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Electric Project

Graham Birt 

 

With the massive range of electric motors and LiPo’s available now in the UK, Hong Kong or the “Big Fish” in Cornwall, I am of the opinion that most IC powered models can be converted to electric without any loss of power or performance. Indeed one prominent and long-standing club member expressed his amazement regarding the power available from outrunner motors today, and you could not find a more die-hard IC flyer!

 

What are the downsides of electric? Well purists say that you cannot beat the sound and smell of a big 4 stroke and I would agree that is a very fair point, indeed I have no intention of disposing of my IC models or engines, but I think that electric flight has certain benefits that should not be ignored. So what are the advantages of electric flight? Well it is clean, equipment is easier to fit in a model and it requires less backup on the field, just a field charger if extended flying is required, and it is environmentally friendly certainly from a noise pollution viewpoint. Electric is also perfect for multi engine models, absolutely no rpm issues and you can contra rotate by just switching a wire.

 

Having recently acquired a Kyoshu Spitfire 90 ARTF designed for IC I wanted to convert it to electric. I needed to understand the “formula” to select the best combination of motor, ESC, and battery and on seeking advise from numerous model shops and flyers of electric models I was faced with a mass of contradictor information, which did nothing but confuse. I decided to work it out for myself, put my trust in common sense and the Internet. My starting point was easy; the model was designed for a 91size 2S or a 120size 4S, with an all up weight of 10lbs max (4.5kg).

 

What do we need to achieve? A 120 4S for example swings a prop of around 16x10 for scale flying, at about 9k rpm, so that is our benchmark for the electric set up and using manufacturers specifications we can calculate the motor, battery and ESC combination for the model.

 

Electric motors as generally rated with the following key information:-

 

Kv                                           The rpm/volt rating of the motor.

Shaft dia                                Important when selecting a prop driver.

Cell count                             Recommended LiPo range   in number of cells.

Max voltage                           Not to be surpassed and dependant on the LiPo cell count.

Power                                    In Watts and important to calculate the correct motor for the required performance.

Length & Diameter               Important to establish that the motor will actually fit in the airframe.

Weight                                    For obvious reasons.

ESC                                        Important to select the correct electronic speed controller to handle the power (W) of the motor.

 

 

Sometimes a manufacturer will recommend a prop range and equivalent IC engine size, which is most helpful.

 

Now for the Spit, how much power do I need? Power is rated in Watts (w) and as a general rule of thumb you will need 70/100 watts per lb for a sports model, 150/200 for a scale model, and 300 for 3D.

 

The motor I chose is for the Spit is a Turnigy Series 50-65 380Kv/1820w, so with an estimated all up weight of 10lbs, with 1820w available I am on the nail with 182w/lb.

 

Next revs, and this is calculated using the number of LiPo cells we are using at 3.7 volts per cell multiplied by the Kv rating of the motor. The motor is specified as 380Kv and I am using a six cell LiPo so the rated voltage is 3.7 x 6 = 22.2volts. Therefore the revs will be 22.2 x 380 = 8436 rpm. In actual fact the LiPo will charge to a max of 4.2 volts per cell so we will initially get 25.2 volts and therefore rpm of 9576. As the volts drop, so does the rpm and at 70% capacity we still get about 6000rpm.

 

The Lipo is rated at 5000mAh this gives us flight duration depending on how fast we use up the available amps. I don’t know the formulae to work this out but by trial and error I use a factor of 35 divided into the mAh rating this suggests that my LiPo will give me about 14 minutes of flying time, assuming that we take off at full chat and then fly at 50-75% throttle. It is important not to take the LiPo down to less than 70% voltage as that will degrade the battery life and they are expensive! This paragraph on flight duration is merely written from my limited experience. I would welcome comment and input from anyone with more knowledge!

 

The motor manufacturer will generally give the peak “burst” current the motor will draw; this will give us the required ESC capacity. Again from experience I add about 15amps to the manufacturers “burst” current as a safety factor, so a “burst” of 65amps requires an ESC of 80amps or greater. Anything less and you risk an ESC burnout in the air, it might look spectacular but it is expensive, and dangerous!

 

Most ESC have Battery Elimination Circuits (BEC) in them to eliminate the need of a separate pack to power the Rx and servos and hence saves some weight. The BEC will syphon power from the main motor battery pack to power the rx and servos. This is fine for small models but in a larger aircraft the servos are going to draw a lot of power, so I prefer to use a separate pack for the Rx and servos and eliminate the BEC altogether. The advantage of this is that the Rx and servos draw their own power from a standard NiMh pack and the motor LiPo can concentrate on the motor, and also you can use a LiPo cut of switch that will allow you to work on the model with the radio live without risking an accidental motor start which will remove a finger with ease!

 

The model came out bang on weight at 9.5lbs, so with the CG checked to plan and in a window of calm weather I sped to AD with Alan, Pete and Nick in attendance, of course Alan offered me a quid for it but, although tempted, I declined. After checks and a taxi, full throttle and into the ether. Minor trimming issue due to CG being too far back, (what do the manufacturers know), and after about a half dozen circuits we landed safely. So it works, bags of power, no vices and no need to clean the model of oil and muck! Back in the workshop now to move the battery and correct the CG and we are waiting for the next suitable flying day!

 

Now I have heard comments that electric is expensive, and I suppose if you have a stable of engines built up over the years, to replace all of those with cells, ESC’s and motors is quite expensive, but if you compare like for like from a standing start it is a different story, for example: -

 

5000mAh 6 cell LiPo                       £60.00            Hobby King

Turnigy Motor                                   £35.00            Hobby King

ESC 80amp                                      £60.00            Al’s Hobbies

Total                                                  £155.00

 

Yes, you will need a charger, but you don’t need fuel so it sort of balances.

 

OS 91 2s                                            £180.00

OS120 4s                                          £359.00

Saito 115 4s                                      £279.00

 

I don’t believe it, I can kit out 2 models for the price of one 4stroke, and electric products are coming down in price all of the time especially with the US$ getting slowly weaker against the £, that’s an even better deal. As an example my first 5000mAh LiPo cost me £150 only 18 months ago!

 

 

A few images

 

No need for a tank or throttle servo, just the LiPo, the blue slab under the radio tray, 299mm x 45mm x 22mm, weighs about 1lb.

 

 
Motor mount and Turnigy motor, on the left is the ESC and poking out of the top is the LiPo balancing lead.

 


 

Bottom of the “engine” bay, on the left is the LiPo isolator. Cunning gadget that allows you to leave the LiPo fully connected to the motor but will not allow current to pass until activated, not essential but a great safety feature.

 


 

 

View showing the Rx and servo NiMh pack and the LIPo visible through the top opening.

 


 

Slots in the cowl are to allow air to cool the ESC, no massive cut outs for exhausts etc.


I hope this is useful if you are considering entering the dark world of electric flight. These are my experiences but my next aim is to try and stop bending retracts on nearly every landing, be it IC or electric power!