ELECTRICAL POWER FOR YOUR AIRPLANE

• The Power Source
• Wiring
• Using a Common Ground
• The "Bus" Concept
• Fuses
• Switches
• Keyed Ignition Switch Installation (Right-click to "Download Link Target" - pdf file)

In the early days of ultralights, much like the early days of general aviation, aircraft designs avoided complex electrical systems. Batteries, generators and wire were simply too heavy and too unreliable. Even as late as the 1950's, accessories such as electric starters were "options" on some production airplanes. And, in keeping with the spirit of light weight and simplicity, which were and are such a critical part of the ultralight movement, for many ultralight aircraft and other simple kit planes, what we refer to as "accessory electrical power" remains an option, even to this very day.

Of course, every spark ignition engine must have some source of electrical power to generate the spark. This engine ignition system is analyzed in another article, and will be discussed only briefly here. The typical magneto used for ignition in most two-stroke engines provides sufficient power to allow a small amount of electrical current to be scavenged from its operation to drive, for example, a tachometer. However, placing electrical accessories in the same circuit with the aircraft ignition system has always been considered a rather dicey proposition. Failure of the accessory may result in failure of the ignition system. Still, in the past, we frequently relied on the magneto impulse to drive a simple tachometer.

It wasn't long into the ultralight movement, though, that many pilots realized it would be a good idea to have a conventional twelve volt power supply, much like that of an automobile, available to operate accessories such as lights, radios and standby fuel pumps. Many of the engines which were adapted for early ultralight use (Zenoah, Rotex and Kawasaki to name a few) were snowmobile engines, and were already equipped with a fundamental electrical system which could be easily adapted to aircraft use. Even without this accessory, it was still possible to build a twelve volt electrical system, based on a small battery which could be periodically recharged when the aircraft was not being flown. One of the earliest ultralight aircraft (the Weed Hopper) utilized, at one time, an engine whose ignition system operated from four D-sized dry cells!

This introduction is a backdrop for the discussion which follows, regarding a simple electrical system for powering aircraft accessories. While the fundamentals discussed are applicable to more complex wiring for more sophisticated home-built or kit-built airplanes, the comments are nothing more than the compilation of several pilots personal experience with ultralight aircraft over the last twenty years.

THE POWER SOURCE

Up to this point, we have been discussing the fact that electrical power for aircraft accessories is a convenient thing and often a desirable goal. So, the first question is, where should we get our electrical power from? There are basically three ways to do it: (1) take it from the engine; (2) take it from a battery; (3) take it from the wind. Of course, a fourth option is to use some combination of the first three.

Let's talk about using a battery first. Batteries have real advantages as power sources. They are simple, they are relatively compact, and they can last a long time. Of course, they have a serious drawback: Over time, the energy they provide is depleted and must be somehow restored. A battery may be disposable and simply replaced when it has run out of power, or it may be a rechargeable type which can be replenished as needed.

Can I use a battery on my aircraft? Of course you can. Keep in mind that batteries tend to be somewhat heavy, another undesirable feature in aircraft. Still, a small motorcycle battery which weighs perhaps ten or fifteen pounds is likely to provide substantially all of the electrical needs of most ultralight aircraft assuming that its regularly recharged.

A by far better solution, however, is to use the engine itself as the power source. While there are not many generators or alternators available for small two-stroke engines, many of them, including almost all Rotex engines, come equipped with an electrical accessory called a "lighting coil" A lighting coil is a tightly wound coil of wire located near the engine flywheel. Mounted on the engine flywheel is a small magnet, the same magnet which energizes the engine magneto coil. As the engine runs, the magneto magnet energizes the lighting coil, producing an alternating current. It is simple to capture this electrical energy using a device known as a "regulator/rectifier". Most engine suppliers these days include a rudimentary regulator/rectifier when they sell the engine. Be warned, however, that the inexpensive regulator/rectifiers supplied with most two cycle engines only do a good job of producing a smooth, clean twelve to fourteen volt output if there is a meaningful load placed on the regulator/rectifier by the aircraft's electrical demands. A better investment is the Key West regulator/rectifier, a newer, high tech device which provides a clean fourteen volt output no matter how much current (up to about ten amperes) the aircraft's accessories require. And, you can use the output of the regulator/rectifier to charge a small battery, meaning that the electrical accessories will be available to use, even if the engine is not running.

