On November 21, 2000, about 0900 Eastern Standard Time, a Robinson R-22, N8117Y, was substantially damaged while landing on a trailer in Londonderry, New Hampshire. The student pilot (SP) was not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the personal flight conducted under 14 CFR Part 91. The SP said he was attempting to off-load the helicopter from the trailer, which was parked at his residence. The trailer was equipped with mirrors that could be used to check the helicopter's skid position just before touching down on the trailer. In a written statement, the SP said that as he lifted the helicopter to a hover, "something didn't feel just right" and he gently started to lower the collective, with the intent of landing back onto the trailer. At that point, the helicopter began to rollover. The SP further stated, "As I gave a quick glance in the mirror, I saw that the right strut had jumped the one inch lip of the trailer and was going down over the side. I did try to force the helicopter back on to the trailer, but the rollover continued as the helicopter went out of control..." The helicopter impacted the ground and came to rest on it's left side. The SP reported 39 hours of total helicopter flight experience, all in the R-22. Additionally, he stated he had landed and departed from the trailer approximately one dozen times prior to the accident. Examination of the helicopter by a Federal Aviation Administration inspector did not reveal any mechanical malfunctions; nor did the SP report any.