AIRCRAFT BUILDING

ELECTRICAL SYSTEMSSIMPLIFIED

BY RON ALEXANDER

Somewhere during the aircraftbuilding process you will encounterthe often-dreaded task of installing theelectrical system. It is likely that youwill not receive a lot of help fromyour plans or assembly manual. Theamount of instruction varies from oneairplane to another. Usually, however,you will be left to design and installan electrical system specific to yourneeds. The first question to ask your-self is, "Do I want an electr icalsystem?" You may not. If you arebuilding a very simple airplane andhave no desire for a radio or a starter,then save yourself a lot of work andthe airplane a lot of weight by not in-stalling an electrical system. Most ofus will want to take advantage of thebenefits derived by having a starter,radios, lights, etc. Putting in yourelectrical system need not be a com-plicated process. Like everything elseinvolved in aircraft building, you needto know a few basic concepts. Thisdoes not mean you are required togain the knowledge necessary to be-come an electrical engineer. Thewiring process can be as simple or ascomplicated as you would l ike tomake it. As an example, the Airframeand Powerplant Mechanics GeneralHandbook contains over 200 pages onbasic electricity, generators and mo-tors. You can spend a considerableamount of time learning about elec-tricity. You may decide that you wantto do that for your own personal satis-faction. Personally, I am not interestedin learning all of the fundamentals ofelectricity. I am simply concernedwith how to wire my airplane. The106 AUGUST 1998

Alternator

point is, if you have a longing to learnall about electricity, the information iscertainly available. Many builders do.

If you are like me and want to knowonly the fundamentals, then read on.Understand that I am not an expert onelectrical systems but I have re-searched the material available and Ihave condensed it down to what I thinkyou need to know to properly wireyour aircraft. As Tony Bingelis soaptly states in his book SportplaneConstruction Techniques, wiring anaircraft involves two basic activities:

1. Installing the electrical equip-ment where it needs to be or whereyou want it to be.

2. Connecting it to a power source(battery/bus bar, etc.) using wires.

I like that. Very simple and to thepoint. Each component part has a powersource that is connected by a wire thathas only two ends. One end is con-nected to the piece of equipment andthe other end to a power source. Is thisan oversimplification? I really don'tthink so. Of course, there is a lot moreinvolved in a proper electrical systeminstallation but if we keep it in perspec-tive it really is not that complicated.

Proper planning of your electrical sys-tem is absolutely essential. Do not waituntil you are in the middle of the buildingprocess to think about this. You will needto route electrical wiring, determine thebest location for your battery, think aboutwhere to put a bus bar, etc. during the ini-tial bui ld ing stages. The basic stepsinvolved in planning your electrical sys-tem are as follows:

• Determine what electrical equip-ment you want to install

• Locate these components withinthe aircraft structure

• Locate your battery, bus bars andcircuit breaker panels

• Protect the components by deter-mining proper wire size and circuitbreaker/fuse requirements

• Connect component parts to anadequate power source

• Ground and bond necessary items• Install proper instrumentation to

monitor the system• Draw a detailed schematic of the

system for referenceAs previously stated, you should de-

sign your system to suit your needs.Are you going to fly in marginal or ad-verse weather (IFR)? Will you be flyingat night? If so, you will want to payclose attention to preventing a system

failure. You must plan your system tooperate the essential equipment for thenecessary period of time you will needto land the airplane. FAR Part 23, thatregulates the airworthiness of produc-tion aircraft, states that essentialelements must be powered by the bat-tery for a minimum of 30 minutes. Stopand think for a minute — 30 minutes isnot that long when you are operating ona standby system. It may seem like along time but in actuality it is minimal.On the other hand, if you are simplygoing to fly day VFR then you are notgoing to install a very elaborate system.You may only need an engine starterand a radio. No matter what your choicethe basics remain the same, only thecomplexity of installation varies.

Prior to discussing the steps of sys-tem installation outlined earlier, I willdiscuss the very basics of electricityand I mean very basics. There are onlya few things you really need to under-stand to accomplish your installation.As I mentioned earlier, if you want toplunge in deeper you certainly havethat opportunity.

