to check if it touches the ground of course...
Assuming the payload is approximately symmetric that should do the trick - we have frame spacing, so we have a Delta t, we have present and last position, i.e. a Delta x, which combine into a Delta v - that's all we need to solve a discretized equation of motion.
It's not actually that complicated
