Technology

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Batteries

There are several different varieties of batteries:

  • Lead/acid - cheap but heavy, no use for flying but good as a power supply for your charger, especially when away from mains power.
  • NiCd (Nickel-Cadmium) - cheap, but don't store much power for their weight. They are capable of relatively high outputs and can be charged in 20 minutes.
  • NiMH (Nickel Metal Hydride) - more expensive and have a higher power-to-weight but lower output power than nicads. Can be charged in 1 hour. A good choice for transmitter batteries.
  • Lipo/Li-ion (Lithium polymer) - more expensive again, and a much higher power-to-weight. They used to have limited power output but are now capable of discharging as fast as Nicads. They take at least an hour to charge. They must not be discharged below 3v per cell, or they permanently loose capacity. If overcharged or charged when damaged) they can 'vent with flame' (i.e. explode) so they must be charged away from any flammable substance. Metal boxes (ammo, tool or cash boxes) are common, but a Pyrex oven dish or steel saucepan will do (until your wife finds out!). Even in a container, extremely hot flames could be released - don't attempt to seal the container or it could burst, causing even more damage. Just make sure it's on a non-flammable surface, and a couple of feet from flammable things. Try to separate your batteries during charging so that if one should ignite, it won't damage the others (or worse, cause them to ignite).

C rating

This is the charge or discharge current as a multiple of the battery capacity.

For 1C is very easy to work out, because it's the same as the battery capacity. (Remember that there are 1000mA in every Amp)

So:

  • for a 500mAh battery, 1C is 500mA (0.5 Amps), 5C is 2.5A, 10C is 5amps
  • for a 1000mAh battery, 1C is 1000mA (1 Amp), 5C is 5A, 10C is 10A
  • for a 2000mAh battery, 1C is 2000mA (2 Amps), 5C is 10A, 10C is 20A

C ratings are useful because they are independant of the battery size - it's possible to generalise and say things like "lipos should be charged at 1C" rather than give a table of charge rates for every possible battery size.

Chargers

Nicads and Nimhs can be slow charged at 1/10C for 14-16 hours, and at that rate a few hours of overcharge will not damage the battery. They can be fast charged if the charger detects the voltage peak and switches off to prevent damage. Nicads can be fast charged at 2C, or up to 4C if you don't mind reducing their life. Nimhs can only be fast charged at 1C.

Lipos are charged by ensuring the voltage never rises above 4.2V per cell. This results in very low charge currents towards the end of the charge, and potentially infinite charge times. Charge rate should also be kept under 1C, even when the voltage is below 4.2V per cell.

It should be obvious that Nickel and Lithium batteries need different chargers. There are plenty of chargers that do both, but accidentally switching it to the wrong setting will result in a ruined battery or even a fire.

Parallel charging

Nicads and Nimhs must never be charged in parrallel, because each pack may peak at a different time and the charger won't notice the individual peaks and won't switch off!

Lithium batteries can be charged in parallel, provided that they are the same type and discharged to the same voltage before they are connected. You should check with a volt meter to be absolutely certain that the packs were discharged, because connecting a charged pack to a discharged pack will result in a large current. If there is a small voltage difference between two packs, you can connect them via a watt metter to see how much current flows between them. If it's less than 1C you can leave them connected until the current drops to almost nothing, then connect them to the charger.

Series charging

It's very common to charge cells in series (nearly all battery packs are connected in series) but as the number of cells increases the chance of an imbalance between the cells also increases.

Motors

Electric motors work by running a current through a coil of wire in a magnetic field so the coil is attracted to the magnet, and then (this is the clever bit) switching the coil off when it approaches the magnet and switching on a different coil of wire so that the motor continues to rotate.

There are two main types of electric motors used for e-flight - 'brushed' and 'brushless'.

  • Brushed motors have been around a lot longer; they use a mechanical device called a 'commutator' that rotates with the coils and 'brushes' that are fixed to the body of the motor to transfer power to the moving coils and to switch the various coils on and off in turn. These motors will run when connected directly to a battery, so their speed controllers are very simple.
  • Brushless motors require a speed controller that electronically switches the power to the coils. The speed controller has to detect the speed and position of the motor and vary the power and frequency of the supply to each coil. This used to be very expensive but brushless controllers are now (amazingly) the same price as brushed controllers.

Brushed motors have two wires, brushless motors have three. A brushed motor can be reversed by switching the wires, a brushless motor can be reversed by switching any two wires. Brushless motors are usually more efficient than brushed motors, and as the power limit of a motor is determined by the amount of waste heat it can dissipate, a more efficient motor will waste a smaller percentage of its power, so the input power can be increased significantly for the same level of wasted power. (i.e. a 100W motor that is 50% efficient is wasting 50W as heat and only generating 50W of useful power. If the motor was 75% efficient the input power could be increased to 200W for the same amount of wasted heat and it would then be generating 150W of useful power - 3 times as much!)

Kits

Brushless motors are mechanically very simple, and there are a number of people selling kits so you can make your own. These often use stators (the star shaped bit around which the wire is wound) from commercial sources such as CDROM drives, and usually contain ball bearing races and small rare-earth magnets that are hard to buy in small quantities. They often also contain machined parts that would be easy for a home machinist to make, but beyond the ability of most aero-modellers.

