Have tested 11 additional tail dragger models.
At this point I think the following:
1 There is a general, programmed ground looping routine in YASIM.
2 Rate of turn is determined by aircraft weight and distance of CG from front wheels.
3 Initiation is a condition of individual aircraft speed (48 for the C18, 30 for the Jenny, 75 for the P40, etc.) .
4. In my opinion. It's somewhat overdone and unrealistic for most planes.
There are a few planes where dealing with it has been relatively successful, the L19 and the Arsenal VG33 being good examples.
With some planes, the tendency has been reduced by moving the CG close to the main wheels, but at the expense of nose ups, the Percival Mew Gull being a good example.
With the De Havilland DH-89 the tendency has been all but eliminated, but only at the expense of its having exceptional flight stability.
I attribute the good behavior of the L19 and the Arsenal VG33 to two things:
1.The CGs in both are only about 1.5 behind the wheels, as compared to 3 in the C18. (Enough to reduce the turning moment; but not enough to easily cause nose ups.)
2.The ground loop routine is triggered to initiate at relatively high (and fairly close to flying) speed for each (50 for the L19 and 104 for the Arsenal), which gives their vertical fins and rudders better ability to resist it.
Kind Regards