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
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.