You may have wondered how best to select the correct components for rc electric planes, such as picking the right lipo battery, ESC and so on. In this article Alex shows you how to do this in four steps, using a RC Decathlon 48″ parkflyer as an example…

Before I begin – credit for a lot of my learning must go to the wattflyer forums specialising in providing great info on radio controlled planes and in particular this thread http://www.wattflyer.com/forums/showthread.php?t=24238 , it really is a great place to start if you are new to the world of electrics and I encourage you to take a look at it.

There are really just four steps you need to follow to select the right bits for your rc plane if you wish to do this yourself. A bit of research IS required, but if you follow the rules I’ve set below you’ll find it makes it very easy to get the right setup for your electric parkflyer.

1. How much power is required?

A good rule of thumb to follow is an RC model trainer needs approx 50-100W per pound, a sporty model needs approx 100-150W per pound, with more powerful 3D models requiring 150W to 200W per pound (and more!).

Once you have this performance figure, add another 50%. My reason is that anywhere between 10% and 30% efficiency appears to be lost throughout all the components in the system, so this helps ensure you won’t have an underpowered model.

Brushless motors tend to run anywhere between 80% and 95% efficiency, there’s resistance within the lipo battery, the ESC (electronic speed controller), each connection, cables and (in)efficiency associated with the propeller too.

Using a newly-purchased Decathlon parkflyer, I estimated it would weigh approx 1.0kg (2¼ pounds) ready to fly with a generously sized battery in it, and I selected 110W per pound for “sporty” performance. The power system thus needed to provide 248W to the prop (110W x 2.25lb). Adding 50% this number reaches a system target of 372W.

So – I was looking for a system that pulled around 370W on full throttle, delivering a useable 260W (with 30% efficiency loss).

2. Pick the battery

Take a look through your battery choices, which will help you determine what sort of ESC you will need. In the case of the Decathlon:

1 cell LiPo: 370W divided by 3.7V = 100A required current

2 cell LiPo: 370W divided by 7.4V = 50A required current

3 cell LiPo: 370W divided by 11.1V = 33A required current

4 cell LiPo: 370W divided by 14.8V = 25A required current

So a 3 cell or 4 cell LiPo battery seems like the right choice for this electric rc plane – that way flight times can remain reasonable, and component costs stay down too (a useful rule of thumb – higher amps cost bigger dollars!). I ended up selecting a Zippy Flightmax 4400mAh 3S battery – on full throttle I predicted the model would fly for 8 minutes (4.4Ah / 33A x 60 to put in mins).

Zippy FightMax LiPo Battery

Please find the next post where this article is continued…

In part one of this article we showed you how to establish the power requirements of your remote controlled plane, and select the correct lipo battery.

In this final section you’re shown how to select the correct RC plane speed controller (ESC) for your park flyer, in addition to finding an appropriate brushless motor and propeller, as well as the actual results from our R/C Decathlon.

3. Select the RC Plane Speed Controller

This step is actually pretty easy – just determine how many amps your power your system will require and then get an ESC that can handle this.

The Decathlon’s system was estimated to run at up to 33A (max) on a 3S battery (look at the table from part one of this post for a 3-cell battery). Thus I picked an ESC that will support 40A, and would burst to 55A for short periods – allowing a wide safety margin for the requirements of my electronics and a good amount of power for this parkflyer.

See, I told you this step was easy!

Turnigy 40A Speed Controller

4. Finally, select the motor and prop

There are plenty to choose from – so it’s best to just read up on some reviews before making your final decision. www.bungymania.com is a good place to access a wide library of test results from different motor/prop combo’s.

For the radio controlled Decathlon, I selected the Turnigy 3536C motor, and matched it with a 10×7 wooden prop, again my research suggested this combo would draw about 30A at full power, and the motor was able to take the required 3S battery (11.1V).

Once I had everything, I secured the motor, ESC and lipo battery in my ARF (almost ready to fly) model kit and got ready to run some tests. I completed it with a Spektrum AR500 full range receiver, and some Parkzone and HXT servo’s I had on hand.

Turnigy 3536C 1100kV Motor - a great fit for larger parkflyers

So how does it perform?

At full throttle my watt meter reads 34A – so pretty in line with what I wanted, and thus I am really satisfied with this result for this RC model plane. Notice even after increasing the required power by 50% I found the motor was a perfect fit, in line with the Amps expected.

When flying the model, there is a good amount of power available but not excessive, it takes off steadily at 2/3 throttle, and it will fly around for about 15 minutes at just over half throttle thanks to the big 4400mAh battery.

The park flyer runs for about 9 minutes when I push the power a little more, and comes down with, at most, a warm battery, with each cell hovering around the 3.80V mark. The electronic speed controller comes down cool as well.

I’ve handed the controls around a little at the MARCS club in Melbourne, Australia, and the comments I have received are consistently positive – not excessively powerful but certainly with enough “oomph” when needed. It certainly manages spins, rolls and loops easily, so very much in line with the sportiness you would expect of a Decathlon.

