Electric Flight in Australia

 

What I wish I knew about electrics before I started!

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Peter Pine
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Marty's Guide for Large Model Electric Flying
by Martin Hillier

Why Electric?
When I was enthused to change over to Electric it was because of the availability of large brushless outrunner motors with power outputs comparative with IC engines, particularly 4 strokes, and the light weight, high capacity LiPo cell packs that became available. The flight pack for my Cub was under $1K in cost, including charger and balancers, and weighed less than 1Kg - comparable with the weight and cost of a Saito or Enya 150/155 stroker and silencer, fuel tank, fuel, fuel tubing, pushrod and servo. Easier to install, saves a fortune on nitro fuel, easier to operate and cleaner! What's (no pun intended) more, my battery choice along with all that charging paraphernalia means subsequent electric models will be considerably cheaper. What follows is some of What I Wish I Knew When I Started.
With any shift in technology there are lots to consider.
  
You have to first pick the motor to do the job, choose a propeller, pick the LiPo cell packs to provide the voltage to drive the motor and the amperage to give duration, ensure the ESC is capable of handling the voltage (LiPo cells), current draw (influenced by prop pitch and diameter) and pick the charger/balancers to do the job. Consider future applications - whether you are starting mid size or large, it is often more economical to consider devices that give you a degree of flexibility with time.

• Motor of choice - many manufacturers and suppliers have a calculation or a table on their website that allows you to do a conversion to find out the appropriate size motor to replace your carbon and noise emitting banger. Many manufacturers now offer alternative electric suggestions to IC power in their ARF's and kits. Electric Motor manufacturers seem to apply different monikers to their product, some based on power output, others on comparative IC engine size. If in doubt, contact the manufacturer and/or supplier for their recommendation. Many suppliers of electric flight products are experienced enthusiasts and are a wealth of information. The internet is a fantastic research tool!

• Propellers for electric flight are different to those used on IC engines. Choose the pitch and diameter to suit your application. Most motor manufacturers have a propeller selection guide in the instructions. Beware - exceeding the recommended diameter and pitch may cause a higher current draw in excess of the capability of your chosen motor and ESC. If you need to experiment stick within the motor manufacturers guidelines. NEVER use electric props on IC engines and vice versa.

• How many Cells? 1 LiPo cell is approximately 3.7 volts, so it is a matter of finding the operating voltage of your chosen motor and dividing by 3.7 to get the number of cells required. Many motors have a voltage range and a choice, based on cost and rpm, has to be made. For example, if a motor has an operating voltage of between 28 and 38 volts, the number of cells to operate the motor would be (rounded out) 8 to 10 cells. Any combination of cells would work - 2 x 4S packs, a 4S and a 5S pack or 2 x 5S packs. 2 x 5S packs are going to operate at a higher voltage therefore would be more suited to a more spirited flying style, the 4S combination more sedate. Naturally, the bigger the pack, the more expensive it is. You pay for power.

• Do not be put off by cost - think of what you are saving in nitro fuel! A $200 battery pack will last much longer than $200 worth of Nitro fuel…
  

• The advantage of split packs are many fold - you have packs that can be used with smaller motors, in the example above running on 14 to 20 volts. Many 40 - 46 IC powered model aircraft can be converted to electric with a single 4S or 5S pack. A suitable electric brushless outrunner motor and ESC is no more expensive than a good quality 40 or 46 so you are ahead anyway. Split packs can be easily fitted into more compact spaces. The packs in my _ scale Cub (2 x 4S packs) fit neatly on top of each other where the fuel tank was - sweet!

• The higher the volts the more rpm for a given prop. Changing the pitch/diameter of a prop will also affect current draw, which will affect duration based on cell storage capacity (Ah or ampere-hours). Basically, the more amp/hours (or mAh) in a pack the more duration you will get for a given prop and flying style. Click here for a fuller explanation of Volts, Amps and Watts.

 Remember the noise up close and personal with your banger? All you will soon hear is the prop noise, oohs and ahhhs!

• Electronic Speed Controller (ESC) - ensure the speed controller is capable of handling the number of cells especially if battery packs are joined in series. Check the requirements of your intended motor (voltage range) and the number of cells required to operate the motor. Some ESC's have colour coded output wires, some not. There is no polarity with brushless motors and ESC's. Line up the wires neatly and connect! If the motor runs backwards swap any two wires…true story! Some ESC's have a motor reversing capability. I would suggest not using this facility to correct reverse rotation, get it right with the ESC on default settings by swapping those wires around.

• The charger must be capable of charging the correct number of cells. Many chargers will only charge up to 6 LiPo cells so if you are charging packs in series or packs larger than 6 cells you will need to spend the dollars to get a charger to suit. Be careful, cost is not necessarily a factor. Some of the more expensive European chargers will still only charge up to 6S LiPo packs. Check the specs. You can always charge one pack at a time, but do you have the time? Many chargers are 12 volt input - make sure your power supply will handle the draw of the charger. Some chargers have a low-voltage cut-off to make sure you do not flatten your car battery (eg. Schulze), but if you are not sure, carry jumper leads and do not be the last to leave the flying field!

• LiPo balancers are recommended when charging. Some balancers will only handle up to 6 cells, but two can be networked to monitor and balance 12 cells.

• Develop a convention on battery pack connectivity. For example, 4mm bullet connectors, male on the positive wires, female negative and colour coded heatshrink, +ve red , -ve black, neutral white. Stick with it for all your models. (Deans connectors are very popular also, because they are polarised and they will handle big amps. Male on the ESC and feamle on battery packs. Ed.).

• Make sure the total length of each wire from the battery to the ESC does not exceed 100mm or 4 inches. Same with the ESC to motor connection. Long cables can pick up stray signals and confuse the motor casuing erratic running. If you must use long cables, fit chokes to suppress stray signals.
  
Some brushless controllers come with colour coded wires, others with same coloured wire. I use female bullets out of the ESC and male bullets on the motor, colour coded with heatshrink to ensure correct reassembly if the ESC is disconnected.

• Network. Talk to other modellers, join on line discussion groups, even ones run by French expats in Dublin, Ireland are a wealth of information. It is encouraging to have somebody agree with your decisions and observations, too. Find someone like minded at the flying field and learn from each other.

Safety
All Propellers are hazardous. Treat with care. They chop, slice AND julienne!
Short circuits can short circuit your life expectancy, too. NEVER short circuit a battery.
Quieter does not mean safer. Remember, the Beaufighter was nicknamed 'Whispering Death”.

Quieter means taking extra care because one of the primary brain triggers for DANGER is missing.

Make the 180 degrees from the propeller tip path and forward a NO GO ZONE. Prop blades look like knife blades and have the same effect on flesh and bone when let loose, accidentally or deliberately.

Hot hot hot! After operation batteries, ESC's and motors are HOT. Batteries will increase in temperature for a short while after use as they radiate core heat so place them in a cool, ventilated area.

Some things rotate for a reason. Let them rotate. It is what they will try to do if you attempt to stop them rotating that causes the damage.