Power Distribution and Buses

[Rynoth]

I'm sure you're right I'm just not electrically fluent enough yet to sketch that out! So far I've been looking for the simplest possible way!

To quote John Monett, You are not building the Space Shuttle. Keep it simple.

[sonex1374]

Other things to note my planned electrical system includes 3 servos, I want to do long leg cross countries so I want to throw in a yaw damper if possible although I suspect that either that wont be possible or I wont find an elegant mounting point for the YD servo so that would improve power budget.

A yaw dampener (or autopilot servo) isn't really needed on the rudder. The plane does not wander at all in yaw, and when a fixed rudder trim tab is adjusted properly the plane will have no yawing tendency at cruise speed. You might need some rudder input at slower speeds, but that's not a problem, nor is that a time when you'd be flying under the autopilot. It's not even worth fabricating an adjustable rudder trim tab.... just not something to worry about.

My advice is to skip the 3rd servo. You'll never miss it.

Jeff

[sonex1374]

The power stabilizer seems very simple to plan and install and it costs $250. Perhaps I could do a backup battery system for less than that, I just don't know yet how to wire that off the top of my head.

Chris,

The folks over at the AeroElectric Connection have been kicking around several ideas for a short-term boost of voltage during cranking that would prevent the voltage sag to the EFIS while the starter is cranking. One solution has been to install a booster transformer that only kicks in when cranking the engine. This would take something like 11 volts and boost it to 14 volts (example....don't know that exact voltages or have a part number). I'm sure you could contact them for specifics, or research your own based on your system's average bus voltage while cranking your starter.

The linked drawing shows how this would be wired in (the section in blue labeled "Brown-out Booster"). http://www.aeroelectric.com/PPS/Adobe_A ... /Z01P3.pdf

Here's a recent message in this thread to get you started. http://www.matronics.com/digest/digestv ... c#MESSAGE1

[gammaxy]

When you operate the starter, you have crazy amperage spikes resulting in voltage spikes up and down as the starter is actuated, and then lugs down a little on each compression stroke as it slows down with the load on the compression stroke, using more amperage, and eases up while the engine is not on a compression stroke and turns more easily. I damaged the gas discharge numbers on a KX155 by accidentally starting my engine with the avionics turned on after the engine stalled on a landing roll out. Should have turned the avionics off before using the starter. I recommend turning everything off while using the starter! Those avionics are expensive, to purchase, and to repair!

Wish I knew what happened in your case. What I measure on my Main/Avionics bus doesn't have any dangerous transients during engine start.



I believe I see voltage dips due to the compression strokes as you describe until the engine began firing at ~1 second. I continued to hold the starter until about 2.5 seconds. You can see a little ripple that I believe is due to the secondary ignition coils charging.

[lakespookie]

Other things to note my planned electrical system includes 3 servos, I want to do long leg cross countries so I want to throw in a yaw damper if possible although I suspect that either that wont be possible or I wont find an elegant mounting point for the YD servo so that would improve power budget.

A yaw dampener (or autopilot servo) isn't really needed on the rudder. The plane does not wander at all in yaw, and when a fixed rudder trim tab is adjusted properly the plane will have no yawing tendency at cruise speed. You might need some rudder input at slower speeds, but that's not a problem, nor is that a time when you'd be flying under the autopilot. It's not even worth fabricating an adjustable rudder trim tab.... just not something to worry about.

My advice is to skip the 3rd servo. You'll never miss it.

Jeff

Thanks for the Info Jeff, That would def save me some power. and some cash =P.

[lakespookie]

Spaceman, wiring the second bus just entails driving a second pair of power wires to the unit. Garmin already has the inputs, so you would need one switch to send power from the battery to the bus 2 power and a second switch to enable charging of the battery, Normal startup you power the avionics from the backup avionics power with the charge cuircuit open, then once you have stable power kick on the full avionics bus disconnect the backup power, or leave it on up to you, then activate backup avionics charging. note you will have to send power to both your G3X and the Engine interface unit but pretty much all the garmin avionics support dual bus power so in theory you could fully power all your avionics via the back ups but I feel like that's a bit much and not worth all the extra wiring. you really only care about the efis and the engine interface unit. If you look at the interconnect drawings it should be clearly labeled.

[Spaceman]

The power stabilizer seems very simple to plan and install and it costs $250. Perhaps I could do a backup battery system for less than that, I just don't know yet how to wire that off the top of my head.

