Im an expert on crash diormas for aircraft, so I think I can help you here. What you do is do just the basic reserch into what kind of damage might occure to a helicopter. If you can find internal diagrams of the chopper so that you can see what might be under the skin, should a fatal blow have been stuck to it. Then what you do is apply your general know how and read up on how to damage models in general. Then take a black and white side view profile of the chopper, and mark out where you want to put the damage. But be carful, in doing the damage, its a far greater sin to over do damage on an aircraft, or in your case a chopper than it is to under do it. This also goes for a destroyed model. All the damage has to be realistic. But dont forget to weather the chopper appropriatly also. Usually it is the paint job that can make or break a good crash scene. So I give my best of luck to you and hope that I have been of some help.