I'm not sure of the origination of the original post copied below, but I thought I
would make a comment. The spring bias method of trim certainly has the benefits described, but it cannot be assumed to be "equivalent" to a standard trim tab. An elevator with a trim tab will "fly" hands-off at the same angle regardless of speed while the spring bias will allow it to go to a more neutral position as the speed increases. If the elevator is biased up it will go up more as speed goes down and if it is trimmed down it will go down more as speed is reduced. Is this good or bad? I don't know. I worry about the low-speed regime where the elevator is biased in the up direction. As speed is reduced the spring will increase the up elevator position, reducing the amount of nose-up force required at the stick (compared to a trim tab in a fixed position). The spring bias also increases the stick forces required to move from the trimmed position. Like I say, I don't know of a spring bias system is
better or worse, but it is certainly different in terms of pitch stability. That being typed I have flown many hours in Cessnas with spring-bias rudder trim and it didn't seem to be a bother.
Gary Casey
ES #157
Previous post:
The earlier LNC2 was designed with a trim system that used a spring bias applied to the elevator pushrod to control the overall
elevator position. It was controlled by a small cruciform trim lever that moved the biasing springs forward and aft. Dick Reichel invented his trim wheel to replace the lever, allow finer and more precise trim adjustments, and to provide folks with the same sort of trim wheel that they had flown with in any number of certified aircraft.
Trimming the location of the entire control surface is probably aerodynamically preferable (less drag) to moving a trim tab to use the airflow to move the control surface, and the removal of the trim tab eliminates its weight and complexity, as well as those of the servo motor and linkages... ie, less counterweight required for balance. There are probably other aero issues, but I leave those things to folks more knowledgable than I in these matters.