BASIC ELECTRICITY

Let's begin with the old familiarohm's law. This law is the very founda-tion of all principals of electricity. Thelaw itself simply shows the relationshipbetween electrical current flow, pres-sure and resistance. Current flow ismeasured in amperes (amps). Electricalresistance is measured in ohms. Thisresistance is simply an opposition toelectrical flow. The pressure of electric-ity is measured in volts. One volt is theamount of electrical pressure requiredto push one amp of electrical flowthrough one ohm of resistance. Or an-other way of stating this is to say thatwhen you pass one amp of currentthrough a conducting object having aresistance of one ohm, the result will bea voltage drop of one volt. The formulais shown in Figure 1. E stands for volt-age or pressure, R equals resistance,and I equals current in amps.

One additional formula that youwill find helpful involves determiningpower. Power is expressed in wattsand is found by multiplying the volt-

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OHMS LAW.

FIGURE 1

Relay

Circuit breaker

age times the current.. . P=E x I. Onevolt of pressure forcing one amp offlow will produce one watt of power. Awatt is a unit of energy. Concerning thechart in Figure 1, if you will place yourthumb over the value you wish to cal-culate you will either divide the othertwo values or multiply as indicated. Asan example, if you wish to calculate R,hold your thumb over the R and divideas indicated. The temptation is to go onwith basic electricity but I will easilyovercome that temptation. Knowledgeof these formulas will assist you in de-signing your electrical system.

BATTERIES AND ALTERNATORSYour system will undoubtedly con-

tain a battery. We are all familiar with a108 AUGUST 1998

battery but what is its purpose?First and foremost it providesthe necessary power to start theaircraft engine. In some aircraftthat do not have an alternator orgenerator it may be the onlysource of power for all electricalitems. The battery also provideselectrical power in the event thegenerator/alternator fails. Whenit comes to choosing your bat-tery you have a number ofchoices. Lead acid batteries areprobably the most common. Thistype of battery has been aroundfor years and is quite commonon automobiles and aircraft.These batteries are usually fairlyinexpensive. Lead acid batteriesemit explosive gases that must

be vented. This will be discussed later.Another common battery is the so-calledgel-cell. This is also a lead acid batterythat has a material added to the elec-trolyte converting it into a gel state. Thistype of battery is less likely to spill andis often used in acrobatic aircraft.Nickel-cadmium batteries are also avail-able. These batteries do present somemaintenance problems and are notwidely used on smaller aircraft. A verygood choice of battery today is the re-combinant gas battery (RG). They donot leak and they can be mounted in anyposition. The disadvantage is the price— they are usually more expensive.

Most amateur-built aircraft will re-quire a 12-volt battery instead of a24-volt. Most designs call for a 14-voltelectrical system versus a 28-volt duelargely to cost and weight differences. A14-volt system utilizes a 12-volt battery.Even though a battery is rated at 12 voltsthe actual voltage will vary dependingupon the charge state. Battery amperagealso comes into play as we make the de-cision as to what type to install. Shouldyou buy a 25 amp or 35 amp battery?Starting performance in addition to oper-ation of essential equipment must betaken into consideration. Generallyspeaking, a 25 ampere-hour battery isadequate for most light aircraft. Whenyou purchase your battery keep thisthought in mind — it may be operatingyour emergency equipment while youshoot that low approach in bad weatherafter the loss of your alternator.

Alternators are simply generatorsthat produce alternating current. Theyare the most common forms of electri-cal power on aircraft today. An

alternator converts mechanical energyinto electrical energy that can powerour electrical components. Most alter-nators today are l ightweight (6-8pounds) and provide a considerableamount of amperage (typically 40amps or more). You must calculate theneeds of your entire system prior toselecting the alternator. Common rat-ings are 30 to 50 amps.

RELAYS, CONTACTORS ANDSOLENOIDS

A relay or contactor is usually in-stalled to activate an engine starter. Itserves to eliminate the need to run aheavy cable from the battery andstarter all the way to the master switch.A contactor or relay is simply an electro-magnetic switch that operates a heavycurrent circuit. These are often called so-lenoids. A large wire (usually 4 gauge) isinstalled between the contactor, the bat-tery, and the starter. A smaller gauge wire(usually 18 gauge) is then run betweenthe relay and the master switch.

