Tuesday, April 6, 2010

I don't speak 'electric'

There's really no reason to ever work on your experimental airplane project without first thinking out exactly what it is you're going to do. I knew that. I've known it since reading Sam Buchanan's excellent essay on the subject many years ago.

So why do I find myself occasionally doing it? Even more scary, why do I do it when the subject is the electrical system in my RV-7A?

The fact is, I don't know avionics and electrical systems very well; that's one of the reasons I'm keeping it fairly simple.

But a build-it-as-you-go approach I found myself taking last weekend has me scared straight on the subject.

I wanted to install the switch for the Airflow High Performance Fuel Pump, which serves as the boost pump for my fuel-injected engine. But I want it not only to be powered via the Vertical Power 50, I want a backup buss so that if something should go wrong with the VP-50 somewhere, I'd still be able to start the engine and get home.

"Simple," I said to myself. I'll install a three way switch, connected to the fuel pump switch. Flip the switch in one direction, and it's powered by the VP-50. Switch it the other way, and it's powered from the back-up electrical buss.

Wise people already see the stupidity of this. But just in case you don't, here's a schematic of what I was doing, which I just created with the knowledge now of how stupid it was.

Had I actually planned this out, I would have seen the problem right away. The fuel pump switch didn't do anything. Just flipping the three way switch up or down would turn it on. What was I thinking? And why was I thinking it?

I only needed the three way switch. When I want to run the fuel pump with standard VP-50 power, flip it up. If I need to turn it on and power it with the backup buss, flip it down.

Mercifully, I don't have many backup systems on the plane. I'm not an instrument pilot. I fly by looking out the window and mostly looking at airspeed. My Dynon D-100 has an internal backup battery that kicks in when ship's power goes out. My GPS is a Garmin 296 which also has a battery (and a GPS isn't a save-your-bacon piece of equipment for a VFR pilot anyway). I'll have a hand-held radio to backup the ICOM A210 (if I can ever get that thing working!).

And only half the engine depends on electrical power. I have a Lightspeed EI in place of the right magneto. The left magneto doesn't need electrical power from the ship.

If things go south, I can fly by looking at my steam gauge altimeter and airspeed indicator, and talking on a hand held. It's not the Airbus 757-type arrangement that most RV builders have, but it'll work for me, I'm sure.

If I think things through a bit better before I head to the hangar.

1 comment:

  1. Nice post, Bob. I think we all have best intentions when it comes to thinking everything through, but then we get excited about working with airplane parts! :)