In cockpits there are a number of rotating knobs, like radio tuning, OBS, etc. Also rudder and elevator trim are rotating inputs.
For radio tuning, I still use the numerical keypad as frequency input. Therefore I mounted a small keypad to the controls console, cut out of an old keyboard, and wired it to the main keyboard number keys. In Flight Unlimited III, OBS needs to be rotated by means of mouse. No keyboard input seems to exist.
For adjusting the elevator trim, you have to press pre-defined trim-up and -own keys repeatedly. Same for rudder trim. Real aircraft use trim disks that you have to rotate to change the trim setting.
Since elevator trim is used very often, it adds lots of realism in the sim when rotating the trim disk a couple of notches in stead of pushing a key a couple of times.
In the Cessna, the rudder and elevator trim disks need to be rotated 2.5 turns for trim limit to limit. In Flight Unlimited III, the elevator trim key needs to be pressed 180 - 200 times limit to limit, the rudder trim about 110 - 120 times limit to limit. (both seem to vary a bit over the different airplanes)
To make that amount of switch pulses over 2.5 turns is best
done with a rotary encoder. This is a little device that looks
like a potentiometer, with three connections, but internally has
two wipers that pass a number of contacts when rotated. The mid
connection is connected to the wipers center, while the
outer connections are the contacts that each wiper passes. The
reason for two wipers is the fact that the circuit that decodes
the on/off cycles of the wipers also has to distinguish whether
the rotary is turned clockwise or anti clockwise. This is
accomplished by setting the inner and outer contacts 90o
out of phase.
This type encoder produces 24 pulses per rotation. For the elevator, 2.5 turns should produce about 200 pulses. I added a gear ratio of 1: 2.5 between trim disk and encoder, giving 150 pulses for 2.5 turns. For the rudder trim I did not add any gear, as it is less used anyway.