Challenger Electrical System

The electrical system on Challenger Ultralights just keep getting more and more complicated as more electronic devices are added and become the standard fare.

Originally the Challenger electrical system only powered the electric starter (if the plane had one) and maybe a strobe light. The radio and intercom had their own batteries if they were even used! Now, a Challenger electrical system powers strobe lights, a landing light, wingtip nav lights, a GPS, the radio and intercom, a EIS (electronic Engine Instrumentation System), an EFIS (Electronic Flight Information System), and maybe some other electrical devices like an electric fuel pump or electrically retractable wheels on amphibious floats or skis,

With all these circuits and loads the electrical system must be designed correctly and installed properly. If not, the pilot will encounter frustrating problems, time consuming refits, aggravating trouble shooting sessions, and in flight distractions.

There are a few simple and easy rules of thumb to follow in installing an electrical system into a small aircraft like the Challenger.

Sizing the System

The Rotax 503 and 582 engines used to power most Challenger Ultralights have an integral electrical generator rated at 170 watts at 12 volts DC. This will produce about 14 Amps of usable DC current to power all the electrical components installed on the aircraft as well as recharge the battery. This generator system is also referred to as the lighting coils in the Rotax manuals. The two generator wires are colour coded yellow/black.

Because the generator system has a relatively small output capacity, a builder must ensure that the total power requirements of the installed electrical system do not exceed the capacity of the engine lighting coils. This will involve totalling up the electrical power consumption of all the gadgets that are going to be installed. This will also involve some searching through all of the various owners manuals for the current consumption of each gadget.

Knowing the current consumption of each gadget will also enable the builder to select the correct size of electrical fuse or circuit breaker for each component. A fuse should be sized just over the maximum current draw of the total number of electrical loads (gadgets) in each circuit.

Most modern electronic communication and navigation gadgets (radios and intercoms and GPS units) have a relatively small current consumption usually well under 1 amp. Likewise the EIS (electronic engine instrumentation system) and EFIS (electronic flight information system) are small consumers of power and draw less than 1 amp each.

The big power hogs are the lighting systems and electrical heating systems. A typical strobe light system will draw a few amps continuously as it charges up a group of high power output capacitors that provide bursts of electrical energy to the strobe lights. Likewise a landing light will often draw 5 amps of electrical current. The Sheer Tech 3 speed heater fan motor often used on the liquid cooled Rotax 582 powered Challengers draws about 4 amps on the high speed setting.

The electrical power consumption of your aircraft can be reduced by installing the new lighting systems that use light emitting diodes (LEDs) instead of the old style, energy gobbling incandescent lights. The second advantage of using the new LED driven lights is that they have a much longer service life expectancy.

Any other electrical systems on the aircraft that use electrical motors or heating elements are going to be power hogs and draw a lot from the available 14 amps of current that the engines can provide. If these power loads occur only briefly, such as when motor driven jack screw actuators extend the wheels below amphibious pontoons, then their current consumption will not be a drain to the system but they have to be considered so that they do not cause the system exceed the total current rating of the Master fuse.

Note that the starter motor system and its initial short burst of power to turn the starter motor is not to be included in this calculation of the total system current draw. This is because the starter motor draws its current (which is actually quite large at 30 to 50 amps) directly from the battery and only for a very brief period of time. This start up power is conveyed from the battery back to starter motor via a large gauge red wire (usually #4 AWG). This large amount of current is controlled by a solenoid operated switch (often simply referred to as “the solenoid”). This solenoid is turned on by the start switch which is usually a momentary (push to operate, spring return to off) push button. When the solenoid is energized by the start button, it closes the contacts and connects the starter motor to the battery power. If the motor takes several tries and a long time to start, the battery will be recharged by the generator when the engines finally starts running. So a small margin of the 14 amps of available current must be left in reserve to recharge the battery. The current draw of the solenoid itself is less than 2 amps and only occurs for the duration that the starter motor circuit is activated.

The ignition system on the Rotax engines is completely independent of the generator system. This is because the ignition system has its own coils generating power to run the engine and is self sustaining. All that the aircraft builder has to do is to connect the two ignition circuit wires to the ignition switch in the cockpit. This switch with its customary right and left circuits will run the ignition wires to ground and enable the engine to run. The ignition wires are identified as two black/yellow wires paired together, each one coming from the electronic boxes on the carburetor side of the engine.

Sizing the Switches and Wires

Switches and wires are selected based on the voltage and total current they will carry while in service.

