In real life MI-8 can do horizontal take-off and taxiing when heavy loaded. However in game its impossible to drive forward without taking off.
I see 2 possible reasons to this.
- Rear suspension is too soft, which makes going forward impossible.
- Angle limits on main rotor are wrong.
Here is a video of real mi-8 taxiing forward.
https://www.youtube.com/watch?v=JZDZ7_f4D3E