During a test flight following a maintenance event, the pilot of the helicopter took off and navigated to a landing zone near the airport. He conducted a normal landing and hover flight without incident for about 5 minutes. The pilot then took off again, climbed to about 600 ft mean sea level, and initiated a straight-in practice autorotation. About 50 ft above ground level (agl), he started the landing flare, while “simultaneously fully rolling on throttle all the way.” As the helicopter approached 15-20 ft agl, the helicopter was level, forward momentum slowed, and “it became clear the helicopter was struggling to maintain lift and we began settling.” Subsequently, he held the controls as still as possible and attempted to maintain level attitude, however, the helicopter landed hard in the grass. The tail boom sustained substantial damage. There pilot reported that there were no preimpact mechanical malfunctions or failures of the helicopter that would have precluded normal operation.