Another option is a wind-powered generator. By mounting a propeller on a small electrical generator and hanging the whole contraption out into the airstream, the movement of the aircraft through the air can generate electricity as the propeller spins. While such a device creates a substantial amount of drag in operation, and is capable of producing only a relatively small amount of current (perhaps five hundred milliamps) nevertheless such may be a sufficient output for a handful of accessories in a modern ultralight. Still, the cost of such generating devices, both financially and in drag is seldom justified in view of the availability of lighting coils on almost all ultralight aircraft engines currently in use.

WIRING

We get lots of questions about wiring. Electrical wiring is, unfortunately, a bit of a black art. Most people ask us pretty simple questions, and what follows are some pretty basic answers. One of the first questions is: How big should the wires be which connect my battery or regulator/rectifier to the accessory? There is a (sort of) simple answer to that question. In most aircraft accessories, you can use eighteen gauge stranded copper wire with a polyurethane or rubber coating. Eighteen gauge stranded copper wire in lengths up to twenty-five feet will handle about five amperes of electrical current, an enormous amount for most aircraft accessories. Keep in mind that most electrically powered aircraft instruments (for example, a clock or a water temperature gauge) draw less than one-tenth of an ampere current. A hand-held radio will draw less than one ampere of current during the transmit cycle and less than one-tenth of a amp while it is receiving the signal. An aircraft strobe light may draw between one and two amperes of current. Even an aircraft landing light is likely to draw well under five amperes.

So, for almost every application in an aircraft, eighteen gauge wire is overkill. Most aircraft accessories will operate safely on much smaller gauge wire (for example, twenty-four gauge) but since eighteen gauge wire is so widely available, and since so few accessories are used in most ultralight aircraft, it is a convenient choice. Keep in mind that current draws are cumulative. If you have three, one hundred milliamp accessories being powered from the same wire, a total of three hundred milliamps is being carried by the wire.

Now that you know this simple shortcut, you may want to evaluate using smaller gauge wire wherever possible, since it is lighter. You will discover that most aircraft instruments will operate very nicely from twenty-four gauge wire, for example. Charts which show current carrying capability of wire at various voltages are widely available in electronic handbooks.

USING A COMMON GROUND

One of the most commonly overlooked shortcuts in electrical wiring in aircraft is the use of a common ground. Using our example of a clock as a typical aircraft accessory, such a device may have a power requirement of twelve to fourteen volts and have two terminals on the back, one marked + and one marked -. Many builders will run a pair of wires from their battery or regulator/rectifier, perhaps through a switch and then to the clock. While this is certainly an acceptable practice, and in a wooden aircraft, probably a necessity, in a metal aircraft it is a waste of valuable wire and weight. Since most ultralights are built from metal components held together with metal bolts, why not let the airframe act as one of the electrical conductors for your electrical system? In other words, connect the negative output from your battery or regulator/rectifier directly to the airframe. Then, wherever in the aircraft you need a negative power connection, simply connect to the airframe. This is what is known as a "common ground" system utilized almost universally in automobiles, boats and aircraft around the world. The only risk associated with the common ground system is insuring that the "hot" wires, i.e., those that are ultimately connected to the positive side of your power supply, are not permitted to come in contact with the airframe. This means insuring that hot wires are properly insulated, and when they pass around corners or through openings in the airframe, that precautions are taken to prevent the insulation from being worn through as a result of vibration. Take these precautions, and a common ground system is safe, efficient and lightweight. It also greatly simplifies the wiring of the many accessories which will go into your aircraft.

