The real Staaken L was fuel trimmed. To allow this in FS2002 it is necessary to provide fore, central and aft tanks.
In real life the forward tank(s) would usually be empty before flight. FS2002 does not permit empty tanks to be defined. All tanks have to start full as the content defines the volume of the tank.
YOU MUST open the fuel menu (aircraft/fuel) and REMOVE ALL THE FUEL IN THE FORWARD TANK BEFORE FLIGHT.
To match the available gauges the forward tank is defined as the left tank and the aft tank as the right tank. The top of the fuel menu is the forward fuel. The 200 USG in the left (forward) tank must be removed.
If you fail to remove the fuel which FS2002 needs to define the volume of the forward tank you will be attempting to take off with a gross overload and so you will probably fail.
In flight select appropriate tanks to feed the engines and also move fuel between the 3 tanks (using the fuel menu) to trim the aircraft so that it will climb or cruise hands off at a safe attitude and airspeed.
Do not alter the total fuel remaining whilst pumping for trim.
The flight dynamics provide as an alternative slow and weak elevator trim. Even if you intend to use this rather than fuel trimming in flight YOU MUST REMOVE THE FORWARD FUEL BEFORE FLIGHT.
If you intend to use fuel trimming as in real life leave the (non existent) elevator trim set to about 15% nose down throughout the flight. The aircraft invokes with this trim set.
