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Electric Aircraft

There is no question that electric powered models are the most common you will find people learning to fly on nowadays.  Electric model aircraft really have come of age and you can get almost any type of scale or sport plane powered by an electric motor. This is mainly due to the availability of brushless motors and Lithium battery packs.  You can get into an electric ARF plane with motor, servos, and speed controller for as little as $100.  Of course you still need a transmitter radio, batteries and a charger but these can all be used on a variety of other aircraft you might get down the track.

Nearly all of our electric models nowadays use brushless motors and lithium batteries. A brushless motor commonly delivers nearly twice the power than your standard old brushed motor and last much longer. The lithium batteries produce huge amounts of power, and are less than half the weight of the equivalent powered NiCad or Nickel Metal Hydride.  

Recently the Precision Aerobatics (Pattern Flying) in the USA was won with an electric powered model. At the time of writing this, the world's fasted model aircraft is electric powered.

A word of caution at this point, Lithium batteries behave very differently to the old NiCad’s (NiCad) and Metal Hydrides (NiMH)  and must be handled correctly.

Please take the following warning seriously.

 NEVER  CHARGE  A  LITHIUM BATTERY WITH A NiCad  OR  MiMH CHARGER  OR  ANY  OTHER HOUSEHOLD CHARGER.  NEVER  PUNTURE  OR SHORT  CIRCUIT  A  LITHIUM  BATTERY.  FULLY UNDERSTAND  THE SPECIFICATIONS  OF A LITHIUM  BATTERY  BEFORE  YOU  CHARGE  OR USE IT,  THEY  VARY  GREATLY.  YOU CANNOT JUST PLUG THEM  INTO  A  THE  CHARGER  AND  PRESS  ‘START’.

I cannot stress strongly enough how dangerous lithium batteries can be if not treated correctly.  If handled incorrectly Lithium batteries WILL catch fire and explode, the chemical reaction will continue after the battery has been disconnected from the charger. Never use a mobile phone or computer lithium charger as you don't know what configuration it is set up for. If you are new to electric flight start off by talking to someone who 'Knows What They Are Doing'.  

Okay, now that I have scared you out of ever going near a Lithium cell.... electric flying is great, no messy fuels, no glow plugs, no noise, no cleaning up after flying etc. And once you are setup it is a lot cheaper than your combustion engines.  Almost every type aircraft that uses a petrol engine can be converted to an electric power.

Okay, a little bit of the technical stuff you MUST understand when working with lithium batteries. There are a number of different types of lithium cells, for example.... lithium polymer, lithium ion, and lithium ferro phosphate.  Each one produces a different voltage and MUST be charged on settings that suits it’s specific type of chemistry. The most common one we use to power our motors is the lithium polymer, so called because each cell is lithium gel in a polymer bag.

There are three attributes that you must get right when working with lithium batteries.  

  1. Number of cells in the pack. This determines the voltage of the pack, the more cells the more volts and the more power we will get.

  2. The milliamp capacity of the pack.  The milliamps is how much fuel we have in the tank, the higher the milliamps the longer the battery will last.

  3. The ‘C’ rating of the pack.  This rating is how quickly the battery can give up it’s power.

Well from the above the obvious thing to do is to use a battery pack with the largest number of cells, the highest number of milliamps and the highest ‘C’ rating.  Unfortunately you can’t do that.  Your motor will be designed for a specific voltage so that will govern the number of cells.  Each cell in a lithium polymer battery puts out 3.7volts under load (typically fully charged they are more likely around 4 volts per cell), the most common motors we use want a maximum of 12 volts, so we must use no more than a 3 cell battery which give us the closest to the 12 volts.  Any higher and you’re likely to damage the motor and speed controller.

The more milliamps the pack has the longer it will last but unfortunately the heavier and larger it is.  We are usually limited to how big the pack can be by how much space we have for it in the model, and more importantly the plane might not be able to carry the weight of a high milliamp pack.

