I borrowed this idea from the GlasAir aircraft and my Piper
Toamahawk. I'm using the MAC trim servo out of the old elevator
to move a shaft fore and aft which pulls on a set of springs to bias
the elevator push rod fore and aft. While this trim system does
work, it doesn't work particularly well and is still in need of some
refinement and tuning.
2007 update- While this elevator trim does work, it doesn't work well and I do not recommend using this method. - Jeff
The issue is that the aerodynamic loads against the springs change with
speed. Full nose up trim was inadequate and full nose down trim
would let it trim for cruise, but could never be flown hands off as the
nose would contantly wander up and down with significant altitude and
speed variations. Converting to a trim tab with the new tail made
for a hands off cruiser.