Electric Guides

= History of this Electric Guide =

Jacko from Parkflyers wrote a wonderful electric guide over here

Slowly but surely we will port it over here. This will make it easier for people to add to it and grow it.

= Dummies Guide to Modern RC Aircraft Electric Power Systems =

Version 0.2 12 April 2009

INTRODUCTION
There seems to be a lot of confusion out there when it comes to powering a radio controlled aircraft electrically. This article hopes to clear up some of the terminology and to give you a decent idea of what it is all about.

Many people seem to have trouble with electrics, particularly those coming to the "darkside" from nitro, If you get it right you won't go back! There are a few reasons for the problems.

The main trouble seems to be is the fact that there are so many motors and a batteries available, all of them having strange numbers attached them.

The truth of the matter is that you can now perfectly design a power system for your plane and one that fits your needs. With nitro power, the main rating for the performance for your average model is the capacity, e.g. a .40 sized plane takes a .40 sized motor. Obviously this doesn't take pylon racing engines and the like into consideration, but I'm trying to keep it simple for the 3rd paragraph.

Note - decimal numbers relating to motor size refer to cubic inches due to the American influence. Decades ago we referred to cc, cubic centimetres.

If you want to, you can't take electric power systems much further than –

How fast do you want to fly?

How long do you want to fly for?

How much do you want it to cost?

How much do you want it to weigh?

Do you want a nice floaty park flyer or a monster capable of showing the space shuttle a real power to weight ratio?

It's all about finding the right balance of weight/cost/thrust/pitch speed/flight time for your model. One of things you may have noticed is when asking an electric flyer for advice on a power system for your model is a sort of distant glazed stare, its the equivalent of asking the meaning of life. There are literally thousands of ways to power a single plane, all of them preforming differently, and so the answer is a hard one.

Another hard question that is often asked is “What is the electric equivalent to a .25?”

There is a general rule of thumb floating around about differing numbers of “watts per pound” (watts is metric, pound imperial – watts with that??) while this works to a degree it is far from an accurate rule and can lead to some poorly powered planes. Its is however the best rule of thumb currently available.

Some examples of where the “watts per pound” rule falls to pieces -

300 W could spin a tiny pylon propeller at silly high rpm, giving very little thrust, but lots of pitch speed

300 W could spin a large slow fly propeller at low rpm, giving lots of thrust, but very low pitch speed

300 W could spin totally the wrong propeller and be making 50% useful thrust and 50% of the watts could be turned into heat.

The aim of this guide is try to help people not to fly a pylon style power setup on a 3D plane, and also try stop global warming...

Add this to the theory that says a nitro engine of “X” cubes is 1 brake horsepower, which converts to 746 watts (1 kW = 1.33 bhp).

So logically one would require an electric power system that is 750 watts to be the equivalent to the previously mentioned nitro. THIS IS NOT THE CASE. The only real way of comparing a nitro to an electric is to spin the same prop at the same rpm.

The prop is doing the work not the motor/engine. So if they both spin the same prop at the same rpm, they produce the same amount of useful shaft turning power.

But why would you want to spin it the same anyway as you can tailor the electric power system to spin it at close to the perfect rpm for the plane and its intended use.

It's not entirely the electric motor manufacturers problem with the way electric motor performance is described when it comes to comparing the two. I have yet to see a nitro manufacture who states the same performance numbers. Many give a bhp rating, an rpm range, a prop range. The thing that is never mentioned is at what rpm, swinging what prop, is that advertised bhp produced? In reality the number of watt/bhp is a fairly useless way to gauge performance with any sort of accuracy. It's ALL about X prop at Y rpm and how that prop preforms at that rpm. Not all props are created equal, or using the same rules.

Go take a look at the back of your electric heater or light bulb, these are both rated in watts, but strangely enough don't swing a prop...

Look out – some maths, its easy though.

Watts is Volts times Amps, (P = V x A) nothing else.

1 V times 1 A is 1 W

and

20 A times 11 V is 220 W.

I will not cover brushed motors, NiMH, NiCd or steam power in this guide. If you are flying these it's most likely in 2009 or it is an RTF aircraft. It is not worth upgrading anything in this setup to anything other than brushless and LiPo/a123. They are not much more expensive and offer far greater performance and efficiency. Simply put, keep it stock, or upgrade to brushless.

SAFETY WITH ELECTRICS
For some reason electrics are falsely considered safer than nitro. I don't think they are at all. Quieter and cleaner? Yes. Safer? No. Unlike a nitro/gas power plant which takes a process to be started before it is dangerous, with an electric, as soon as you plug the battery into the circuit your throttle is armed. Knock it and the prop spins. There are ways to make this safer -

The easiest is to ALWAYS treat the plane as if the prop is spinning when the battery is in it, combined with a conscious effort to put your thumb over the throttle stick, holding it at zero whenever handling the model.

Other ways are – Programming a throttle hold on your radio and using an arming switch on the ESC. These work but can lull the user into a false sense of safety. Forget to flick the switch and the prop can spin with a little knock of the throttle stick.

One of the cool things about electric motors over nitros is the fact the electrics give virtually 100% torque from zero rpm. A nitro has to get into the right rpm band before it gives 100% torque. This combined with the fact that a nitro will stop once the piston stops moving (i.e. the prop hits something), and unless something in the electric system blows, an electric will not, it will keep doing the funky chicken dance until you disconnect power or it self destructs.

