Save a flight with the problem aircraft, make sure the trim is set to the position you need it to be at when flying.
Open the saved file and look in the [Controls.0] section at the ElevatorTrim= setting:

[Controls.0]
SpoilersHandle=000.00
FlapsHandle=000.00
LeftFlap=000.00
RightFlap=000.00
GearsHandle=000.01
Gear1=100.00
Gear2=100.00
Gear3=100.00
YokeY=050.00
YokeX=050.00
Rudder=050.00
LeftBrake=100.00
RightBrake=100.00
ParkingBrake=100.00
ElevatorTrim=050.00
RudderTrimPct=0
AileronTrimPct=0

The number should be between 0.00 and 100.00. With 050.00 being the middle section of the trim wheel/lever. I'm betting that the range of the Trim (in the MDL file itself) is something like UP 10 and DOWN 30, which would mean that ElevatorTrim=050.00 would be at DOWN 10. So to fix it, you would change it to something like ElevatorTrim=075.00 to fix it. AFAIK, you can only do this with a saved flight, not in the aircraft.cfg.

Mark