A supplementary power drive system for providing additional power to the drive system of rotary wing aircraft that powers the aircraft main lifting rotor and tail mounted torque compensating rotor or propeller in a manner as avoids overloading the power capacity of the main rotor transmission. The supplementary power system, utilizing the power of a usually installed auxiliary power plant as a source of additional power, bypasses the main engine powered main rotor transmission in providing power to the tail mounted rotor or propeller and main rotor if the main transmission cannot absorb it. This is accomplished by incorporating a supplemental power transfer unit comprising a one-stage gear train into the tail rotor/propeller drive system that extends from the main transmission to the tail rotor/propeller in which a gear from the gear train connected to the main transmission and meshing with gears connected to the tail rotor/propeller drive shaft also is connected to the power output from the auxiliary power plant. This allows power from the auxiliary power plant to go directly to the tail rotor/propeller and bypass the main transmission, or is available to the main transmission for driving the main rotor in certain flight modes when the main rotor transmission can absorb the additional power.