For anyone who doubts the ability of an electric to maim you seriously, go talk to the nurse who stitched my shoulder/neck/chest up.... Be careful, Electrics can bite hard.

TOOLS OF THE ELECTRIC FREAK
There are a number of tools I suggest you look into getting if you're serious about flying electrics, they aren't necessarily expensive in the whole scheme of things. Some just make life easier, some WILL save you money in busted/burnt expensive electric components.

WATTMETER
These are becoming a must when it comes to trouble shooting, understanding your setup and getting the most out of it. The most basic simply tell you (in real-time on an LCD) what is going on. They measure voltage and amps and multiply the two to give watts. Simple ones go for around $60-80 NZ and are all that you really need when starting. Trust me, as soon as you plug one in between your battery and ESC, power up and start playing with the gas stick, you will say “Wow.... now I get it!”.

Top of the range ones, data loggers, like an Eagletree V3 or Hyperion's Emeter & RCU do much, much, more. They can be left in the plane and will record many parameters of your flight so you can have a look-see/brag once you've landed. Both of the above will give you a direct readout once landed and can be plugged into your PC for much more informative information... and some pretty graphs. Airspeed, Altitude, Temperature, rpm and many more.

The Hyperion Emeter II is a serious piece of gear. It includes an optical tachometer and has built in expandable prop charts, so it can tell you exactly how much thrust and pitch speed you're pulling, as well as the efficiency of the system.

All of these can be done without the Emeter, but it just makes for less work and it would be handy to have it all in one magic box. One of the cheaper ones would be the HobbyKing HK-010 wattmeter and voltage analyser for about $20US.

TACHOMETER
A tach is a handy thing when it comes to judging your power system's performance. It is the old prop and rpm thing again. Either an optical job, or part of your data logger setup work equally as well, and don't cost much.

SOLDERING IRON
There is a choice of soldering station or soldering iron. A solder station $70 - $400 or a temperature controlled soldering iron ~$35. There are cheaper non-temperature controlled irons also available. 25-30 watt should be sufficient.

Jaycar is a good source. Note Dick Smith only have a couple of soldering irons and appear to be getting out of that type of equipment.

A soldering station or soldering iron is a personal preference. Both have cords though on some soldering irons the cord will be thick and heavy. The soldering station's iron usually heats fast and is temperature controlled. It can be higher wattage meaning it won't cool as it is "loaded". Temperature controlled soldering irons can be as light or lighter than some soldering station irons.

Soldering guns, trigger action, are generally heavy though are almost instant heating.

ELECTRIC POWER SYSTEM CALCULATOR
I don't mean an abacus, but a genuine electronic calculator. There are many of them, hundreds? Some are basic as they come, some are much more complex, some are quite accurate at predicting results, some aren't. My favourite? ScorpionCalc, it's quite complex and possibly tricky to get your head around to start with, but it takes everything into account and spits out arguably the most accurate results. It also is a downloadable stand-alone calculator so you don't have to be online to have a play. Its obviously focused at the Scorpion line of motors, but has a custom option where you can plug the numbers of any motor you like in and see the results. It also includes a propeller efficiency section, which is rare. I highly suggest you download it and start plugging in numbers. The best thing is it free. If you want to try others – Google is your friend, and if you find a better one – Tell me!

Get it here - Scorpion System Downloads

Rcgroups thread - Scorpion Calculator

UNDERSTANDING BASIC ELECTRICS
Go grab a cup of coffee and get ready!

The most common way to learning to understand an electric circuit is to think of it like water. Bruce Lee suggests that we should become the water, I've got no idea how it relates to flying planes.

THE BASICS
Voltage is pressure. ( V ) Measured in Volts. Amperage is amount of water that is flowing. (I) Measured in Amps. Resistance is well, resistance. Like leaves in your gutter, less is better. ( R ) Measured in Ohms

Resistance is a bad thing, it is wasted power that gets converted into heat. The bad thing about this is that as your average conductor heats up, its resistance increases, so it heats up more and you can see where this ends up.

This is all you really need to know, not hard at all.

ELECTRIC FLIGHT COMPONENTS
This is where it might start getting interesting. I'll break each bit down, explain how it works and the crazy numbers that are sold with it. Then hopefully I'll stick the system back together and explain how the different parts affect each other. Unlike a nitro, the engine and the prop are not the only things that affect the resulting performance.

BRUSHLESS MOTORS
The Business end of system. Motors are the cool bit that takes the magic smoke and turns it into a spinning thing. A few terms for you get your head around -

Stator – The bit the doesn't move, this is the bit you can see that is covered in wire

Rotor – The bit that does move, and has magnets stuck to it

Available in two configurations, the outrunner and the inrunner. They work on exactly the same principle, but the outrunner's rotor is the exterior can of the motor, with the stator sitting in the centre of the motor. The inrunner is the exact opposite, rotor in the centre, stator is the exterior. Both designs have their own benefits, but the majority of motors are outrunners. Just to make it more confusing there are outrunners that have another external can around the outside, which makes them at first glance to be an inrunner (align helicopter motors are an example of this). They are relatively rare however.