Therefore the switches used in the aircraft electrical system should be rated to carry the sum total electrical current of all the components that it turns on. This current rating varies with different voltages so that a switch rating will state something like “20 amps at 12 volts, 10 amps at 24 volts”. Do not under size a switch or it will get very hot and may even fail in the closed position so that you will not be able to turn it off. Do not use a switch that has an unknown rating.

Switches can be used, however, if they have a much larger current rating than they will encounter in service. Over sizing the switch is not a problem, In fact, switches with larger current rating may be more suitable because they are more physically robust or a more practical size for use in an aircraft cockpit. remember that you may be flicking the switches on with a gloved hand in cold temperatures if you intend to be doing winter flying in Canada or the northern US states. It may be advisable, therefore,  to avoid using switches with plastic toggles. Plastic may break in cold temperature or after a small number of operations.

Wire should be sized so that the smallest practical size is used that is necessary to carry the total current for that circuit. Over sizing wire on an aircraft will result in a weight penalty. Typically the master switch circuit on a Challenger will be carrying less than 15 Amps, therefore 14 or 16 AWG (American Wire Gauge) size wire is suitable.This wire will carry the power from the battery to the voltage regulator and from the voltage regulator to the master switch, its fuse, and on to the main power bus which feeds all the individual switches and their circuits.

Individual circuits carrying current in the realm of 5 amps, such as for strobe and landing lights and the heater fan motor, can be wired with 18 AWG size wire. Circuits with smaller current draw such as the radio,  intercom, GPS, EIS and EFIS can be wired with 22 gauge wire.

Smaller gauge wires (24 AWG and above) and their connectors have less physical strength and are often not suitable for the environment found on an ultralight aircraft. Many builders chose to use 18 AWG on all their circuits as it is readily available from automotive suppliers for chassis wiring and is very durable.

Labelling, Recording and Identifying

Wires and connections should be clearly labelled for ease of identify circuits and for trouble shooting. It is best to use good quality adhesive labels such as produced by an electronic label maker. These labels can be attached to wires and affixed to surfaces on or beside connections on components or wiring terminal strips. Avoid using masking tape as it degrades quickly, falls off, and the pen scribbles fade and become unreadable in a few short years.

One of the most helpful tips for wiring up an aircraft is to use a variety of colours of wires. If each circuit has its own wire colour it makes following the circuit through the maze of wires, switches, fuses and connections much simpler. The initial wire up task and future trouble shooting is made much easier with colour coded. It is usually quite easy to source 18 gauge wires in at least 10 different colours from an automotive or industrial electrical supply house. Colours available typically include red, black, white, orange, blue, green, brown, yellow, purple, and grey. Ground wires are usually black. Wire harnesses with a grey cover can be colour coded by the installer with coloured marker pens or by black dots and dashes with a black marker. Be consistent throughout the wiring process.

A clearly labelled diagram of the electrical circuits for the entire aircraft electrical system should be kept in the aircraft near the instrument panel, avionics shelf, or electrical distribution box.

Electrical Noise

Electricity and Magnetism are closely related. You could say that where the is magnetism there is electricity and vice versa. If a magnetic field is passed over a coil of wire electricity is produced. If electricity passes through a coil of wire, a magnetic field is produced. When the voltage level in a coil of wire changes, a reverse voltage is produced to resist that change. It all gets very noisy very quickly with wires carrying AC (alternating current) such as generator wires or bursts of electrical current such as strobe light systems or rotating interacting coils of energized wires such as in motors.

Certain electrical components on an aircraft and the wires carrying power to them create and carry electrical noise. Electrical noise will interfere with other sensitive electronic equipment. The most common problem with electrical noise occurs with radio and intercoms. Electrical noise will cause static, popping sounds and and a high pitched whine over the audio system. It may render transmitting useless and reception difficult and painful.

The unfortunate problem with electrical noise in a homebuilt aircraft such as the Challenger is that it is often only discovered at the very end of the project during the initial flight phase when everything is assembled, completed, covered over, and buried deep in the airframe behind many permanent and semi permanent devices. This makes effecting a cure difficult, aggravating, and time consuming. The best cure then is actually prevention.

There are several electrical circuits that are big producers of electrical noise in a Challenger. These include the two generator wires (yellow and yellow/black) coming from the lighting coils on the engine to the voltage regulator and battery, the ignition wires (both black/yellow) running from the engine to the ignition switch and from there to the ground connection, any power or ground wires going to strobe lights or electrical motors. The Rotax engines also provide a tachometer wire (gray) that carries electrical pulses indicating engine speed. This is another noisy wire.

