Saturday, March 5, 2016

Wiring Discrepancy Identified In Trim Circuit When AP Panel Module Installed

Time to break out the popcorn this another long post ... believe I have tracked down a wiring discrepancy in the plans when incorporating the Dynon Autopilot Panel module during new construction. There appears to be a discrepancy involving the trim motor wires in the Fuselage connector at pins 31 & 32.


When the AP Panel is installed, the trim motor’s power wire and the wires from the panel mounted trim switch are moved to the AP panel  module because it now becomes the controller for the pitch trim motor. Step 2 on page 31B-4 of the plans instructs the builder to place a battery on the two white trim motor wires  from the WH-P30 trim cable to determine which of the two white wires in the cable moves the trim tab UP when a positive voltage is applied to the wire. Once identified, that wire is called the “Trim UP” wire because it moved the trim tab up when a positive voltage is applied to it … (do not confuse this with nose up trim). Next the plans instruct the builder to insert the now identified “Trim Up” motor wire in pin 31 of the Fuselage connector and the remaining white  trim motor wire goes to pin 32. Of course, following the plans the “Trim Up” wire was placed in pin 31 of the Fuselage connector as instructed. I can’t say with certainty this is a mistake for all RV-12’s but it appears to me it is a mistake for RV-12’s under construction and installing the optional Autopilot Panel module. Below is the rational for that conclusion.


Return from the future 4/9/2016: Vindication!!! The trim motor worked precisely as I predicted it would based on the  circuit analysis below. The Van's plans are indeed in error ... when installing the Autopilot & Knobs Panel modules during the building process, the wire identified as +Trim Up definitely needs to be placed on pin 32 of the fuselage connector to make the trim move correctly when pressing the panel mounted trim switch.


As a side note for builders: When the trim tab on the stabilator moves UP it produces nose down trim. So don’t confuse the “Trim Up” movement of the trim tab with actual nose up trim. The physical movement of the trim tab is opposite to that of the desired trim. Another words, pressing the panel mounted switch calling for nose down trim should result in an upward movement of the trim tab. Likewise, pressing the panel mounted trim switch calling for nose up trim should create a downward movement of the trim tab.


As mentioned above, when installing the Dynon AP Panel, the wiring for the trim motor gets changed in such a fashion that the AP Panel module becomes the controller for the trim motor and the trim up/down switch on the instrument panel is wired directly to the AP Panel module. Because of this, I wanted to verify how the panel mounted switch is wired so I can wire in the corresponding wires coming from the hat switch on the Tosten CS-8 grips to the co-pilot trim inputs on the AP Panel module.


This brought me to studying the latest Van’s schematic showing the Dynon AP Panel's wiring diagram and comparing that to how the Dynon SkyView instillation manual says the AP panel works with the trim motor. This got me suspecting the polarity appears to be wrong at the Fuselage connector ... seemingly there is an error in the instructions on page 32B-4 in that the "Tab Up" wire should go to pin 32 and not 31. Of course, the Dynon manual's description may be incorrect, but doubt it since the manual has been out for a few years and revised at least once ... and the wiring call outs remained the same.


Felt it was time to do “ a little FBI 007 type investigatin' ” to coin a phrase from a Dr. John song. As an exercise to understand and verify the wiring, I used an Ohm meter to check the entire circuit beginning at the panel mounted trim switch. First it was verified that pressing the panel trim switch forward calling for nose down trim placed a ground on pin 18 of the connector on the back of the AV-50001 Switch & Fuse Module … it does. Next the brown wire (F-2035) attached to pin 18 of the connector on the WH-00031 wiring harness that plugs into the back of the AV-50001 module was tested. Per the schematic, the brown wire (F-2035) applies the ground coming from the down trim switch to pin 4 of the connector on the back of the AP Panel module labeled “Pilot Trim Down” in the Dynon manual. OK ... so far so good.
Testing to verify pin 18 on the switch connector wiring harness that plugs into the AV-50001 Switch and Fuse module is connected to pin 4 of the connector that plugs into the back of the AP Panel … it is. Looking closely, one can see a yellow/blue wire was placed on pin 18 of the connector and connected to the Ohm meter … at the connector for the AP Panel a wiring pin was placed on pin 4 so the meter reading could be taken.


