delta,
If the TR gets to VRS at 15 knots, it means that the TR passes through the zero thrust point at this paltry speed. In other words, the helo needs no TR thrust because the weathercock stability is so strong at 15 knots that the fin provides all the anti-torque. I find this hard to believe.
Most TRs that I have analyzed reach the VRS point at 25 knots or more (30 to 35 knots for the S76, for instance.)
Also, it is my opinion that LTE is not achieved when the helo is at 15 knots, because at 15 knots the MR torque is low, since the MR is near/above ETL. LTE occurs when the MR torque comes rushing in as the speed decays to near zero. The concept of LTE as a mystery and caused by wondrous aerodynamic effects is a necessary component of the myth that single rotor helos can all get LTE, and the accompanying myth that we can't design to get away from LTE. Both myths are simply wrong.
I have a 1970 Russian helo aerodynamics text that shows the 5% decay in TR thrust available when the MR wake finds the TR. No big mystery, IMHO. Also, let me state that any TR that runs out of poop at 17 knots IGE will cause LTE if the helo is maneuvered near OGE under the same conditions, since MR torque is the biggest cause of LTE, because when MR torque is high, the TR thrust is fully consumed.