I would be interested

in other ideas on how to protect against runaway trim

Art,

 

In my 320:

 

1. Elevator:  Spring and Dick Reichel Walnutty geared trim wheel system - Run away trim is only a problem should I develop an uncontrollable tic in my trim hand.  Of course, flying from low humidity to high results in the wood expanding and the wheel getting awfully hard to move.

 

2. Rudder: Spring bias/servo operated system - The spring is just strong enough to provide right rudder assist at takeoff and just capable of keeping the ball centered at max cruise, both situations are at the opposite servo stops.  My leg (whichever one is required) can easily overcome the spring pressure at a reverse setting (actually flight tested) if, and only if, I remember to look at the ball.  A mis-trimmed rudder only induces yaw (usually a yawn) and the A/P (if used) is quite happy to provide the slight bank to overcome my forgetfulness.

 

3. Aileron:  Uh Oh!  Also a spring bias/servo operated system (like the Cirrus or Bonanza) - Requires very small servo movement to laterally trim the craft.  For safety sake, I used two springs for each direction (as in redundancy).  If the servo would run away I should have to hold two or three pounds (English System) in order to counteract this anomaly.  Hmmmmmm, maybe I will try just one spring per side thus requiring more servo activity to trim the plane.  No, upon second thought, if my equal wing emptying fuel system should hiccup I might actually need more trim to account for wing weight imbalance. 

 

If you have trim tabs, perhaps others will have suggestions like make them smaller or provide servo movement limits.  Slowing down should lessen the force required to overcome the run away trim tab.