One of the first things i could see from the pilot seat in a BO-105 was a 110 degrees dive to demonstrate the possibilities of my favourite ship up to date. I will never forget the first look at the Attitude indicator going into the deep, deep, deep brown... " (really i looked at the instrument, why i couldn't believe the incredible dive angle) The instructor came calm with the words "... look to the mast bending moment, allways stay in the green.... "
I will admit now, i was shocked!!! I had flown up to this point some really different types of helicopters but it wasn't to imagine what the small beast could do! Ok, some 100 hp and a little bit more cabin room needs the "pilotship", but thats impossible due to some constructional and CG problems.
The hydraulics and the fuel system are able to work proper on negative Gs. They are full pressurized and closed.
Aerobatics are prohibited due to some simple reasons.
1. Most pilots are not aerobatic experienced. That could easy kill you! The green area is small on some situations. In the described dive you have only round about 6-8 sec from a IAS 0kt to bring the ship back to level or you will exceed the VNE with all the problems in aerodynamic and solidity.
2. The a/c will be overstressed, especially with the rigid rotor. Such a rotor brings the stress and vibrations direct to the fuselage with a lot of cracks as the result .
3. You lost all your insurance and may be your job.
4. The a/c is not certified for such "flights".
Thats the reason why EC pilots will today never show (such) aerobatics on official dates. The German Army shows sometimes such procedures, but only very limited to special trained pilots and they have problems with the cracks and other damages. No civil operator will pay such bills.