Guys, this is my initial thought as I'm designing the electrical system. I'm trying to keep it as simple as possible but well protected. There's a few other items not shown because they will be connected to circuit breakers. Those are the ECU, Flap Motor, Fuel Pump 1, and Fuel Pump 2.

Please let me know if you see anything wrong. I'm not sure which side of the Master Relay the power IN to the key should be connected to. The ignitions run through the key so it seems that I would want that powered even if I have to turn the master off incase of a fire. So perhaps it should be connected to the other side of the relay?


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Nice diagram.  What software did you use to produce it?  Initially I noticed that you do not have an alternator field power source.

John-there's nothing difficult or tricky to making this. I just used MS Paint which is free on every computer. I just drew it line by line and added the text. Takes about 17 seconds to learn to use MS Paint!

You're right I don't show the alternator in the diagram. The UL Power engine has a regulator/rectifier with a wire going directly to the battery. It's only a 12 gauge wire though so either that's large enough for what it does, or I'm missing something somewhere!

Here's a pic of the wiring diagram that's in the manual for the UL Power engine. I'm sort of using this and other sources to draw my own simple diagram I can understand. You can see here how the alternator connects to the battery.

You show a 30 amp fuse in line to the battery -- the starter motor draws hundreds of amps (that's why you use very heavy wire for the battery and starter motor cables) and it all comes from the battery. Most aircraft wiring has the heavy power lead going directly to the master relay. A fuse is a failure point and if you had one there it would have to be rated to carry the full load of the starter motor which would be a truly massive fuse.

If your engine is electrically dependent you should NOT have the ignition going through the master relay, for sure. I would not even have it use the heavy wire from the battery to the master relay, myself. I would have a power lead hotwired directly to the battery to power your iginition, ECU, etc that needs power at all times. You are correct in wanting to have it be able to keep running with the master switch turned off. You might even want to consider a secondary battery that you can switch your hot DC buss (the circuits hotwired to the battery) over to. It could be as simple as a 12 volt drycell lantern battery of some kind of rechargeable battery. If a rechargeable battery it should be kept charged while isolated with a diode to make it impossible for it to feed power to the normal electrical system. Whatever your backup battery is it should be strong enough to run your ignition, ECU, fuel pump, etc long enough to get on the ground with the engine still running. In a "normal" electrical problem you can run your hot DC stuff off the normal ship's battery until it runs down and that should be enough to fly someplace and land. That emergency battery would be your last resort, something that will power the needed stuff just long enough to get you on the ground if you run down your primary battery or if the normal battery should suffer a catestrophic failure of some type.

You might want to look over some of the wiring diagrams people with Corvair conversions have posted. The Corvair does not have magneto ignition or an engine driven fuel pump if converted the William Wynne way. You need electicity for the ignition and the fuel pump or your engine stops. Rather than trying to build from scratch see if you can find some proven systems to borrow ideas from.

I would also suggest you use the UL Power concept and their suggested wire diagram as a base - they know what their engine likes. I would still prefer to see a secondary or emergency battery included but that would be simple to patch into their concept. The UL diagram appears to not bother with a master relay or an external starter relay which is very common in homebuilt aircraft. A 30 amp fuse and a toggle switch (or a 30 amp switch/circuit breaker) provides power to the electrical system (lights, radios, etc) and the starter motor heavy power is hotwired to the battery as it is in modern cars. The starter relay is built onto the starter motor and you run a light gage wire from the starter button to the starter relay on the motor. You do not need a master relay or an external starter relay. I would also prefer to see a "hot battery buss" that powers the stuff you NEED for flight - fuel pump, ECU, ignition. I don't like UL power running them through the master switch. Those things should run no matter what position the master switch is in.

Your diagram is very pretty, that's a good idea to do it with Paint. I might borrow your good idea there!

I've noticed that the UL Power manual seems to conflict with itself on different pages. Here's a pic of another page in the manual that shows the ECU connecting directly to the battery (through a switch and 15A fuse). Or maybe where it says "Power Supply Batter +" it means to the + batt bus. But I like the idea of connecting the ECU directly to the battery with a switch. Now if anything fails (except the battery) the ECU is still powered.

I like this switch from SteinAir. Since it would connect the ECU directly to the battery, I don't want it turned off by accident in flight, and I don't want it turned on on the ground. This is a lockable switch-you have to pull the switch to activate it.

Mark, love the videos! I too am uncomfortable with the ECU and pumps powered after the contactor.

I have an always hot feed to the ECU and pumps and would love to compare with yours. Any snip its you can share?


Hey Bob.

Any links to some of those Covair schematics? I too have a UL350 and am looking at main wiring designs for a VFR machine. Good advice above.



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