THE PILOT REPORTED THAT SHORTLY AFTER TAKEOFF, HE NOTICED THE RPM'S BEGAN FALLING. HE STATED THAT HE INCREASED THE THROTTLE AND LOWERED THE COLLECTIVE, BUT THE RPM'S CONTINUED TO DECREASE UNTIL THE LOW RPM WARNING HORN CAME ON. AN AUTOROTATIVE LANDING WAS MADE. DURING TOUCHDOWN, WHILE THE HELICOPTER STILL HAD FORWARD SPEED, THE SKIDS DUG INTO THE GROUND AND THE TAIL MOVED UP INTO THE PATH OF THE MAIN ROTOR BLADES. THE HELICOPTER THEN TURNED AND CAME TO REST ON ITS SIDE. AN EXAMINATION REVEALED THE AIR-FILTER CARTRIDGE WAS COVERED WITH OIL, DIRT AND OTHER CONTAMINENTS; THE 4 V-DRIVE BELTS WERE FROM DIFFERENT MANUFACTURERS AND WERE OF DIFFERENT LENGTHS; THE ENGINE DRIVE ACTUATOR (PULLEY) VALLEYS WERE DEEPLY GROOVED AND WORN, THE EXHAUST MUFFLER HAD PIECES OF THE FLARE TUBE FLOATING LOOSE INSIDE THE MUFFLER. DURING A TEST RUN, THE ENGINE OPERATED NORMALLY. A TEARDOWN OF THE TRANSMISSION REVEALED NO PREIMPACT FAILURES.