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My friend's L235/320 had a Mac servo in
the left aileron with a small tab just in back of the aileron trailing edge.
What I found most troublesome about this method of trim is that it is totally
dependent on airspeed to accomplish the trim force. By that I mean that if you
have a slight lateral unbalance from fuel, passenger, or baggage, the amount of
trim tab deflection must change when you change speed. So if you have an
unbalance, and are in trim during cruise, as you slow for approach and
landing you will continuously have to increase the tab deflection. I decided to
get rid of this feature in the plane I was building, so I incorporated a
trim based on applying a constant trim force to the ailerons which
caused the aileron deflection to change with speed to always match the aero
force on the aileron from dynamic pressure. To this end I made a trim system
consisting of a phenolic drum in the console with 1/16" cable wrapped
twice around it which went down and then out to a spring on each side
attached to the bottom of the control stick. The springs I used are the
over-center springs from the L235 main-gear, which has a spring- rate of
5lb./in.; this gave a combined force of 10 lb./in. for the two. A pre-tension
force of about 10 lb. was included in the springs when attached. This drum in
the console was driven through a knob which turned the drum through 4:1 gears.
It's an absolute joy to set it and forget it.
By the opposite token, I installed a
mechanically-driven trim tab in my elevator in place of the spring-driven force
on the elevator push-pull tube, since speed-variation of trim is a desired
feature of elevator trim since it tends to maintain a constant speed, whereas
the spring trim maintains a constant force. As CG changes with fuel depletion,
it is necessary to change the trim force. This is almost automatic with the
elevator trim tab. I drove the tab through nylon-lined bicycle brake conduit
with solid wire instead of the stranded. Much, much lighter and more
flexible than Bowden cable. Additionally, I have attached a spring from the
flap-actuating arm to the elevator push-pull tube in the center console. This
spring gives an up-elevator drive as the flaps are lowered to help, almost
completly, to maintain speed trim during landing.
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