Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/96284
So, how do you decide whether to go with a 14- or 28-volt system? Let's kick that around a bit. The advantage of using a 14-volt system is that it is common with most automobiles, so you can use many of the less expensive electrical components available for cars, such as lights, switches, etc. Also, 14-volt systems are the most common in homebuilts and smaller GA aircraft. The great advantage of the 28-volt system is that it uses wire that is significantly smaller (approximately 70 percent less weight) than in a 14-volt system, which is why the higher-voltage system is used on larger aircraft. With all of the new avionics aircraft are acquiring these days, even smaller, single-engine aircraft from Cessna, Mooney, and Beech are coming with 28volt systems, probably for the weight savings. Consider this: If your plane had 1,000 feet of wire in it and you were able to use a 28-volt system, you could reduce the wire size from 12 gauge to 20 gauge, and the weight savings would be about 16 pounds! Our average homebuilt uses much less than 1,000 feet of wire, and much of this wire is "signal" wire (like encoder to transponder) and the same size irrespective of the system voltage. On the other hand, the alternator would be lighter for the same power in a 28-volt system, but manufacturers seem to make the 28-volt alternators on the same frame as those for 14 volts, and with the same amperage, which means that you get twice the power (whether you need it or not) for about the same weight. One other consideration for our 14 versus 28 discussion that favors 28 volts is that some newer avionics are designed to run on anything from 10 to 30 volts. If you have a 28-volt system and have an alternator failure, it will take longer to drain the battery down to 10 volts than if you started at 14, so the 28-volt system should give you a larger margin of safety in case of an electrical power generation emergency. Let's also consider battery costs before we leave this discussion. For a 14-volt system battery on a smallerengine aircraft, say below 150 hp, the Gill G-25 or its equivalent from Concorde, the CB-25, can be purchased for about $150 to $200. For larger engines, the G-35 or CB-35 is standard for an additional $40 to $50. On the other hand, for a 28-volt system you will either have to use two of these batteries in series (twice the weight and twice the cost) or buy a battery made for a 28-volt system. The cheapest one listed in Aircraft Spruce costs more than $430 and is roughly equivalent to the G/CB-25. So, you can plan to spend up to twice as much for a 28-volt system battery, and it may weigh the same or twice as much as your 14-volt system battery. One other consideration for our 14 versus 28 discussion that favors 28 volts is that some newer avionics are designed to run on anything from 10 to 30 volts. Let's pause here and look at system voltages. A 14-volt system actually is nominally designed to operate at 13.75 volts of direct current. This should be the output of the alternator when the engine is operating at a high enough rpm for the alternator to carry the full electrical load. The battery for this system is nominally rated at 12 volts. The higher voltage of the alternator allows it to recharge the battery. So, if you look at the buss voltage in your plane and it reads 12, the battery is carrying the load. And if it reads higher than that, the alternator is online and carrying the load. For a 28-volt system, the same idea applies, but the fully charged battery puts out 24 volts, and the alternator produces 27.5 volts to recharge it. Another thought is jump-starting and troubleshooting. I know you are not supposed to jump-start an airplane, but if you ever had to, it would be easier with a 14-volt battery system and your car than if you have a 28-volt aircraft system. Which voltage system should you use? You have to decide. But that decision drives everything else in the electrical system. Only you can provide the answer, but remember, most homebuilts have 12- or 14-volt systems. Hope all this discussion on voltage helps you with your project. Battery 12 volts 24 volts Alternator 13.75 volts 27.5 volts Called 14 volts 28 volts Dick Koehler, EAA 161427, is a retired Navy carrier pilot, A&P;/IA, Technical Counselor and Flight Advisor. He teaches the electricity course at SportAir Workshops and can be seen in many Hints for Homebuilders. EAA EXPERIM ENTER 35