Helicopters appear to interact with objects somewhat randomly as far as impacting is concerned. Lightly bumping a building may result in hull/engine failure and ramming a tower at 20mph with the intent of destroying the tail rotor results in no damage at all! I do not know if this is helicopter interaction or terrain/object interaction.

Also, there appear to be "sweet spots" when firing at helicopters, for example, when firing threw windshield of AH-9 into upper bulkhead behind the crew with a small caliber weapon I noticed that only 1 shot resulted in engine and main rotor failure.