So there you have it – four steps to pick the correct electronics system for your plane. I hope you’ve found this useful – and feel free to add any comments / questions below that you may have on this methodology.

I’ve had a few questions from fellow flyers at my flying field (and seen questions also on forums), regarding the “C rating” on a LiFePO4 and RC Lipo battery,  and in particular what it means for us radio control pilots, so here’s an explanation to get it all wrapped up.

For us electric RC fans, the C rating is simply the measure of maximum safe dischange a battery can take over a sustained period of time.

As you’re probably aware, RC planes and helicopters draw significant amounts of current when we’re using them so this is an important concept to be aware of.

In simple terms, “C” acts as a multiplier of the mAh capacity of the battery. Here’s a quick table I’ve put together:

Discharge Rate    Time to Discharge   
     1C                1 hour
     2C                30 mins
     4C                15 mins     
     8C                7.5 mins
     20C               3mins
     etc...

As you can see, the higher the discharge rate (or current, or throttle position), the quicker it takes to discharge a battery… common sense really

A Park Flyer example

When on WOT (wide open throttle) our DHC Beaver rc plane draws approx 18A*. Depending on the battery capacity and “C” rating will depend on whether we are causing damage to the battery discharging at these Amps.

Radio Controlled Beaver

In this case, I use a 1500mAh LiPo battery, rated to 20C (just like this one).

So let’s work out if we’re causing too much stress on the battery:

• We know that this airplane draws 18A on full throttle (with the 3 bladed 8×6 prop I’ve fitted)
• We know my battery has a 1500mAh capacity (or 1.5Ah)
• Dividing 18A by 1.5A gives us 12…  this is the discharge rate “C”

Maximum current draw at WOT    /     mAh battery capacity    =   actual discharge rate in “C”

As the battery is rated to 20C, if I’m only discharging at 60% of it’s maximum (12C) when at full throttle, we can be sure I won’t be causing damage to my battery. And that’s assuming I fly around the flying field  at full throttle the whole time (which I don’t given it’s such a light model).

At a glance – determining the maximum discharge rate of a battery

“C” rating    x     mAh capacity    =    max Amp draw

A simple way to determine the maximum Amp draw a battery can take is to multiply its capacity by its C rating. So using my 1500mAh LiPo as an example again, discharging at 20C would draw 30A of current. So that is the maximum safe amount of draw the battery can take, assuming the manufacturer isn’t overstating anything!

Again, 18A is only 60% of 30A so we’re not stressing the battery. In reality it comes down cool after most flights, generally not even warm, so all is good here!

What’s the Burst figure?

You may see that some LiPo’s actually have two C ratings – the “burst” and the “constant”. Often the burst figure is not labelled on the battery, but is rather in the sales literature (example).

The manufacturer is essentially stating that for periods of no longer than 30 seconds, the battery can discharge at the “burst” C rating. Personally I don’t like to discharge any more than approx 60% of the stated “constant” figure, as doing high rates of discharge repeatedly tends to shorten the life of your battery.

Furthermore, by buying getting a higher voltage battery, you can drop the Amps required to achieve the same wattage, which is generally a more cost-effective solution (a lesson for another day perhaps – for now just remember from your high school lessons that Amps x Voltage = Watts).

So – even though you may see burst figures, my preference is to ignore them, and do all your calculations based on the “constant” maximum discharge rate – you’ll be financially better off in the long run.

And finally, what’s the difference between LiFePO4 and LiPo Batteries?

Without going into the chemistry, LiFePO4 batteries, also known as A123, allow much higher discharge rates than lithium-polymer batteries. RC heli pilots who require serious amounts of discharge (40C and higher) are starting to look at these, and as the price comes down it is likely these will become more mainstream.

I hope this all makes sense – and please feel free to comment on this post, or drop in some examples of your own for us to work with!

* we measured this with this watt meter, connected it between the battery and ESC .

Zippy Flightmax LiPo Battery

Being electric R/C pilots, we benefit from some of the latest technology, in particular, batteries. Improvements in the past 5 years in battery technology is probably the main reason driving the growth of park flyers.

We use LiPo (Lithium Polymer) batteries for two main reasons – they’re light, and they’re powerful. More often than not we’ll be discharging our batteries at 15Amps or more, and with the more powerful aircraft (such as the Pitts), this figure will be closer to 30A.

Although they are stable, these Lipo batteries do pack quite a punch, and as such, care needs to be taken when storing them.

Check out this video to see what happens when you treat your LiPo’s badly:

[youtube=http://www.youtube.com/watch?v=SQ0SNESIkWk]

So here are a couple of precautions to ensure your don’t burn down your house, or your car when you’re charging or storing your packs:

a) charge them in a fireproof bag, such as one of these

b) don’t charge them too quickly (never more than 1C – the mAh rating of the battery).

c) store them uncharged in the same fireproof bag (at between 3.7V and 3.8V per cell)

This way you’ll keep your property safe, and your batteries lasting for plenty of time to fly your planes!

Until next time, blue skies!

Alex

LiPo storage gone bad (image from RC Universe)