12/14v vs 24/28v systems

Don · #1 08-11-2006, 10:58 AM

While attending Oshkosh I heard Greg Richter (sp? - the Blue Mountain guy) give a talk on aircraft wiring for smart people. He made a strong case for using a 28v system. His argument is most electronics (radios and efis's...efii?) work on either and will stop working at 9.6v more or less. If you lose your alternator he argues that you'll have a lot longer use of your electrical system as your battery drops from 24v to 9.6v than you would if you had a 12v system. Actually he thinks 48v is ideal but 24v is the best you can readily do now. He also argues that you can save 6-10 lbs of wire using 24v and the lighter guage wire required.

I'm midway along with the fuselage and nearing a point where I need to finalize my wiring plans but early enough that I can change plans without any real impact (other than maybe a little money). I've got a set of Whelen position lights and strobes that are 12v that's my only real 12v electrical investment so far and I'm sure I could sell them for near what I have in them if I go 24v. But as I look around, the light weight alternators seems to be 12v (though I'm sure if l look harder I can find a 24v). The LED lights I've been thinking of switching to for weight and other reasons are strictly 12v. So right out of the gate I seem to be heading uphill. I believe my planned radios and purchased EFIS (a dual GRT Series 1) can operate on either.

So, this not is too long but my questions are:

1. Am I right that 12v systems are predominant today? This includes the assumption that putting together a 24v system will involve more scrounging and searching.

2. Is there a simple way to get a 12v system to last longer in the event of a battery failure other than adding a second battery?

3. Other than night flying, I don't really see an electrical failure as a big issue. About the biggest concern would be loss of the GPS and I expect I'd have a handheld that would suffice for getting me to an alternate airport. If all else fails there's always the chart and pilotage to get me to an airport. Does anyone see something I'm missing?

My inclination at this point is to resign myself to the 12v system and finish the airplane. In theory, Greg appears right. In practice, he may be right in the future. For now theory and practice don't seem to agree.

Don

Mel · #2 08-11-2006, 11:11 AM

Go with 12 volts! 6-10 lbs saved on wiring? I doubt it very much. Weight of 12v vs. 24v battery? Expense of 24v alternator system? Expense of 12v vs 24v accessories? Most amateur-built aircraft take advantage of many 12v automotive parts. 24v parts are almost exclusively aircraft. Compared the price difference?

joeboisselle · #3 08-11-2006, 11:30 AM

I agree with Mel, go 12 volt. Your brand new aircraft with all new parts shouldn't be having any electrical problems any time soon. And if you do have problems...at night, how long will a 25 ah battery last you while running minimal equipment for minimal amounts of time? I doubt you'd have enough gas to run it all the way down. Good question tho!

Paul Eastham · #4 08-11-2006, 12:17 PM

Bob Nuckolls dismantles this and a few other dubious statements from this guy at:
http://www.aeroelectric.com/articles...response_1.pdf

PJSeipel · #5 08-11-2006, 12:29 PM

I am going with a 28V system. I expect some will try to talk me out of it or tell me I'm stupid, but they're wasting their breath.

That said, yes, you will have a harder time finding some things in 28V. Most things can be found in both voltages, but if you plan on using automotive parts, go 12V.

I doubt the wire savings will really be 6-10 lbs. I know mine won't because I wired fairly conservatively. If you bought the Whelen System 6 that Van's sells, then it works on 28V, just need to swap the bulbs on the position lights. Plane-power makes a 70 amp alternator that does 28V and costs the same as their 14V model.

If you go 28V you are bucking the current trend and you will find yourself scratching your head over some items (landing lights, trim system, flap motor). All of these can be solved, it just depends on whether it's worth it for your situation and mission profile.

PJ
RV-10 #40032

PJSeipel · #6 08-11-2006, 12:38 PM

Quote:

Originally Posted by Paul Eastham

Bob Nuckolls dismantles this and a few other dubious statements from this guy at:
http://www.aeroelectric.com/articles...response_1.pdf

I don't disagree with much of what Bob wrote, but Bob's response was written to a draft that Greg later revised, and Greg did fix a lot of the areas Bob had an issue with. There's a big difference between what Greg talked to at OSH and the document Bob tore apart.

w1curtis · #7 08-11-2006, 07:05 PM

You have to understand where Greg is coming from. That 6-10 pounds may be accurate on the composite plane that he is use to (Cozy Mk 4) but in our metal planes we can use the airframe as ground and effectively cut our wiring in half compared to his composite. While his thinking is technically correct, in the practical world of single engine airplanes, a 12/14 volt system beats out the technical superiority of 28 volts.

Side note; the composite Cirrus uses 28 volts but the Columbia uses 12/14. A couple of years ago the auto industry tried to move to 36/42 volts. But like the metic system, technical superiority lost out to day-to-day practically.

Mel · #8 08-11-2006, 07:14 PM

One other small thing that has happened to me. Ever been stranded at a small airport needing a "starter boost" and the airport doesn't have a 24v booster. Just about any car in the parking lot can give a 12v boost.

Kyle Boatright · #9 08-11-2006, 07:38 PM

Automobiles use a 12v system. That means that there are many suppliers of instruments and other components that will work on 12v. The economies of scale mean that 24v stuff will be more expensive for equivalent products. The battery many (most?) of us are using these days costs about $70 on Ebay. I don't think you'll find a similar deal on a 24v battery.

In addition with 12v, you can easily find a battery tender or a battery charger that will work on your system. Shoot, you can jump the battery in your airplane from your car or from an automotive jump starter.

Finally, there are plenty of accessories (cell phones, CD players, etc.) you can run off of a 12v power plug installed in your panel. Not sure if you can do that with a 24v system.

24v is superior in theory (more power, less weight, etc.), but the practicalities all point you to 12v.

Don · #10 08-12-2006, 09:16 AM

Thanks guys. This was exactly the discussion I wanted to get when I posted. I'm going to proceed ahead with a 12/14v system but I'm doing it knowingly. If there was an emoticon with a smile and a morter board it would be perfect for me - an education without having to go to the school of hard knocks.

Don