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Trim Switching

penguin

Well Known Member
In another thread an RV-9 builder relates a trim runaway experience, rather than cluttering up that thread I have started another. Its in the electrical section as the most appropriate place.

The first RV-6 I flew had mechanical trim (pitch & roll).
Initially I though it was poor, I later realised it is probably the best - since flying with electric trim.
My conclusion is simple is best. The fewer switches the less confusing and the less likely to break.
Don't put switches behind the throttle when using a quadrant.

We all know that trimming at cruise speeds needs only a quick press on the trim switch. Because the speed the trim motors run at trimming at cruise speeds requires only a short press, the bonus is trimming at slow speeds is quick enough to be useful. A speed controller for the trim motor can also be useful, they can also include include a time limit to constrain any trim runaway (what ever the cause). My own view is to avoid inserting systems such as the VPX in the trim system, the draw backs outweigh the benefits. I am also yet to be convinced on the need for autotrim offered by some autopilots - keep it simple.

My current RV-6 has electric pitch trim and no roll trim. Electric roll trim going in soon as I can't install the manual version. My view is roll trim is very useful. Pitch trim switching was via a 4 button stick top and also a rocker behind the throttle. Throttle is in a quadrant between the seats. A selector switch close to the rocker selected either the stick top or the rocker. The stick top was difficult to use as the buttons had very light spring tension, it was never clear by feel if the switch had been pressed. All of a sudden there was too much trim. Using the rocker was easier but open to an interesting snafu. Not much trimming is needed, once set after take off re-trimming is usually not required until entering the pattern. It is comfortable to rest the heel of your throttle hand just behind the quadrant to adjust the levers with fingers only. At that point it was easy to press the trim rocker - all rather confusing for a few seconds. I could work around it but why. The other problem was the trim indicator had lost a few segments so it was never clear where the trim was set, and the indicator was next to the rocker requiring a look right into the cockpit.

My solution is to use only one trim switch mounted just ahead of the throttle - easy access for both pilots - and works forward and back, so sense is clear. The switch (2 way momentary, centre off) has relatively strong springs so requires a definite push to engage. It was $25 or so, but has a distinctive top so is clear what it is. I find it much better than the previous set up. The trim indicator is now in the panel so is always easily in view.

I do understand the desire for stick top trim switches but are they really necessary?
What is wrong with one breaker and one switch near the throttle?
I don't see a VPX like system in my future, if I did I wouldn't run trim power through it (too much to potentially go wrong).
The LED indicators are rather poor, EFIS indicators would be better, but that is separate from the switching (and assumes an EFIS is installed).
A switched speed controller (airspeed or flaps) would make life easier but is by no means required. I haven't yet used on and am debating on whether to install or not.

Hope this helps some who are still building

Pete
 
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