VOLTAGE REGULATORSA voltage regulator must be present to

protect the battery. The charging voltagegoing into the battery must be controlledwithin a relatively small range — as anexample, 13.8-14.2 volts for a typicallead acid battery at normal temperatures.This voltage varies with the ambient tem-perature. The voltage regulator is goingto prevent battery overcharging by de-creasing the alternator output as thebattery nears a full charge state. Mostother equipment within your airplane canwithstand a wide range of voltage. Justabout any solid-state regulator will con-trol most alternators.

CIRCUIT PROTECTIONWe must protect individual electri-

cal circuits from overloads. This isusually accomplished using a fuse or acircuit breaker. Fuses and circuitbreakers are not intended to protect theequipment rather they are installed toprotect the cable (electrical wire) at-taching the power source and theequipment. Of course, in protectingthe cable the equipment is also pro-tected. Fuses wil l be cheaper topurchase and lighter in overall weightthan circuit breakers. During flight itis easier to reset a circuit breaker thanchange a fuse. If you closely analyzemost electrical faults they are pro-

duced by failure of an electrical devicethat in turn causes it to draw an ex-tremely high amount of current. Inreality, how often will you be able tosolve the problem by resetting a cir-cuit breaker or replacing a fuse?Probably not very often. A short in awire will also cause an electrical fault.

The bottom line is that you must se-lect the right size circuit breaker orfuse for the size wire you are runningto the component part. The circuit pro-tection should open before the wiregets hot enough to begin smoking.Another point worth mentioning isthat if the circuit protection is toosmall for the wire size you will get anuisance protection requiring you toreset the circuit breaker or replace thefuse even though a major problemmay not exist. The following chartfound in FAA Advisory Circular 43-13 shows the comparison betweenwire size and circuit protection.

Wire ANGaugeCopper

22 ..........2018161412

•>''i<t& >.*>••*'•'.;.-<rg.".r • : - • ; ,

CircuitBreakerAmps

....,,...,5..... ._..,7.5101520

25/30: 35/40

50

FuseAmps

.,.—:„, 4, ........5101015203050

What would happen if we did notcomply with these guidelines? Let'sassume we install a 10-amp circuitbreaker in a circuit connected by a 22-gauge wire. As long as everything isnormal you will probably not knowthe difference. However, should anelectrical fault occur in this circuit, thebreaker may not trip until the wire getsso hot that it causes an electrical fire.

ELECTRICAL WIRING AND\ .A . - - I I CONNECTORS ,

The primary concern is that you usethe proper type of wire. Do not go downto your local electronic store and expectto get the proper wire for your aircraft.The best type of wire to use in your air-plane is MIL-W-22759/16 unshieldedwire or MIL-C-27500 shielded wire.Both types of wire are manufactured tothe military specification number givenand they are insulated with a Teflon

type of material called tefzel. The wireis rated for 600 volts. Teflon and PVCwire is also often used but after re-searching the problems involved I wouldstay with tefzel wire. Sure, you canprobably save a few dollars on surpluselectrical wire but is it really worth it?Wires are manufactured in various sizeswith 22 gauge being the smallest youwill normally encounter on your air-plane. To provide an example, 22-gaugewire is about .025 inches in diameterand is comprised of several strands ofsmaller wires.

As mentioned under circuit protec-tion, the size of the wire is of utmostimportance. The wire size will be se-lected based upon the amount ofcurrent the wire will carry and the re-quired length of the wire. The voltagedrop resulting from a long length ofwire can be significant. Fortunately,AC43-13 provides charts showing therelationship between wire length, sizeand amperage.

A large number of electrical prob-lems are the result of poor connectionsbetween the electrical cables and thevarious components of the system.Most cables are connected with "crimp-on" connectors, such as ring-endterminals or butt splices. Often a solderjoint will be used to complete the con-nection. I would recommend the use of"crimp-on" connectors whenever pos-sible. Solder joints are subject tobreaking or weakening when subjectedto vibration. If you use a solder joint,be sure to support the wire near thejoint to prevent flexing. You will findthe need for soldering to be more com-mon during your radio installation.