The main advantage of building a motor from a kit is that you can choose the wire thickness, the number of turns, winding and termination pattern and even the number of magnets. These factors all determine how fast your motor will turn at a particular voltage, so the same kit could be used for high speed plane with a small prop, or a low speed plane with a large prop. You can usually buy spare stators cheaply (or just rewind the same one) and experiment with small changes until you find the optimum configuration for your plane.

The other advantage is that kit motors are easy to repair and all the parts are available as spares.

Winding the stator for a kit can take a few attempts but is within the abilities of most modellers. It can take up to an hour, which is why kits are cheaper than complete motors.

Timing

The brushes on a brushed motor can be rotated around the motor shaft to optimise the motor to run in one direction. Many brushed motors are not designed for model planes and will run at equal speed and efficiency in either direction; in these cases the motors can be improved by re-timing. There are usually some metal tabs that need to be unbent to allow the backplate to rotate, then the motor should be run under load while the RPM and power input are monitored and either the maximum power or efficiency can be selected. Typically timing changes are less than 10 degrees.

Suppression capacitors

The brushes in a brushed motor can generate sparks, which cause radio interference. This is usually fixed by adding small capacitors between the motor terminals (and/or the motor can).

Breaking in brushed motors

Brushes do not fit the commutator perfectly when the motor is made, they wear to a better fit, but during this period they spark more than usual and this can damage the brushes. This can be avoided by running at very low power for a while, typically a motor that would usually run from a 7 cell pack would be run-in with a 2 cell pack for a hour. This can be done faster by running the motor immersed in water (fresh water does not cause short circuits at the low voltages used for e-flight).

Speed Controllers

Brushed and Brushless motors required different electronic speed controllers (ESC). Brushed motors have two wires and use DC, brushless motors have three wires and use 3-phase AC.

Low Voltage cut off (LVC) must be set to at least 3v times the number of cells in your lipo pack (6v for a 2 cell pack, 9v for a 3 cell pack, etc) or you will damage your battery by over discharging it.

For 3D flight (hovering, etc) use the 'soft' cut-off option to prevent crashing when the LVC cuts in.

Can you run two brushless motors from a single controller? Sometimes! It works for some people and not for others. Identical motors and wire lengths seem to help.

If your brushless motor stutters and doesn't run up properly, you probably have a bad connection or a short. Try wiggling the connections to see if anything changes.

Connectors

Connector ratings - use this to decide what size connector to fit to your electronic speed controller.

  • Gold bullet connectors
    • 1.8mm to 25A,
    • 2.5mm to 50A,
    • 3.5mm to 80A+.

The only limitation with Gold Connectors is the size of wire which will fit into 'em. 1.8mm (up to 18ga), 2.5mm (up to 16ga), 3.5mm (up to perhaps 12ga).

Anderson Powerpoles are fully insulated and available in a variety of colours. They come as individual connectors that may be assembled in many different configurations; you can rotate or align the contact pins. Any number of connectors can be joined together to make multiway connectors. They are available in a variety of current ratings, from 10 to 1000amps.

Tips

When attaching battery connectors, be careful not to let the wires short together. Don't cut through both wires at the same time because the wire cutter will make the connection. Cut and strip one wire and attach it to the plug (with solder or a crimp) and ensure it's completely insulated before cutting and stripping the other wire.

Put female connectors on batteries, so they can't be shorted by random metal objects.

Use a black marker pen to colour one side of JST/BEC plugs and sockets to make it easier to get them the right way around.

Servos

Servos are available in many sizes and configurations. Those with plastic gears are often cheaper and lighter, while those with metal gears can usually sustain more violent crashes without stripping the teeth from the internal gears. Servos are also available in analog or digital varieties.

Radios, Transmitters, Receivers

There are lots of different makes of transmitters and receivers but most of them are compatible (i.e. you can use your transmitter with a receiver of a different brand) but you not all the crystals are compatible - i.e. you need a 'brand-x' crystal for a 'brand-x' receiver but it'll work with a 'brand-y' transmitter.

If you are in the USA, it's illegal to change the crystal in your transmitter but you are allowed to swap the entire radio module with a different module that has another crystal installed. You are also allowed to use a synthesizer module to switch frequency. Elsewhere in the world you can change transmitter crystals and if you fly at a busy club it's a good idea to have a couple of alternate frequencies. 2.4GHz systems alleviate frequency problems and typically reduce glitching.

Receiver types

Some receivers are 'double conversion' - as you'd expect these are better at filtering out interference and correspondingly more expensive.

Another technique is receivers with Digital Signal Processing (DSP) - these use digital techniques to filter out the interference.

Both these technologies work and are worth investing in for larger, more expensive models, or if you have a problem with glitches in your area.

Installing receiver antennas

Ideally your antenna should be straight, extended to its full length and well away from metal or carbon fibre that can shield it from the signal. You should also avoid taking it through metal objects, which can choke the signal.

Radio theory says that antennas have to be an exact length (usually 1/4 wave length) to work properly and even a small difference will have a big impact on range (or more accurately, the 'gain'). RC gear is designed to be installed by hobbyists and is deliberately less critical.

It's common practice to have a few bends in an antenna; if it's taken to a wing tip and then trails, or out of the fuselage and up to the fin. This is fine so long as the overall length of the antenna is close to the original length.

It's also common to wind the antenna back and forth across a wing, or to wind it around a straw. This has a serious affect on the range and isn't recommended, but it's possible get away with it on small, slow planes where range is less of an issue.

There's a lot more information and some interesting test results here:

Does Rx Antenna Length Really Matter? ]