The inrunner is generally taken to be the most efficient of the two, and is also better at swinging a small prop at higher rpm, or used through a gearbox to swing a larger propeller at lower rpm. Outrunners are good at swinging larger props directly, and are creeping up on the inrunner as efficiency king as of recent times.

To start with I'll explain the labelling numbers most manufacturers use. Most motors are badged in what seems like a code – It's not.

As an example : 3040-6

The first 2 digits are the stators diameter, i.e. 30 mm

The second 2 digits are the stators length, i.e. 40 mm

The last number is the number of turns the stator has (we will get to this later).

Some manufacturers use a slightly different system.

As an example : 3040-1400

The first 4 numbers are the same as the example above, but instead of a turn count, a Kv is shown ( I'll explain what this misunderstood number is later.)

The above two are the most common, there are often slight differences between manufacturers but it can be generally taken that the first two are diameter, second two are length, last is number of turns or Kv.

What does this actually mean?? Well not much apart from a rough idea of the size of the motor. In general the diameter effects the amount of torque it can produce, the length is the amperage handling capability. Often a manufacturer will make a series of motors of the same diameter and increase the length of the stator to handle more power. This is where the term the Z30 / Z22 / Z40 etc. series comes from, they are all respectively the same diameter, but available in different lengths. Make sense?

The number of turns is the number of times the wire is wound around the stators core for each of the three phases. Each of the three wires coming out the back of your brushless motor is a phase. Lower numbers suggest a “Hot” motor – it spins faster and can take generally take more current as the lower the length of the wire means thicker wire can be used (thicker wire handles amps better). The more wire the manufacturer can stuff inside your motor the better it will work, so you can have a short length of thick wire wrapped a few times, or a long length of wire wrapped many times.

So you want a “Hot” motor for your plane? Not necessarily.....

The more turns of wire the motor has means it swings a prop slower, but this also means you can swing a bigger, more efficient prop.. ahhhhh you say.

Some terminology, and getting rid of some misunderstandings.

The first three are often referred to as the motor constants.

How Brushless Motors Actually Work in Practice
A brushless motor is a constant speed motor, i.e. it will always TRY to spin at its Kv multiplied by how many volts its getting. 1000Kv @ 10 V with no load will spin at 10,000 rpm.

The confusion seems to be with that TRY bit.

When you put a load on the motor (prop) it will slow the motor down proportionally to how much prop is on the front of it. And the amount of current it draws is also proportional to the load. As an example (not a real motor) -

If we stick a 1x1" prop on our 1000Kv motor at 10 V we might get 9000 rpm and it sucks 10 A (10 A x 10 V = 100 W)

If we took the 1x1" prop off and stuck a 2x1" prop on it at 10 V, say we get 8500 rpm and it sucks 20 A (20 A x 10 V = 200 W)

If we then stuck a 50"x1" prop on the front, at 10V, it might make 200 rpm and suck 200 A (200 A x 10 V = 2000 W)

The motor is trying to spin at its Kv x V but the load slows it down, and the more it slows it down the more current it draws as it tries to accelerate the prop up to its Kv x V. It will NEVER achieve its Kv x V with a load (unless you are in a terminal velocity dive, i.e. the load is essentially gone, but we won't go there)

The Cubed Power Vs Speed principle applies to props as well as planes. To go twice as fast it takes 8 times the power. To spin a prop twice as fast takes 8 times the power.

The amount of watts it CONSUMES (not creates) compared with how many rpm the motor is swinging the prop at, is its efficiency.

If a motor was a 100% efficient, all of the watts going in are going to equal the watts coming out at the shaft.

A motor sucking back 100 W and running 80% efficient. The motor is converting 80 Watts into spinning the prop, and 20 W into making heat.

Clever people have worked out exactly how many watts it takes to spin certain props. Not how many watts are put into the motor, but how many watts it takes coming out to achieve that particular rpm with that prop.

By knowing how many watts it takes to spin the prop, and how many watts we are having to put into the motor to get that same rpm, we can work out the difference between what we are getting out and what we are putting in, and this is the efficiency, expressed as a percentage.

So why can't a 10 gram 1000Kv motor swing the same prop as a 100 gram 1000Kv motor? Because the 10 gram motor cannot take the same current as the 100 gram motor can. It gets inefficient and too many watts get turned into heat and it burns up.

Kv (motor velocity constant, or back EMF constant)
This is the most misunderstood of the lot. For some reason, some people think this has something to do with power. It does not, Kv is simply a theoretical number that the manufacturer often guesses (and often poorly). It is the number rpm that the motor should do per volt with no load. If I had a 2000Kv motor and put 1V into it I should see 2000 rpm with no prop. Get the "no prop" bit? No prop, no useful power, nothing to do with power.

NOTE - do not confuse Kv (capital K) with kV (capital V). The latter is kiloVolts = 1000 volts.

Io
This is the amount of current the motor draws with no prop on it at a certain voltage. The problem is that not all manufacturers state the voltage. Think of it as like the load it takes for the motor to spin itself.

Rm
The internal resistance of the motor – obviously less is better, the lower the resistance the less electrical power gets converted into heat. We want to spin propellers, not contribute to global warming.

