Chapter 17 Pitch and Roll Trim, Landing Brake and Center Console

This section is not per plans.

There are several options for elevator trim.  Most that have implemented electric trim use either the Atkinson or Strong trim systems.

There are a couple of plans for the Atkinson system.   The later one uses a rod that is about an inch longer than Vance's original.  

Getting the spring pressure and the stick travel so it feels right and having enough movement if the trim motor runs away has been a challenge that is still being worked on. 

I used the mill to make some smaller springs that go inside the original springs to help provide more force at full trim down.

   I rethreaded the longer rod and Vance will test and see if it works when the weather is nice again. -Update - We're still not getting the right compression when allowing more travel. Update 5/4/2018 - the 30 .lb springs still don't allow enough travel.   I found a source with some 8" springs that will compress to 3". They're 3/4" in diameter and to avoid having to move the actuator mount, will require two parallel tubes. I'll try a POC today....Another update 5/18/2018- Vance said he'd probably announce his quick fix soon.   I'd like to come up with an improved design which would seem to need springs that are a larger diameter and take more than 3 inches to compress.   Or multiple tubes with a more complex interaction of components.  Final update - Vance opted to use a pin like gliders use for disconnecting controls if there's a run away trim.  He also rewired the trim switch and breaker so that trim power could quickly be turned off.

The elevator tube travels 1.6".   I used a scale and plotted the force to compress and stretch the springs that have the right force.   It takes about 30 .lbs to fully extend and about 16 .lbs to compress.   The original tube's length changes 1.7" from max compression to stretched.  The original actuator is about 24" long and has 3" stroke.

 Here's a simple graphic with some of the forces that has a nice "feel":
https://docs.google.com/drawings/d/1r6xrTR4MKgW9Uh6U7p5Yd-u8x8raV6ehccOanaHypew/edit?usp=sharing

 A little research on compression springs reveals that springs should be selected so their load is about 45% of their full capacity.  I found a bag of 10 ea.  60 .lbs springs on Amazon for $18 that have 3.5" travel and should have them Wednesday.

Also, while testing, the motor became intermittent.  Some strained and broken wires were found at the motor. 

The wires on an older Motion Systems Actuator broke so needed repaired.   There's only six strands each in black and red OEM wires.   I replaced the old wire with high strand wire of the same gauge.  Some little U's made of .042 safety wire holds the brushes and springs in place.   The original wires were cut where they broke.  Insulation pulled off to make little pig tails that connect to the brush leads.  The new wire was twisted and soldered to the stubs that were left.

Some shrink tube and a wire tie should keep the wire breaking in a bad place.

I made a few springs that are wound the opposite direction so they will work properly inside the larger spring.   The first couple of springs didn't turn out perfectly, but I found if I set the mill to a right hand feed at 20 threads per inch using .050 music wire, it took about 7' of wire to make about a 5" spring.   The mini-lathe doesn't have enough torque to wind .080 wire, so I turned the chuck by hand.  The small springs didn't provide enough force.

My first tube had +/- 2" of travel with about 14 .lbs of force.   I rethreaded the inside rod so it was extending an inch less and put one of the smaller springs I made inside the larger spring to provide more force near the end of its travel.  

In order to increase the rod travel a little more than the original design, I cut about 1/4 shoulder on one plug and on the sliding bushing.

This is the cheap HF mini lathe.   I upgraded the tool post to a quick change model and quickly found out the tool holders were poor quality.  It's easy to spend hours learning how to use this.   It will wind .050 music wire to make a spring but .080 was to stiff so I had to turn the chuck by hand.   The arbors to make different sized springs were fun to make.