Paul Visk wrote:

>> Mark L., I noticed on your wiring schematic on your website you have a 7 Ahr backup battery. Would you still recommend that size? Is it large enough to turn the engine over while airborne?I'm at a point now to where I can pretty much put everything on and do a weight-n-balance and find out if I need to move my corvair engine forward to pinpoint my cg.<<

I never intended or expected that little 7Ah battery to start the engine. It's there mainly as a backup power supply for fuel pump and ignition system if the rest of the electrical system goes up in smoke. It's isolated by a diode to keep it from being compromised by a main bus melt-down. I do test it by starting the engine on it, which also takes some electrical load off the main battery while it's spinning the engine over using the main battery.

As a data point, my experience has been that a high compression Corvair with a 54" prop will not restart in a glide if there is no power applied to the starter, not even if you point it at the ground and run it up to Vne. The starter circuit won't be the thing that makes your engine quit, and it's so simple it's difficult to see how it could fail in flight unless as a victim of collateral damage. Running out of battery "juice" due to an alternator failure and not noticing the voltage drop is probably when you'll find a backup battery the most useful. When you do notice, you should have some idea of how far you can fly before you kill the backup avoid unpleasant surprises. You'd probably want to head for nearest airport with something like a 7Ah battery. Your Corvair ignition system probably burns 3A at 3000 RPM, with another amp going to the fuel pump, but that doesn't mean you can fly almost 2 hours if you are running purely on the 7Ah battery.

For those who haven't seen it, the electrical system Paul is asking about is at .

Mark Langford

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