Max efficient current
This is what the manufacturer suggests to aim for. It is the point at which the greatest amount of electrical energy is going into spinning the prop and not making heat.

Peak current
The most you should ever see or you risk burning up your motor. Depending on your prop this is most likely to occur statically. However high pitched props cavitate with no forward airspeed so they won't actually achieve full load until the plane is moving forward. See propellers for more information.

Max continuous watts
Similar to peak current, but a voltage is also thrown into the mix, using the good old, watts = volts multiplied by amps, its possible to work out a how hard you can expect to push the motor before failure. A general rule of thumb for the average brushless motor is 3 watts per gram of motor, i.e. a 100 gram motor is likely to be happy at 300 watts.

Most manufacturers don't suggest a maximum voltage, some do however. The thing to remember is that voltage is not transferred as readily into heat, where amperage is. So therefore it is more efficient to run a high voltage, low amperage power system. But this isn't always the most practical for a number of reasons which we will get into further on.

Motor Mounts
Motor mounting is done in four ways - front mounting, back mounting, “stick” mount and clamping.

A front mount is when the front of the motor is attached to the mount, the shaft extends through mount forward. This has the advantage of hanging the motor by the middle, the rotor is spinning at the rear, the prop in front. Nice and balanced.

A back mount is when the back of the motor is attached to the mount, and everything is in front of it. The advantage of this is the motor can be spaced and shimmed by the user to clear the cowl etc.

Clamp mount is the ugliest of the lot. It is only done with inrunner motors and essentially is a circular clamp the grabs the case of the motor.

A stick mount is favoured by some companies. The motor hangs off a stick by a bracket. The stick can be cut down to length to properly position the motor. This is mainly used in low power applications.

BATTERIES
Currently there are two types of commonly used batteries Lithium Polymers (LiPo) and A123s (Nano-Phosphate LiFePO4).

Both have far greater energy density and performance than NiCd and NiMH chemistry batteries, but in saying that both have advantages and short coming over each other.

Some more terminology -

mAh – milliAmp hour
This is the capacity of a battery, a 2000 mAh battery will supply 2 amps for 1 hour, or 1 amp for two hours. This is one hour rate is the same number as C.

Note - milli (m) is 1/1000, Mega (M) is a million.

C - rating
This is the big one when it comes to performance. The C is the "fuel tank size" (mAh) and has a multiplier in front of it e.g. 20C. The "size" (capacity) of the battery times the multiplier gives the rating, that is the amount of current the battery can supply.

It is often described as, for instance, 10/20C, 10C is continuous, 20C is maximum short burst. So our 2000 mAh 10/20C battery can supply 20A continuously or 40A in a burst.

What happens when we double the battery's capacity C with the same multiplier? Max discharge rate is doubled with it.

Example - 2200mAh 20C and a 4400mAh 20C. These batteries equal 44 Amps and 88 Amp current output. There will also be a serious difference in physical size to handle the capacity and current increase.

Max discharge rate = number x C

Max short burst rate = second number x C

S - number of cells
As C has already been used, S is used to give the number of cells the battery has, which affects the voltage. More cells = More voltage. Common is a 1S for UM (Ultra-Micro indoors or parkflyers) or 3S – or 3 cell for larger parkflyers. Sometimes you will come across something like this 3S2P Or 3 cell 2 pack. This means two 3S packs are linked in parallel, voltage remains at 3S but the capacity has doubled and therefore the maximum discharge rate as well.

Voltage Sag
Voltage sag is what happens when you draw current out of a battery, the more you draw – the more it sags. Your average 11.1V LiPo shouldn't sink below 10V while being discharged. C rating effects this to a great degree. A 30C LiPo will hold greater voltage at a set current level than a 10C LiPo, and a larger battery should hold a high voltage under load than a smaller one. The best way to see this in action is to get a wattmeter.

Lithium Polymer LiPo
Lithium-ion polymer batteries, polymer lithium ion, or more commonly lithium polymer batteries (abbreviated Li-poly, Li-Pol, LiPo, LIP, PLI or LiP) are rechargeable (secondary cell) batteries.

This type has technologically evolved from lithium-ion batteries. The primary difference is that the lithium-salt electrolyte is not held in an organic solvent but in a solid polymer composite such as polyethylene oxide or polyacrylonitrile. The advantages of Li-ion polymer over the lithium-ion design include potentially lower cost of manufacture, adaptability to a wide variety of packaging shapes, reliability, and ruggedness.

Lithium-ion polymer batteries started appearing in consumer electronics around 1995. The cells we use were first released on to the market in March 2008.

Specific energy 	130–200 W·h/kg

Energy density 	300 W·h/L

Specific power 	up to 7.1 kW/kg

Charge/discharge efficiency 	99.8%

Energy/consumer-price 	2.8–5 W·h/US$

Self-discharge rate 	5%/month

Time durability 	24–36 months

Cycle durability 	>1000 cycles

Nominal cell voltage 	3.7 V

The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination.

LiPo Wiki

Lithium Polymer packs are the most commonly available today. They still offer the greatest performance in terms of energy density that is currently available. This means they have lots of “go” for the size and weight, comparatively speaking.