THE "BUS" CONCEPT

Many aircraft also utilize the so-called "bus bar" approach to the positive side of the electrical system. Lets say that you want all of your electrically powered accessories to operate from a single switch. Using a carefully designed and protected copper bar (the bus bar) you can connect the positive power leads of a large number of accessories together. Then, use a single switch to supply power to the bus bar. In this fashion, when you turn on the switch, all of your twelve volt accessories will be receiving power simultaneously. Just be sure that the wire going to the bus bar is sufficiently large to carry the current required by all of the accessories which are fed from it.

FUSES

Another common question we have to deal with on a daily basis is the necessity for fuses (or circuit breakers) in a circuit. As a general rule, electrical accessories and their circuits should be protected by fuses or circuit breakers. But how big a fuse should I use? Again, there is a "shortcut" answer. Figure out how much current an accessory draws, and double it. For example, an instrument which draws one-tenth of an amp (one hundred milliamps) should be protected by a .2 amp fuse. Since fuses of this rating are not readily available, go up to the next higher level, one-quarter amp. Electrical accessories frequently draw more current during startup, and if fused at too low a value, will allow the fuse to blow when there is nothing wrong with the circuit. By the same token, "over-fusing" a circuit is dangerous. A five amp fuse in a circuit powering an instrument drawing only one-tenth of an amp will probably not fail before the instrument fails (although it will probably protect the wire, if you are using eighteen gauge).

It is not necessary to fuse each individual component in a circuit. For example, you may wish to group together all of your engine instruments, which collectively draw five hundred milliamps, on a single circuit with a one ampere fuse.

SWITCHES

Good circuit design dictates that aircraft accessories be connected to the power source by switches. But how many and what kind? Well, switches, like wire, have electrical current ratings. You can't, for example, use a switch designed to carry a maximum of one ampere to power accessories which will draw more than that.

The physical layout for switches is another important consideration. We prefer simple, old fashioned toggle switches: Nothing fancy, and very easy to tell whether they are in the "on" or "off" position. In general, try to pick switches which can be mounted in a round hole; it is much easier to drill a round hole for a switch, for example, then it is to cut a square hole for a fancy rocker-type switch. Rocker switches can also be confusing, i.e., should depressing the top or the bottom half of the switch be considered "on"? "Toggling" push-button switches (first push off, second push on) are even more confusing, unless they contain a toggling indicator which clearly shows their operating state.

Once again, it is not necessary to have every accessory on a separate switch. In a typical ultralight layout, use one switch for instruments, one for radios, and one for lighting. Set up a single master switch which feeds twelve volt dc to all of the foregoing switches, and to one terminal of your starter push button or key switch, if you have an electrical starter.

The only tricky part of the accessory wiring for instruments has to do with the hour meter. If you have an hour meter which is integral with a tachometer, it should be internally or externally wired so that the hour meter runs simultaneously with the tachometer. Many modern electronic tacks are available with such integral hour meters. If your tachometer is not so equipped, you need to devise a mechanism whereby the hour meter only operates either (a) when the engine is running or (b) when the aircraft is in flight. Four cycle engines can use a readily available oil pressure switch which can be mounted in an oil line. Such switches activate only when the oil pressure reaches a specified level, thereby completing the circuit to turn the hour meter on. Another commonly used switch for hour meters is a so-called "wind vane" switch. Fabricate a small, free-swinging metal handle which is hinged and dangles freely from the aircraft so that it will be blown backward when the aircraft reaches flying speed. Mount a small micro switch in such a way that the lever of the micro switch will be activated by the wind vane when it is pushed backward and upward. In this fashion, anytime the airplane is moving forward at any appreciable speed, the micro switch will be closed, and the hour meter will run. Make sure and check the freedom of movement of the wind vane periodically. If the hinge is not well lubricated, it can stick in the up or down position.

For those of you inclined to delve deeper into the mysteries of aircraft wiring, we commend to your reading the Aero Electric Connection.