Yes, free. And yes, that good.
What HTR does is process dozens of user-defined parameters and make forces act upon your aircraft in a realistic matter. HTR uses Fred Naar's own physics library, which took him quite some time to develop and tweak making it one of the most complete and accurate pieces of code for helicopter flight simulation physics.
This is a companion discussion topic for the original entry at https://www.helisimmer.com/articles/htr-helicopter-total-realism/