LiPos have a bit of a reputation for being dangerous little packs of explosive chemicals, and they rightly do have.. IF YOU TREAT THEM POORLY! If caution and a bit of respect are used in handling LiPos, they are just as safe as a bottle of nitro, if not more so. A quick search of YouTube will show many videos of LiPos exploding – all of them being mistreated...

If you observe a few simple rules you will get a good safe life out of LiPos.

The LiPo rules -

- Do not charge LiPos with anything other than a LiPo balance charger (get a GOOD one!!)

- Buy good LiPos! Don't have to be expensive – Just good!

- Always balance charge LiPos, I'll cover this more in the charging section

- If you are not flying for more than 24 hours – Store LiPos a 60% capacity, good chargers can do this automatically. You will notice your packs last longer and go harder if you do this! In this regard think of a LiPo like a rubber band – If you store it stretched it doesn't keep its stretchiness for long!

- NEVER fly beyond 80% capacity (see Robs flight time guide for more information)

- NEVER charge a LiPo in your model

- NEVER leave a LiPo charging with no one watching

- Double check your charging settings; most accidents seem to be caused by having an incorrect voltage/capacity setting programmed on the charger. For instance charging a 2S as a 3S is going to make a nasty burn hole in your carpet/home/body

- Do not discharge or charge a LiPo at more than its C rating. Remember manufacturers like to put big numbers on packs. Experience will show you what they really are. 90% of LiPos should be charged at 1C, some high end LiPos are capable of being charged at up to 5C however

- Do not stab your LiPos with sharp things (Strangely enough they don't like being stabbed.)

- Treat you LiPos gently for the first few cycles, it can increase their lifespan

- Store and charge LiPos in a suitable fireproof bag/container.... Just in case you mess up one of the above rules...

Individual LiPo cells have a nominal voltage of 3.7V, so 2S is 7.4V – 3S is 11.1V etc. A fully charged LiPo cell will sit at around 4.2V, a discharged cell should never drop below 3V. Any lower than 3V and they start to go bad quickly.

Most good quality LiPos when treated correctly will last upwards of 150 cycles. The worse they are treated – the quicker they will die. Some new generation LiPos are advertised as being capable of over 500 cycles before a noticeable performance drop – This ROCKS! Personally I've had some LiPos that last 40 cycles before dying, some are knocking on 250 cycles. “Treat 'em mean keep 'em keen” does not apply to batteries....

A123 LiFeP04
The lithium iron phosphate (LiFePO4) battery, also called LFP battery, is a type of rechargeable battery, specifically a lithium-ion battery, which uses LiFePO4 as a cathode material.

Energy density 	220 Wh/L (790 kJ/L)

Specific power 	>300 W/kg

Energy/consumer-price 	0.5-2.5 Wh/US$ (US$0.11–0.56/kJ)

Time durability 	>10 years

Cycle durability 	2,000 cycles

Nominal cell voltage 	3.3 V

LFP chemistry offers a longer cycle life than other lithium-ion approaches.

The use of phosphates avoids cobalt's cost and environmental concerns, particularly concerns about cobalt entering the environment through improper disposal.

LiFePO4 has higher current or peak-power ratings than LiCoO2.

LiFeO4 Wiki

A123s are not directly built, or at least directly available in any number, as “plug and play” batteries for toy planes. The cells are available from many sources, including Dewalt power tools! So if you are playing with A123s you are going to be assembling your own packs, and therefore hopefully have some clue to what you are doing!

A123s have many benefits over LiPos – They are much more stable, are mechanically stronger, are not as easily damaged through abuse and can safely be fast charged. They are however, lower voltage per cell (3.3v), noticeably heavier, only available in 1100mAh and 2300mAh sizes. Running cells in parallel can solve capacity limitations though.

Instead of covering A123s more in depth – I'll point you at some of the resources, they are far better written than what I can do anyway.

http://media.hyperion.hk/dn/a123/123-brief.pdf

http://media.hyperion.hk/dn/a123/packassy/A123packassy.pdf

http://www.A123racing.com

And if you still can't find out what you are after, search Rcgroups.com

CHARGING
The batteries that most people are familiar with are lead-acid batteries in cars. Generally the charge rate for them is 0.1C. A 20Ah battery is charged at 2 Amps for about 10+ hours. Similarly for NiCad and NiMH batteries.

Lithium batteries can be charged at 1C. That is a 1000mAh (1Ah) battery can be charged at 1 Amp for 1 hour. As the chemistry and technology is advancing the allowable charge rates are increasing. Read the label on the battery before "pushing" it.

Note that though a battery might theoretically be fully charged in 10 hours (lead-acid) or one hour (LiPo), it will take a small percentage of extra actual time to reach that full charge state.

CHARGERS
Chargers come with their own set of reasonably confusing numbers and abbreviations. Hopefully I can clear that up here and help you get the charger you want/need. I'll only focus on decent chargers here. I did my thing with the RTF so called balancing chargers, 90% of them don't work as they should. If you are serious about electrics, get a serious charger. Like batteries, a good charger doesn't have to be expensive. Most good chargers will cover a wide range of battery chemistries, but in the interests of keeping it simple I'll cover LiPo charging here. Charging A123s is for all purposes much the same, but with higher charge rates and a different charging “philosophy”. You don't really need to know how it does it, just to know how to make your charger do it!

