12/14v vs 24/28v systems

[Bryan Wood]

Originally Posted by w1curtis

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.

Using the airframe as a ground all over the airplane instead of running ground wires and using a common airframe ground is something that will cause so many ground loops in your airplane that you will most likely chase gremlins forever. It's very important no try to prevent ground loops in all of your installations.

Regards,

[mgomez]

Originally Posted by Don

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.

Doesn't that sound like a law-of-physics violation? I'd wantt to see either some analysis to back it up, or better yet, a test using two batteries of identical weight, one 12 V and one 24 V.

If your essential avionics load is drawing P watts, and your battery stores E watt-hrs of energy, you will run out of energy P/E hours after the alternator dies.

Why does it matter if you draw P watts as 12 V * I amps, or 24 V * I/2 amps? To first order, the energy stored in a lead-acid battery is approximately proportional to its weight, isn't it?

[w1curtis]

Originally Posted by Bryan Wood

Using the airframe as a ground all over the airplane instead of running ground wires and using a common airframe ground is something that will cause so many ground loops in your airplane that you will most likely chase gremlins forever. It's very important no try to prevent ground loops in all of your installations.

Regards,

As with all things, that depends. I'm not suggesting chassis ground for avionics. For avionics it is wise to ground everything to a common point. Since avionics are usually in close proximity, this does not represent a great increase in wire weight. But conventional design in metal cars and light plane is to go from the negative side of the battery to the chassis. Then all major power consumers, starter, flap motor, etc use the chassis for the negative side of the wiring. Maybe there are some, but I know of no production metal aircraft (or car) that run TWO massive #2 AWG wires from the battery to the starter? Same for the Pitot Heat, Flap motor, etc. These are the things that have BIG (heavy) wires that you have to run two of in a composite plane. These are the same wires that would be smaller (lighter) in a 28 volt system.

I've noticed some RV-10 builders running DUAL #2 AWG cables from the rear mounted battery to the firewall. I don't think this will reduce the number of electronics gremlins but I guess the think they are offering the electrons a better path--maybe they are since I've noticed some obsessive builders also dipping EACH rivet in primer prior to riveting.

[Bryan Wood]

FWIW, I knew better and still wired my landing lights without a seperate ground wire to the lights. In a former life I used to earn my living fixing problems like these in airplanes that had problems like we are talking about. Now when the landing lights are switched on the hash appears in my headsets every time. Add more systems wired this way and enjoy more of these kinds of problems. Do one of these complete wiring jobs and it will drive you and your avionics nuts.

JMHO,

[N395V]

For many of the reasons (pro 28V) stated above I have built my current project with a 28V system.

If I had it to do over again I would not.

Probably saved 2-3 pounds in wiring picked up a heck of a lot more in battery and converter. Saved a little on the heated pitot.

No vacuum system and ship will be IFR so have to have 28v backup and you guessed it a separate 28v batt and converter for 14v for the backup bus. As well as a way to get the 14v and 28v from the backup busseS to their respective busseS.

My 14v items are trim fuel boost and the oil scavenge pump.

I do not consider trim or fuel boost non essential for IFR.

Because trip is 14v and flaps 28 V I need a separate relay board for the flaps.


28V equals more weight, more stuff, more complexity
and when it comes to trouble shooting its a what voltage at what point when nightmare.

Stick with 14Volts and save yourself a lot of grief.