Generally speaking you can never have too high a ‘C’ rating pack, but there is a minimum you must have.  The minimum ‘C’ rating is governed by how quickly the motor needs the power from the battery pack.  If you have a really powerful or fast spinning motor it’s going to draw heavily on the pack, if it can’t get the power quick enough the motor will surge and the battery pack will get very hot and you will rapidly damage the pack. The worst thing is the higher the ‘C’ rating of the battery pack the more they cost and the heavier they are.  Many manufacturers offering budget price batteries will state a high ‘C’ rating on the pack but realistically it is nowhere near what they say and the batteries don’t perform.  Experienced electric flyers have learnt to buy battery packs from manufactures who indicate the true ‘C’ rating rather than wasting buckets of money on dodgy and usually cheaper packs.

Never fly your battery packs to dead flat, it will severely reduced the life of the battery and they will get very hot.  Most charges will not start charging a battery if it is dead flat. Do not take them lower then 20% or 30% left.  We all set timers on our radios to stop us over flying the battery  

When charging a battery pack we must set the charger to the number of cells of the pack and how quickly the battery can be charged at... the wrong number of cells or too fast a charge and the battery will more than likely get very hot and then explode and burn.  Most chargers now have a ‘balanced’ charge option which means while it is charging it will match the voltage in each cell, most people do this every charge but it must be done every 3rd or 4th charge, it takes slightly longer to charge but is well worth it for the safety, maintenance and performance of the pack.

Okay one last thing on the basics of lithium batteries, they do not like to be stored fully charged.  Storing them fully charged dramatically reduces the life and performance of the battery.  It is recommended that they be stored at about 40% of their capacity.  Most chargers have a storage setting on them which will bring the pack down to the correct storage voltage or you can also buy smaller attachments that will bring them down correctly.  In the early days we never used to think storage voltage was important and we were always wondering why our battery packs were puffing up and becoming weak, it was because we weren’t reducing them to storage voltage when we weren’t using them for a few days.

Treating your batteries properly will ensure you get the maximum amount of use and power out of them.  Charge rates, balancing of the cells, cycling, minimum discharge levels, storage discharging are all important.

Electric powered models use the same radios as gas powered models with one exception, the throttle of the motor is operated by an electronic speed controller (ESC), this replaces the throttle servo in the gas engine powered models.

Speed controllers have to be matched to the type of motor you plan to use and the amount of current (power measured in Milliamps or Amps) the motor is capable of pulling out of the battery. If you don't get this match right you'll just be burning up money... literally. Brushless motors must be used with a brushless speed controller, he older brushed motors are totally different.  The easiest way to tell if a motor is brushless or brushed is to check how many wires are coming out of the motor, a brushless motor has 3 wires and a brushed motor has 2.

Motors come it all different configurations, some have reduction gear boxes to swing big props and a huge variety of armature windings. The internals of the motor can get quite technical but as a rule of thumb, the less the windings (Kv) the slower it will turn but the more torque it will have, but it will also require more current to turn it.  The higher the Kv the faster the motor will turn but the less power it will produce. For example, a jet with an electric motor spinning a turbofan will need to turn really fast so it requires a high Kv motor, however a slow old Tigermoth that swings a big slow propeller would be better suited to a lower Kv motor.  Gets a bit technical doesn’t it, not to worry as most electric aircraft will come with the recommended motor and speed controller or will tell you which one to use.

Be very careful, the brushless motors nowadays can draw huge currents and the battery packs can be extremely powerful,  if short circuited the consequences can be serve.  One of my helicopters can draw up to 100amps... you can weld steel with 100amps !

Electric models have come a long way very quickly and are an excellent way when  getting into the hobby. There is huge selection of 'Ready To Go' models in the Hobby shops and online. The same principles of what makes a good trainer applies to electric models as they do for gas powered ones, make sure you buy the right type of plane if you are starting out, see the section on Aircraft Types.

                                            Contributed by Brian Carson - SMF Senior Instructor (Note from Bob Cottle Webmaster)n  - SMF Senior Instructor (Note from Bob Cottle - Webmaster)