All chargers are sold with a maximum watt rating. This is the main thing to consider. It works in exactly the same manner as we have discussed above 1W = 1A x 1V so to charge a 2200 sized 3S LiPo safely at 1C we will need a charger capable of supplying 12.4 Volts (Fully charged 3S LiPo voltage) and 2.2A (1C) we need a charger capable of around 27 W (12.4 x 2.2 = 27ish). Obviously the larger the capacity, the higher the cell count, the faster you want to charge, the more you have to spend!

A lot of confusion exists around the idea of “Balancing” batteries. If charging without balancing is done you can get into some problems! Your LiPo is made of more than one cell and that cell likes to to stay within a set voltage range. Go out of that range and stuff burns and/or breaks.

So when you have a battery that is at 12.4 Volts fully charged, how does your charger know that each cell is happily at its fully charged state? One cell could be in a discharged state, one could be happily charged, and one could be massively overcharged. The pack would still register the same voltage as a perfectly happy one.

This is what balancing aims to solve, the charger will monitor each individual cell while charging and make sure they all arrive at the right voltage. It's simple and VERY necessary!

Adding to the confusion of balancing, there is no set standard for the balancing plug (or “tap”). Different manufacturers use different plugs just because they can! It's annoying, but is easily solved. The easiest way is simply buy a range of balancing boards for you charger or batteries. Another way is to rewire all your LiPos to the same standard of plug. I'd recommend simply buying adapter boards however as there is less likelihood of making sparks and smoke! I would recommend getting a Polyquest/Hyperion style adapter and a JST-XH style adapter – these two will cover most LiPos available in NZ today.

For more information on balancing taps and which batteries use which style -

http://aircraft-world.com/images/BalConnApps.gif

Another commonly asked question is how long does it take to charge. The simplest answer is an 80% discharged LiPo being recharged and balanced at 1C, a bit less than an hour. I would recommend not charging at above 1C until you are confident of your charger and your LiPos. A typical 2C charge takes around 40 minutes. The time keeps dropping as you up the charge rate, but not at a linear rate, as the final balancing of the pack is not done at a high rate. You could expect a 3C charge to take 30 minutes.

Most chargers do not come with a 240V AC input or adapter, and certainly no high powered ones do. Most charge at the field off a deep cycle marine gel cell battery or off your car battery. When charging at home, a bench top power supply is most commonly used. Be sure to get one that has the capacity to support your charger's thirst! PC power supplies can be modified to power charging setups.

Details can also be found at -

Computer ATX power supply to lab supply

Computer power supply to bench supply

This page shows the wire links needed to link the start-up pin on an ATX supply -

Startup pin linkage

BRUSHLESS ELECTRONIC SPEED CONTROLLERS AND BECs
The brushless ESC is the really clever part of the system. Unlike a brushed ESC which is essentially just a big variable resistor, a brushless ESC spits out “pulses” to the individual phases of a brushless motor. The frequency they are spat out at controls the rpm of the motor. Think of it like the ignition system on a gas engine, firing off the right electric pulse, at the right time. All these pulses are done at an incredible rate, and result in rotor turning around the stator.

Brushless ESCs also come with a wide variety of associated numbers and acronyms. The main one is the amperage capability. This is the maximum continuous current that the ESC is recommended to flow, Keep it conservative. If your system draws 30A, go for a 35A ESC as an example (in most cases an 80A+ would work just as well, but is going to be heavier – and more expensive!) Other factors that are less well advertised - The recommended maximum voltage, The maximum switching frequency, and the internal resistance. 99% of the ESCs on the market are perfectly acceptable for flying your average park flyer, its when you start getting into large, high pole count motors, running at high rpm that the switching frequency can come into play. And as always – Less resistance is always better!

Keeping an ESC cool should be very high on your priority list when setting up a model, keep it in moving airflow! Some manufacturers uprate the current handling ability of an ESC (by as much as 20A) by simply fitting a bigger heatsink. Keeping them cool is a BIG factor.

Most smaller ESCs include a built in BEC, or Battery Eliminator Circuit. The whole idea of the BEC is to provide a lower voltage (5V or 6V) from your flight battery to your radio system, through the ESCs throttle connection. Most built in BECs are what is known as linear BECs, the excess voltage is simply converted into heat, the more voltage – the more heat. Most are rated at around 2-3 A on 3S, which is plenty to fly your average park flyer with 4 average 9 gram servos. When running a higher voltage (4S and above) or more than your average 4 servos, I highly recommend using a separate switching BEC (SBEC or UBEC). A switching BEC controls the output voltage by switching it on and off really fast! And so are not practically limited by input voltage. When running a separate BEC with an ESC that has a built in linear BEC, it is recommended to disconnect (cut and then insulate) the red wire on the ESC's throttle cable to prevent the linear BEC from supplying current and causing problems. This will not affect the function of the throttle!

Another term associated with ESCs/BECs is OPTO or opto-isolated. The opto ESC has no built in BEC, and goes one step further by separating the “high voltage” side of the ESC from the radio side of the ESC by using an optical coupler (think fibre optics). Disconnecting the red wire does not apply here.