If wires carrying electrical noise pass close to communication wires or communication components especially antennas and antenna wires, interference and noise is induced into the communication circuits.

Electrical noise can be minimized and muffled by running these noisy wires as far away as possible from antennas, antenna wires, and communication components. A good procedure in the Challenger Ultralight is to run the alternator wires, the ignition wires, the tachometer wire, and any wires going to strobe lights and electrical motors, along the left side of the airframe. Communication wires between the radio, intercom, push to talk switches, and antenna, should be run along the right side of the aircraft.

Keep the strobe light and its associated power box mounted away from communications equipment especially the radio antenna and the antenna cable.

The wires carrying steady DC voltage signals such as from the EGT and CHT senders, the fuel level sender, and the coolant temperature sender are not noise producers because the voltage is very low, the current very very small, and the levels change very slowly. Therefore noise is not produced.

Because pulsating electricity creates a magnetic field, these noisy components and there associated wires should also be kept away from the magnetic compass or any solid state motion sensor or electronic compass. Twelve inches (30cm) is usually the recommended minimum distance between such devices.

Running the Wires

The electrical wires and connectors must be carefully installed so that they will not corrode, encounter pull forces or flexing, be subject to abrasion, or interfere with other controls.

Electrical wires are usually made from copper covered with a sleeve of plastic. Copper corrodes when exposed to moisture. Therefore do not place wires in an area that will see moisture. Ant connectors or junctions exposed to the elements should have rubber covers or be shrink wrapped to keep moisture out.

Electrical wires and connectors should not be installed in a way that they will see pull or push forces either from the weight of the wire or the component. Avoid a situation that would allow continuous flexing of the wire or subject the connectors to vibration. Wire and connectors should be assembled with the appropriate electrical tools. Connector size should match wire size to ensure a solid connection.

Wires should not be installed so that they rub against sharp metal surfaces. Wires in areas where abrasion might occur should be protected with an abrasion resistant sleeve. Wires passing through holes in metal should be protected with rubber grommets.

Electrical wires should be kept away from moving control cables and push rods so as to prevent any interference. An area of particular concern are the vertical aileron control cables below the aileron bell cranks. Wires passing through this area to and from the engine must be retrained so that wires will never rub against the control cables. Wires can be restrained and held in place using tie wraps or wire guides.

Electrical Connections

Electrical connectors (also called terminals) used on an aircraft should be of a type that will endure vibration without coming apart. This includes the wire coming away from the connector or the connector coming loose and falling off of the component.  Many installers use shrink wrap on wire to connector junctions so as to give some extra physical strength to the bond.

Ring style connectors are preferable to fork or spade style connectors as they are less likely to fall off of a screw connection.

Any push on connections should have sufficient friction to preclude the junction from disconnecting over time or due to vibration.

Wire connections should be covered so that a short circuit will not occur due to falling metal debris, liquids or other loose metal objects coming in contact with them. Electrical tape is not recommended as it can come loose or deteriorate over time.

Buy good quality wire crimping and wire stripping tools.

Any solder joints should first be cleaned then well soldered so as to avoid cold solder joints that will break off over time. Excess solder should be removed. A solder sucking tool is made that helps do this. A good quality solder gun is needed to do a good soldering connection. Butane powered soldering guns have the advantages of heating up surfaces very fast and not requiring a pesky power cord. Make sure that no microscopic strings of solder carry over to other connections. A good practice is to cover solder connections with heat shrink tubing.

The Battery

Batteries that are used in Challenger Ultralights come in all types, sizes and shapes. Some use old style lead acid batteries. Others use the fancy new light weight gel cell units. You can use a cheap lawn tractor battery or an expensive battery from an aircraft supply house.  Builders find all kinds of interesting and creative places to locate the battery on the airframe.

There are some important considerations when selecting a battery and mounting it to the airframe.

The first consideration is the power rating of the battery. The Rotax engines do not require a huge battery to turn the starter motor. Batteries are often vastly over sized on Challengers. The Rotax Installation Manual states on page 18-8 that the battery must have a minimum 16 Ah (Amp Hour) rating and be a high discharge type. A larger capacity is preferred.

The size of cable running from the battery to the starter solenoid to the starter motor is also important. An undersized wire will not convey the power effectively.  The aircraft kit supplies 2 lengths of 4 gauge wire, 3 feet and  11 feet, for this task. Do not substitute a smaller wire size for this or you may encounter starting trouble.

