I guess it has something to do with the station loads.
Go to the aircraft.cfg file and open it with notepad. Scroll down and look for section that says "Station Loads".
From here on, it's up to you. You can either move, add, or remove a station load [weight].
I am making an F-117/A Have Blue [predecessor to the Night Hawk] and it's dynamics. This aircraft did go nose down at the start of the flight. It's flight dynamics are now being finished by someone who is helping.
Anyways. Try it. It just might work. If it doesn't, then look for the position of the aircraft's engines in the file. If the engine position is on the rear, then the thrust of the engine is probably pushing the aircraft up by the tail. Try moving it forward.
If this does not help, then I am stupified.