The best practice when working out if and what sort of BEC you require is to be conservative! You will not harm it by having a BEC capable of supporting twice the current draw that you plane requires! With some radio gear (i.e. Spektrum) being very sensitive to input voltage, it is wise to use a more than capable BEC if you value your plane and gear, hoping to bring it back to earth in one piece!

ESCs can come with a wide range of programmable options for setting them up different models – timing, cell count, prop brake, soft starting, a governor mode for helis are all common examples. You will need to consult your motor's documentation and the ESC's programming guide for right setup. Some are very easy to program with a simple remote, or plugging into your PC via a USB cable. Some are total pigs to program...... Luckily in most cases the ESC's default programming should work fine for the average Parkflyer.

PROPELLERS
Propellers are probably the most poorly documented and understood part of the whole system, and are arguably the most important.

Not all propellers are made equally, or even apply the same rules. The best advice I can offer you in setting up a system is get a rough idea of the propeller range that an electric calculator suggests, get a bunch of them in that range and do you own testing with a wattmeter to make sure you are within the systems limits and still getting the maximum amount of bang. Propeller charts (derived from testing) are available online. Some searching will find then and some calculators include this data in their outputs.

Props are sold by two numbers, a diameter and a pitch, usually in inches i.e. 9x6. The prop has a diameter of 9” and a pitch of 6” (well it should do anyway!) Pitch isn't really measurable, and so should be taken as a rough guide. Some manufacturers have trouble even getting the diameter right! The bigger the diameter, the more thrust will be produced, the higher the pitch, the higher the pitch speed. Props are sometimes described as being under square or over square. As an example, an over square would be a 4.75 x 5.5 prop, an under square as a 10 x 5.

The larger the prop, or the higher the pitch, the more current will be drawn. In flight a low pitched prop will unload as it approaches its pitch speed and will draw less amps. A high pitched prop will do the opposite. As it approaches pitch speed it will come “on-step” and current draw will increase. A datalogger with an airspeed probe is the only way to see this in action, but take my word for it. As long as you are running an under square prop as most (if not all) park fliers will be, you can generally assume that your static current draw (i.e. the plane is not moving at full throttle) will be the highest current it will draw throughout the flight.

Think of pitch like the gears in your car. Low pitch props make good thrust at low forward airspeeds (good for 3D flying and the like). High pitch props make good thrust at high airspeeds (good for going fast!). Pitch speed is a derivative of rpm and the pitch and is expressed in speed i.e. a pitch speed of 50 mph. An online pitch speed calculator is here.

As a general rule, stay away from Master Airscrew (MAS) props (they are generally totally rubbish compared to the rest). But other than that, feel free to experiment cautiously with various manufacturers and their sizes until you find the right prop for the job.

Its a bit beyond the average parkflyer but not all props use the same aerofoil, and as everyone knows different aerofoils preform differently at different speeds. Likewise propellers preform differently at different rpms. This is a massive subject on its own, but if you are interested start looking deeper into propeller charts and stand-alone propeller calculators.

Two bladed props are the most common, however in the interests of scale planes, and just looking cool, multi-bladed props are available. They are however less efficient and turn more watts into heat than useful thrust. Interestingly enough, at the scale most RC planes are, a single bladed prop is the most efficient.

ELECTRIC DUCTED FAN UNITS
EDF section kindly supplied by Daryl Choat (KiwiKid)

With the advent of brushless motors that can sustain rotation speeds of over 30,000 rpm and lighter weight LiPo batteries the popularity and variety of EDF models has expanded rapidly over the last few years. The choices for EDF products range from smaller 50 mm (2") ducted fans on up to a more powerful 120 mm (5") unit. The prices go up with size and power but new entries in the market are constantly lowering the cost through competition.

The general "watts per pound" rule of thumb still applies to EDF which makes it easier to select an appropriate power system based upon the models weight and desired performance. These numbers are generally accepted as providing the desired level of performance:


 * 150w/lb for Fair EDF performance
 * 200w/lb for Good EDF performance
 * 250w/lb for Great EDF performance
 * 300w/lb (or greater) for Extreme EDF performance

Manufacturer’s or model shop’s web sites should have detailed performance figures.

The picture at the end of this summary shows the innards of a standard EDF set up on a Skyhawk with a description of the components.

Here is a brief summary about how the system works:

How a ducted fan works

Ducted fans operate as they sound: they are an internally ducted high performance fan which takes in air from the inlet opening and blows air out an exhaust tube to generate forward thrust. The air enters an impeller spinning at high rpm which propels pressurised air out of its exhaust at increased pressure and velocity. A ducted fan thrives on the air fed into it so too little air will starve the fan, thus preventing it from performing at its optimum design point. Conversely, inlets that are grossly oversized can have the same effect: a ducted fan can only process a given amount of air at any particular time, so too much air will hamper performance -- not to mention the increased drag from the larger inlets. See FSA below.

External Inlet Shape

The ideal inlet lip shape for a ducted fan aircraft is a 2 x 1 ellipse. This represents the most aerodynamic shape while maintaining good airflow into the inlet system.

Fan Swept Area

The fan swept area is calculated by subtracting the area of the impeller hub from the area of the inside of the fan housing. Most manufacturers will give you this information within the instructions for the fan unit or you can dig out your fifth form geometry notes! As mentioned above, matching the inlet(s) size the to the FSA is important to obtain maximum efficiency - too little air and the fan starves, too much and it chokes. You could just tack an EDF fan onto the fuse of a model and it would fly, but by doing the things described here we increase efficiency by up to 30/35% compared to bare fan operating in fresh air.