Most builders locate the battery in the nose of the aircraft. Some locate it between the pilots feet on aluminum angles running from the 2CT-2 and 2CT-1 cross tubes. Others build a battery box on a frame work that extends forward from the two main longerons. The advantage of nose mounting the battery is that it offsets the tail heavy C of G of the aircraft. This is preferred unless the pilot is very heavy and thus needs some more weight in the rear of the aircraft to offset his body weight up front.

Batteries should be held down by very strong restraints. Batteries are generally very heavy. If the aircraft encounters turbulence, the battery may break loose if it is not properly restrained and this would be disastrous. The battery should be restrained with several very heavy duty cable ties, or a pair of marine style battery straps. Bungie cords are not acceptable as they are elastic and are susceptible to weakening over time and may break.

If a lead acid style battery is used, the sealed variety is preferred so as to prevent leakage. If a battery leaks the highly acidic electrolyte will cause corrosion to the aluminum frame and will deteriorate the fabric and discolour or bubble any paint it  contacts.

If a gel cel is used, it should be located in a position that will not cause any harm to the pilot if it should catch fire or overheat. This has happened on rare occasions. If the rare occasion should occur, you would not want the gel cell mounted to the bottom of your seat.

Finally, the battery should be located where it can be reached easily for inspection, recharging and removal.   Many builders cut a hatch in the top of the nose cone to allow easy access to the battery for these reasons. Having a nose cone that is easily removed with 1 half dozen screws is a better idea. Having the flip up nose cone option is best as it allows quick and easy inspection of all of the equipment and controls located in the nose.

Compass Location

As mentioned in the beginning of this article, electrical equipment generates magnetic fields. These magnetic fields can severely affect the accuracy of your compass whether it is a conventional liquid float compass or a modern solid state compass. A compass is a very sensitive instrument. It is designed to move with the very weak lines of magnetic field produced within the planet. It will also move when it senses the magnetic field that is generated within a nearby gauge to move a needle. Yes, within those gauges is a tiny coil that produces a magnetic field to move the needles on the gauges.

To prevent the compass from being affected by the magnetic fields produced by electrical instruments, it should be located on the instrument panel as far away from any electrically driven instruments as possible.  A good practice is to locate the compass at the top of the instrument panel with the purely pneumatic instruments (the Altimeter, ASI and VSI) below it. Then placed underneath these non-electrical instruments are the gauges and other gadgets that use electrical power. These electrically driven gauges include the following: EGT, CHT, Water Temperature, Fuel Level, Volt Meter, Ammeter, Tachometer and Hour Meter. Other electrical components that should be located away from the compass include the Ignition Switch, the Master Switch, Radio, GPS, EIS, EFIS, and Transponder.

Mount the compass using stainless steel or aluminum fasteners not steel steel fasteners as these may carry a residual magnetic field. Do not locate any steel objects or magnets near the compass.

The compass instructions will usually quote a minimum distance of 10 inches (25 cm) between the compass and electrically driven gauges. This distance is often very hard to achieve on the small instrument panels found in Challenger Ultralights, but try your best to maximize this distance.

Summary of a Good Electrical System

Keep alternator, ignition, tach, strobe, and motor wires away from radios, intercoms, antennas and their associated  wires.
Keep strobe light systems and electrical motors away from antennas, radios and intercoms.
Determine the current consumption of all your electrical equipment.
Size the wires, switches and fuses according to the load they will handle.
Use good robust metal toggle switches.
Make sure all connections will not come apart under tension or vibration.
Protect wires from abrasion and flexing.
Ensure wires will not interfere with control cables or push rods.
Label your wires and terminal strips.
Use colour coded wires.
Place electrical instruments as far away from the compass as possible.

Click here for a schematic diagram of a typical Challenger Ultralight Electrical System.

Colour Key to the 6 Rotax Engine Wires:

Brown: Ground Wire – Connect this firmly to the metal of the engine and the airframe.
2 Black / Yellow: Ignition (Magneto) Wires – Connect to Ignition Switch then to Ground.
2 Yellow and Yellow / Black: Generator (Lighting Coil) Wires – Connect to Voltage Regulator Input Wires.
Gray: Tachometer Wire – Connect to analogue Tachometer – not used with Tiny Tach.

Recommended Wire Gauge Sizes

Battery Positive to Solenoid to Starter Motor – 4 Gauge
Battery Negative to Airframe – 4 Gauge

Battery to Voltage Regulator to Master Switch to Fuse to Power Bus – 14 or 16 Gauge
Generator to Voltage Regulator – 14 or 16 Gauge
Loads 4 Amps and above – 18 Gauge
Loads 3 Amps and below – 20 or 22 Gauge
Common Ground Bus to Airframe – 14 or 16 Gauge