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Date:  2-23-2014
Number of Hours:  26.00
Manual Reference:  
Brief Description:  Autopilot Autotrim

My Trutrak autopilot pitch control has been troubled by frequent requests for pitch trim adjustments, occuring every few seconds in other than smooth air, requiring minute trim adjustments, typically a 1/8 inch movement of the trim wheel. Annoying over time, I took the aircraft to TruTrak, and they thought it was due to floorboard movement when the pitch servo applied corrections. Accordingly, I made a structural reinforcement which was placed over the floorboards to increase stiffness:

http://www.youtube.com/watch?=R3X1R8IzIo8&list=UUIQaOnwx3erRNlGnnLsu3NQ&feature=c4-overview

That made little or no improvement. So, I purchased the TruTrak Autotrim module, and found a relatively inexpensive Edo-Aire autopilot servo IC465-1-303R on Ebay. This particular servo had a 12 v motor, but a 24 v clutch solenoid. However, I found that moving the clutch return spring to a less "springy" setting allowed it to work with 12v. I used an ELK-924 solid state relay powered by the autopilot master switch and triggered by pin 6 of the pitch servo connector to actuate the clutch solenoid when ALT is selected on the TruTrak unit. The servo came with a capstan bridle. I had to fabricate capture blocks out of 6061 so as to fit the trim cable and ultimately had to shorten them to fit in the space available, so that only two bolts were used per capture block. A phenolic cable comb was used to splay the trim cables so as to avoid conflict between the roll cables and the two trim cables.

After the first test flight, low speed on the module seems to do ok for level flight; the initial intermediate setting applied way too much correction and caused porpoising. After numerous flights, I now longer need to manage the trim, so that's good. Unfortunately, the Tru Trak unit still announces the need for trim changes from time to time, so I have turned the annunciator off.
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Rear view showing cable comb

Rear view showing cable comb

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Top view of fold down servo panel

Top view of fold down servo panel

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Pitch servo, Autotrim module and floorboard stiffener

Pitch servo, Autotrim module and floorboard stiffener

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