Selection of the proper solderlessconnector is easy because the connec-tors are color coded. The coding is asfollows:

Color of ConnectorRedBlue

Yellow

Size of wire18-20-22 gauge

14-16 gauge10-12 gauge

A special tool is used to crimp theconnection. Approximately 3/16 inchis stripped away from the wire, usingwire strippers, and the wire insertedinto the connector. The tool is thenused to make the crimp. You thencheck the connection by sl ight lypulling on the terminal and wire.

It is also recommended that you in-sulate the terminal connection using

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IConnectors

heat shrinkable tubing. You must re-member to place the tubing over thewire prior to assembly.

BUSBARSNo doubt you have heard the elec-

trical term bus bar. A bus bar is verysimply a central point where wiresfrom electrical equipment are groupedtogether and attached to a metal barthat is then attached to a power source.Without a bus bar we would have toconnect every electrical component di-rectly to the power source. This wouldbe very complicated and impractical.Normally, one terminal of each circuitprotector (circuit breaker or fuse) run-ning from the electrical component is

Tefzel wire and connector

attached to the bus bar. The power tooperate all of your electrical items isthen obtained from one common point.This is generally accomplished using astrip of copper large enough to tap ahole for each circuit breaker required.This would usually be about 1/8 inchto 1/4 inch thick and 1/2 inch wide.They must be long enough to accom-modate the number of breakers. Ifusing fuses, a heavy copper wire is of-ten used that is soldered across oneterminal of each fuse. Doing either ofthese creates a bus bar.

SwitchesA lot of text could be written con-

cerning electrical switches. You will

Grounding wires

need to determine what type of switchyou desire and where you want it lo-cated. Even though there are manyswitch types they all provide the sametype of function. That function is thecontrol of an electrical component.Listed below are various types ofswitches you may encounter and theirdescription. ">• ;

• Single-pole, single-throw ". 'Used for one circuit

• Single-pole, double-throwTwo circuits but not at the same time

• Double-pole, single-throwTwo circuits at the same time

• Double-pole, double-throwTwo circuits with each throw

A pole is the movable metal con-

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110 AUGUST 1998

E A R N A F R E E R I D E I N A B - 2 5 M I T C H E L Ltactor of the switch. The number ofpoles will equal the number of exter-nal terminals on the switch. Thethrow is the number of circuits eachpole can complete. As an example, asimple toggle switch with an on-offposition is an example of a single-pole, single-throw. A rocker switch isanother example.

Another available switch option isthe combination switch/circuitbreaker. This switch also serves a dualfunction as a circuit breaker. Manycustom builders are using this type ofswitch. Whatever type of switch youinstall you must be sure the voltageand amperage ratings are adequate forthe circuit you are completing.

GROUNDING AND BONDING

Grounding is the electrical con-necting of a conducting object to theprimary structure to provide a returnpath for the electrical current. Onmetal aircraft the main frame, fuse-lage, or wing is used. The battery isgenerally grounded to the enginecrankcase and from the crankcase aground wire is run to a ground busbehind the instrument panel. Bond-ing, on the other hand, is the electricalconnecting of two or more conduct-ing objects not otherwise connected.Often, in composite and wood air-craft, the bonding connection willalso provide the ground connectionfor an electrical component. Bondingprovides a conductor for the purposeof eliminating the build-up of staticcharges. This will also reduce radiointerference. Specific methods ofgrounding and bonding will be dis-cussed in the next issue.

So, as you can see, installation ofyour electrical system need not becomplex. There are a number ofsources for information concerningbasic wiring practices. Tony Binge-lis' books are excellent. BobNuckolls is also a recognized electri-cal expert and has writ ten manyarticles concerning the subject. Healso has a regular newsletter calledThe AeroElectric Connection. In-formation obtained from thispublication will be very helpful tothe first time builder.

Next month I will expand upon theactual steps needed to install yourelectrical system. Those steps arelisted earlier in this article. ^

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