hmmm... the experienced pilot who flew my first shuttle for the first time after i had trimmed it out had to fly a wide circle around himself to wait for it to descend 20 feet to land. then he took out a pitch gage and showed me that i had only -1 degree at low stick. i took it down to -3 and had no problem with boom strikes while learning to hover. it sounds like your throttle trim may be set too low, which would slow your head on descent and not give your engine enough room to recover before you bottomed out.