Chris,

The folks over at the AeroElectric Connection have been kicking around several ideas for a short-term boost of voltage during cranking that would prevent the voltage sag to the EFIS while the starter is cranking. One solution has been to install a booster transformer that only kicks in when cranking the engine. This would take something like 11 volts and boost it to 14 volts (example....don't know that exact voltages or have a part number). I'm sure you could contact them for specifics, or research your own based on your system's average bus voltage while cranking your starter.

The linked drawing shows how this would be wired in (the section in blue labeled "Brown-out Booster"). http://www.aeroelectric.com/PPS/Adobe_A ... /Z01P3.pdf

Here's a recent message in this thread to get you started. http://www.matronics.com/digest/digestv ... c#MESSAGE1

I'll give that a look, thanks!

Wish I knew what happened in your case. What I measure on my Main/Avionics bus doesn't have any dangerous transients during engine start.

That's a handy graphic to refer to. None of your instruments have trouble with that drop to like less than 8.5 volts? I'll have to dig through the Garmin encyclopedia later and see if there's a specified minimum.

[Spaceman]

Spaceman, wiring the second bus just entails driving a second pair of power wires to the units. Garmin already has the units, so you would need on switch to send power from the battery to the bus 2 power and a second switch to enable charging of the battery, Normal startup you power the avionics from the backup avionics power with the charge cuircuit open, then once you have stable power kick on the full avionics bus disconnect the backup power, or leave it on up to you, then activate backup avionics charging. note you will have to send power to both your G3X and the Engine interface unit but pretty much all the garmin avionics support dual bus power so in theory you could fully power all your avionics via the back ups but I feel like that's a bit much and not worth all the extra wiring. you really only care about the efis and the engine interface unit. If you look at the interconnect drawings it should be clearly labeled.

Definitely true most of the Garmin stuff has at least two power inputs. Do you know of any diagrams of how this would be wired up? I think I understand what you're saying but I am terrible at visualizing electrical stuff. Big picture-wise that makes sense though, thanks!!

[Spaceman]

That's a handy graphic to refer to. None of your instruments have trouble with that drop to like less than 8.5 volts? I'll have to dig through the Garmin encyclopedia later and see if there's a specified minimum.

I looked through the G3X manual but I couldn't find an actual min voltage spec. Just this note on page 27-1:

14. TO MINIMIZE THE CHANCE OF THE SYSTEM RESETTING DURING ENGINE CRANKING, THE OPTIONAL REDUNDANT (DIODE OR'D) POWER INPUTS MAY BE CONNECTED TO AN AUXILIARY BATTERY (SUCH AS THE TCW TECHNOLOGIES INTEGRATED BACK-UP BATTERY SYSTEM) OR STABILIZED POWER INPUT (SUCH AS THE TCW TECHNOLOGIES INTELLIGENT POWER STABILIZER IPS-12V-8A) TO MAINTAIN THE NECESSARY LRU MINIMUM INPUT VOLTAGE. HAVING A STABLIZED SOURCE OF POWER DURING ENGINE CRANKING SHOULD ALLOW THE SYSTEM TO PROVIDE CONTINUOUS ENGINE INDICATING SYSTEM (EIS) OPERATION DURING ENGINE START AND MAINTAIN ANY DESIRED PRE-FLIGHT SYSTEM SETUP OR FLIGHT PLANNING THAT WAS ACCOMPLISHED PRIOR TO ENGINE START. VISIT
http://WWW.TCWTECH.COM

So this actually says to connect the optional power inputs to the power stabilzer... I'll have to figure out if what I was planning will work or maybe I need to change some stuff!!

[gammaxy]

That's a handy graphic to refer to. None of your instruments have trouble with that drop to like less than 8.5 volts? I'll have to dig through the Garmin encyclopedia later and see if there's a specified minimum.

All my instruments stayed on just fine during that startup. This is not always the case, especially in the winter with a weak battery, but I'd say it's true 90% of the time at least. All of the instruments have built-in power supplies that probably smooth out the big dips for a little while. Another thing to consider if you are really just interested in oil pressure during startup is to install a mechanical oil pressure gauge. The recommended sender mounted to the engine block ends up going wonky after 100-200 hours or so anyway.

I thought about all this stuff when building and tried to err on the side of simplicity. Partially because the airplane is so small and every addition is one more thing you have to tuck away someplace and one more wire you have to reliably join. I just have a single bus using automotive fuses. As simple as mine is, I'll be honest and say it's still an embarrassing rat's nest behind the panel.