On March 23, 2003, about 1215 eastern standard time, a Bell 47D, N74120, was substantially damaged during takeoff at a private heliport near Princetown, New York. The certified flight instructor was not injured, and the student pilot received minor injuries. Visual meteorological conditions prevailed for the local instructional flight. No flight plan was filed, and the flight was conducted under 14 CFR Part 91. According to the instructor, the helicopter was kept in a hangar at the student's house. The instructor preflighted the helicopter, checked weather and NOTAM's, and then boarded, with the student occupying the right seat. During the first of three flights, the student conducted upper air training, and then flew to a private airstrip for fuel. During the second flight, the student conducted some traffic pattern training, and then flew to Duanesburg Airport (4B1), Duanesburg, New York, to practice takeoffs and landings. The instructor had the student conduct approximately 18 takeoff and landings. Each was executed to and from a hover, and the student had difficulty maintaining rotor RPM during the maneuvers. They then departed, and flew back to the student's house. The student completed an "approach to the ground," and the instructor asked him to practice hover taxiing, before terminating the flight. As the student started to apply collective, the instructor looked inside to check rotor RPM. When he looked back outside, the helicopter was in a nose high, right side low attitude. The instructor lowered the collective and applied full left cyclic, but the roll to the right continued. The main-rotor blades impacted the ground, and the helicopter came to rest. The instructor added that during all three flights, and accident sequence, he did not experience any mechanical problems with the helicopter. Examination of the wreckage revealed that the rear of the right skid was in an area of mud.