Aircraft Electrical Diagram
Aircraft Electrical Systems, VII
Almost all aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more COM and NAV radio's, transponder, intercom and other electronic systems. The electrical system consist of a battery and an alternator or generator on older aircraft. All of this is connected through several meters (kilometers in large aircraft) of wire.
Even for the private pilot it pays to have some basic knowledge of the electrical systems of his or her aircraft, which could be a life saver in case of an emergency.
Cars and your average homebuilt aircraft run on a 12 volt electrical system where the battery, alternator and electrical equipment is designed for these voltages. But other, usually larger aircraft, have 24 volt systems.
So you might ask: why is that and isn't possible to standardize on one system? This article will explain the differences and the dis- and advantages both systems in use today.
How many volts
Which system you may choose for your kit aircraft does not really matter. Either one will run reliable and operate all equipment you may wish to add to your aircraft. Be that under IFR or VFR conditions. So at first glance there is not much of a difference.
Also, most manufacturers have equipment which can run on either voltage or are selectable, this is not really an issue. But should you wish to use your car to jump start your aircraft, this requirement might steer you towards a 12 volt system.
As explained in our generating electricity article we need a power source in the form of a generator (not very common), alternator and a battery to store the energy. A 12 volt system (battery) runs actually on 13,8 volt (alternator) and a 24 volt system runs on 28 volt. The difference is set by the voltage regulator needed to charge the battery when the engine is running.
The choice of engine usually dictates which kind of electrical system you will need. Rotax engines are sold with a 12 volt system (starter and alternator). Other engine manufacturers might have an option for either system. If you already have an engine: check its battery, alternator and starter motor to see which system it is.
28 volt system
Having twice the amount of volts means literally that the current is half with equal power drawn, see volts, amps and ohms law. As a result wiring can be thinner thus you will save some weight.
Another advantage is that 28 volt systems have more reserve in cold weather where the 12 volt battery looses its power more quickly. Not too mention the fact that a 24 volt battery has a lot more cranking power for starting. Which is really helpful when starting turbine engines.
Some would say that in cold weather engines are preheated so this is a non-issue, but for most of us this is not standard practice (unless you live in really cold places like Alaska, Canada or Siberia).
Turbine powered aircraft normally have a 28 volt system for these reasons. Starting these can drain several hundreds of amps for minutes at a time!
A drawback could be that not all personal electrical accessories are able to take 28 volts. Think about hand-held computers fed from the ships power system. You will need a converter for those items or a separate 14 volt system in your aircraft.
A 28 volt system can be more expensive, especially the battery. Radio equipment used to be more expensive but most avionics manufacturers now design radio's, GPS and NAV systems that can take any voltage from 10 to 30 volt.
14 volt system
As in your car, everything runs on 12/14 volt, wires are thicker for the same amount of power (a bit more weight). But the advantage is less cost in batteries and sometimes equipment. For some kit builders it is a plus that they can use their car to start the aircraft should the battery die some unforeseen day.
12 V batteries do not have the amount of reserve power as a 24 volt type, but they are easier to come by as every car has one, so this can be important should your battery die.
Should you fly or wanting to build an aircraft with a Rotax engine, the choice is easy: it will be a 14 volt system. But if you have the option, do investigate if a 28 volt system is feasible for you.