OK, so having done this the inlets on a model may be found to be too big or too small. If they need to be reduced a neat trick is to add intake strakes. These also give the model a more scale look and smooth the airflow into the fan unit by diverting disturbed boundary layer air.

See the picture of the same Skyhawk below.

These things only really need to be addressed when you start "messing" with stuff. In this case we were replacing a 75 mm 3 cell rated fan unit with a smaller but more powerful (because it was better engineered and could spin faster) 69 mm 4 cell rated fan.

Now, if the jet intake(s) are less than the FSA in size we need to get more air in somehow. Depending on the model it may be possible to increase the inlet size, if not the only alternative is to add "cheater holes". These are holes made in the fuselage that allow additional air into the fan chamber. They will need to be bigger than the calculated extra area needed as they will probably be side on to the air flow. Quite a few commercial models have cheater holes to maintain a scale look to the air intakes.

As long as you can get within a 95%-110% range of the FSA there shouldn’t be too much adverse effect.

Ducted Fan Outlets

Ducted fans produce best results with an exhaust outlet sized to approximately 80% to 85% FSA. (If you look at the picture below you can see the thrust tube narrows towards the outlet). Choking down airflow at the exhaust by means of a thrust tube increases exhaust velocity - a bit like putting your thumb over the end of a garden hose. This in turn will increase top end speed of the aircraft. There is, however, a point of diminishing returns: choking down the exhaust too much will back pressure the fan resulting in degraded performance. Additionally, a larger outlet area will increase the static thrust of the system, but lower the top end speed of the aircraft. The percentages suggested above represent a good compromise between static thrust and exhaust velocity.

The thrust tube can be made out of anything smooth and light. Acetate sheets are good if they can be supported or something stronger if the EDF unit is going to be out in the open. Commercially produced ones are available.

Inlet lip

This is another little trick to smooth air flow and also accelerate it. The lip is a strip attached to the edge of the fan housing and is shaped like an aerofoil with the flat part on the outside. What this does is smooth the peripheral air as it goes into the fan unit and, as it is an aerofoil shape the air is accelerated over the curve of the aerofoil and enters the fan unit at a higher velocity thereby increasing its outflow speed. Theoretically speaking, that’s the theory. Most people just make them out of Depron or you can buy commercially made ones.

Both the thrust tube and the inlet lip can be applied to free standing EDF units mounted on the back of a model.

Splitter

EDF fan units like to have smooth air to work with. If they receive turbulent air they do not perform efficiently. Two things to do to help out here, firstly make sure the inside of the fuse and or inlets is very smooth, fill any dents, sand and varnish with water based polyurethane if necessary. Secondly, if you have two inlets, the air will meet and get jumbled up in front of the fan unit. To stop this we can install a splitter plate (just made out of Depron or balsa) to keep the two air streams apart so they enter the fan separately. Some larger commercial models have specially designed fibreglass ducting to achieve this.

EDF/ESC/Battery cooling

As with all electric systems cooling is important and, with high powered EDFs operating at high stress levels, this is even more important than in lesser powered models. The motor generally sticks out the back of the EDF unit and some heatsink can be added here if needed. Make sure the battery has adequate inlet and outlet vent holes and that adequate airflow is getting to the ESC. In the model below the ESC is positioned on the bottom of the fan chamber where it gets a good dose of airflow. You can also place the ESC in the thrust tube. As the airflow here is much faster than elsewhere you can actually have a lower rated ESC (by maybe 10 amps) as it is being kept cooler by the accelerated air flow. There would be some small penalty in output thrust resulting from disruption to the outgoing airflow, but it is apparently minor.

Launching EDFs

OK, you’ve got everything balanced and she’s ready to go. No more BS - time to get it in the air. Unless you are fortunate enough to have a paved runway or well prepared field, park sized EDFs can generally be hand launched. The trick is to hit full throttle and launch about half a second later so that the fan has a chance to spool right up. You have to time it just right - too early with the launch and she will drop like a stone - too late and the thrust will try and pull it out of your hand. The other alternative is a bungee launch. Once you get the hang of it a bungee probably gives you a more consistent launch. The bungee can just be a simple bungee cord and line secured to the ground or a more complicated pedal operated contraption. The larger heavier EDFs need the bungee assist as they can’t really develop enough static thrust for self launch. The alternative of course is to have a fixed or retractable undercarriage with attendant weight, drag and complexity

Flying Site

Be aware that, while you might think your screaming EDF sounds cool, the family having a picnic on the other side of the park may not agree, so choose your flying site carefully with thought for other users.

SERVOS
Servos, those things that make the control surfaces move. The HXT900 is a good Parkflyer size and there is a cheaper orange version that appears just as good.

Reversing Servos

When fitting flaps with the servos pointing in opposite directions but required to move in the same direction, either fit a servo reverser circuit or do some soldering. Note that Servo Reversers are not recommended with Spektrum systems.

There is a thread on RCGroups

For a YouTube video showing doing a solder job on a servo.

or specifically reversing an HXT500