AFTER REFUELING WITH 12.9 GALLONS OF FUEL, THE PILOT TOOK OFF WITH ONE PASSENGER ON BOARD. THE PILOT STATED THAT AFTER LIFTING OFF WITH 104% PRM, HE BEGAN A NORMAL SHALLOW TAKEOFF WITH 25 INCHES OF MANIFOLD PRESSURE. HE REPORTED REACHING ABOUT 75 FT AGL AND 50 KTS WHEN HE NOTICED THE ROTOR AND ENGINE SPEED STARTED DROPPING TO APPROXIMATELY 100 RPM. HE STATED THAT MORE POWER WAS APPLIED BUT THE RPM CONTINUED TO DECREASE. HE STOPPED CLIMBING, LOWERED THE COLLECTIVE, AND BEGAN DESCENDING. SUBSEQUENTLY, THE HELICOPTER TOUCHED DOWN WHILE STILL TRAVELING AT ABOUT 15 KTS WITH ABOUT 85% RPM. THE MAIN ROTOR SEVERED THE TAIL CONE AND THE HELICOPTER WENT OVER ON ITS NOSE AND TO THE RIGHT. THE DENSITY ALTITUDE WAS ABOUT 2800 FT. DURING THE INVESTIGATION, AN INSPECTION AND OPERATIONAL CHECK OF THE ENGINE REVEALED NO PREIMPACT FAILURES.