In response to the posts from NavyEEL and Harnu, I decided to do some testing with the AH-64 at the lovely Malden airport.
I found 2 things, Harnu is correct, you can certainly complete a turn without loosing airspeed. However, it's a really unnatural feeling, totally flat turn (even a little visual roll, if it did nothing else, would make it feel a lot more normal). Also, I apologize for exaggerating the speed loss in my initial reply.
Second, the hit point damage model seems to consider the damage of the helicopter more than the speed of the aircraft at impact .This can also be made very clear in game by ramming into trees at 200 kph with nary a scratch as a result (it's not as rare as you may think). At about 21 meters, a damaged helo is very likely going to explode after a crash. A healthy one, which has simply had it's engine turned off, will survive with only minor damage from a greater fall height. This may seem reasonable at first thought, but a proper autorotation can make an emergency, engine-out landing as soft as a normal one.
As to the "usually explodes in real life", I doubt it's likely to happen. Fuel/Ammo probably only explodes after a crash one would not have escaped from otherwise. Autorotations are much safer than most people think, I've been through a 180 auto (granted, intentional), and it was quite enjoyable.