THE PLT & A TECHNICIAN WERE PERFORMING REARWARD FLT TO TEST AN EXPERIMENTAL ENG INSTLN. THE MANEUVER USED WAS A REARWARD HOVER TO 20 KTS, FOLLOWED BY A RIGHT PEDAL TURN TO REVERSE COURSE & FLY AWAY. THE PLT WAS USING RWY 16 & THE WIND WAS FROM 260 DEG AT 12 KTS. THE PLT RPRTD THAT DRG THE RIGHT TURN, THE HELICOPTER ENTERED AN UNCOMMANDED RIGHT SPIN. THE PLT WAS UNABLE TO MAINTAIN ANTI-TORQUE CONTROL & LOWERED THE COLLECTIVE TO LAND. HOWEVER, THE HELICOPTER TOUCHED DOWN WITH SIDEWARD MOVEMENT, COLLAPSING THE NOSE & LEFT MAIN GEAR. THE TAIL ROTOR BLADES CONTACTED THE RWY DRG THE GEAR FAILURE & THE TAIL ROTOR GEARBOX WAS TORN FROM THE TAIL SECTION. THE PLT ORIGINALLY SUSPECTED A HYDRAULIC PROBLEM, BUT AN INSPN FAILED TO REVEAL EVIDENCE OF ANY PRE-ACDNT FAILURE OR MALFUNCTION. ELEVATION & TEMPERATURE WERE 9927 FT & 65 DEG, RESPECTIVELY.