I've decreased, at least I believe, the sensitivity of the joystick down to approximately 26%. The C-172 is much more stable. It will take more experimentation to determine what the best settings would be.
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To test what is happening by changing the joystick properties, I put Flightgear into Fly-By View. I was able to observe that changing the joystick's property values changed how the rudder and elevators mover
proportionality instead of full movement but with less joystick sensitivity. To rephrase, when I reduced the Joystick property value for the rudder by 50% the rudders movement was decreased by 50%. I had hoped that the rudder would still move by 100% with the joystick property value reduced by 50% thereby effectively making the joystick less sensitive.
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Since posting, I found this article:
Writing Joystick Code: Part 1. I have not yet experimented with it, but it would seem to be the correct approach to reducing joystick sensitivity. (December 17, 2015 edit)