Here is where the wheels fall off: For nose down trim, the trim tab needs to move UP meaning a positive voltage needs to be applied to the previously identified "Tab Up" wire that was placed in pin position 31 on the Fuselage connector earlier when completing the instructions in section 32B-4 step 2.


Reading the description given in Section 18 of the Dynon SkyView Instillation manual when a ground is placed on pin 4 (Pilot Trim Down) the AP module applies voltage to the trim motor by placing a negative voltage on pin 7 and a positive voltage on pin 8 of the connector on the back of the AP Panel module.   Following the wire from pin 8 of the AP Panel module back through the AV-50000A connections module it goes to pin 32 in the Fuselage connector … WHICH IS INCORRECT because as previously determined on page 32B-4, the wire placed at pin 31 needs the positive voltage to move the trim tab UP.
The Autopilot wiring  harness was plugged into the AV-500000A control module and as can be seen where my finger is pointing, the Ohm meter was connected to pin 4 on the connector for the AP Panel module to verify there is was continuity to pin 32 of the Fuselage connector … there is.


Assuming the Dynon Instillation manual is correct, and the AP module will place a positive voltage on pin 8 when down trim is called for, that positive voltage is going to pin 32. I submit to the reader that when the AP panel is installed, the instructions given in step 2 on page 32B-4 are incorrect. The correct wiring when the AP module is installed in a non-flying RV-12 should place the previously identified "Tab Up" wire into pin position 32 of the Fuselage connector ... and the remaining white wire goes to pin 31.


Today I flopped the two wires so now the “Tab Up” wire is on pin 32 and the remaining white motor wire is on pin 31. Now when nose down trim is called for, the positive voltage on pin32  will run the trim tab up.
The white wire with the red sharpie marking my finger is point to is the “Tab Up” wire that was moved from pin 31 to pin 32.


If moving the trim motor wires turns out to be wrong (strongly doubt it if the AP Panel works as Dynon says it does), I will be sure to return from the future in a week or so and edit this post accordingly. But at this point in time, if the Dynon SkyView Instillation manual is indeed correct, by switching the trim motor wires at the Fuselage connector the movement of the trim tab should be correct when power is applied to the avionics and testing begins. The goal: Trim tab moves up when nose down trim is called for … Trim tab moves down when nose up trim is called for.


While the 15 pin connector for the AP Panel module was open, two wires were added. The nose up trim wire from the hat switches on the Tosten control grips was placed on pin 5 (co-pilot pitch trim in-UP) and the nose down trim wire from the hat switches was placed on pin 6 (co-pilot pitch trim in- DOWN). The wires were covered with heat shrink, installed and the connector was closed up.
Installing an additional two wires in the connector for AP Panel. The wires are from the hat switches in the Tosten grips. Nose up trim was attached to pin 5 (co-pilot pitch trim in - Up) and nose down trim was attached to pin 6 (co-pilot pitch trim in - Down).


Regular readers of the DOG Aviation Blog may recall when wiring the switches on the Tosten control grips, with the exception of the push to talk switches, all the stick grip switches were wired in parallel. I chose this method for wiring the hat trim switches on the grips because the RV-12’s panel mounted trim switch is wired directly to the Pilot pitch trim inputs on the AP Panel module. Logic inside the  Dynon AP Panel module’s built in trim controller gives the pilot's input overriding precedence over the co-pilot inputs. During critical phases of operation the pilot should have a hand on the throttle ... as such the panel mounted trim switch is within finger reach and capable of overriding the trim inputs from either stick since they are both wired to the co-pilot inputs on the AP Panel module. I felt this was a good safety feature should a passenger or myself accidently get snagged on the hat switch during critical phases of flight. The trim controller in the AP Panel module also has a built in adjustable two step speed controller ... so the trim motor speed can be slowed down when flying faster so as to not be so sensitive ... yet the motor can speed up when flying at slower speeds. The built in trim controller also has run away trim prevention logic … in that, it will only run the trim motor for five seconds at a time. Should a trim wire get shorted or a switch stuck, the logic will only run the motor for five seconds, as opposed to letting the motor run until the trim tab is moves to its full limit. All nice stuff in addition to all the extra features the AP and Knobs panels will add … which makes adding the Dynon AP Panel a much welcomed addition to the DOG Aviation RV-12’s avionics.