Lag filter implementation

I am implementing a RC UAV in FlightGear and need a little help with
the control system...

The control system is fly-by-wire, with a receiver and standard RC servos.
The servo response is similar to a first order lag, so I am using a
lag-filter with C1 = 600.

This is the elevator command system:

<COMPONENT NAME="Pitch Trim Sum" TYPE="SUMMER">
       INPUT        fcs/elevator-cmd-norm
       INPUT        fcs/pitch-trim-cmd-norm
       CLIPTO       -1 1
</COMPONENT>
<COMPONENT NAME="Elevator Control" TYPE="AEROSURFACE_SCALE">
       INPUT       fcs/pitch-trim-sum
       MIN          -20
       MAX          20
       GAIN         0.01745
</COMPONENT>
<COMPONENT NAME="Elevator Actuator" TYPE="LAG_FILTER">
       INPUT      ................   
       C1            600
       OUTPUT    fcs/elevator-pos-rad
</COMPONENT>



What should the input be for the lag filter? 
Should there be an output from the elevator control?

Thanks, Sam (again!)

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