During the last five years or so I have had a couple of attempts at "getting into" the increasingly popular electric flight aspect of our hobby. Until recently these have ended in disappointment with poor performance due to heavy batteries or excessive brush wear. I have to say that the recent maturing of Lithium Polymer ("LiPo") battery technology and brushless motors has transformed my recent experiences. I now fly a scaled down "Wot 4" which is just as lively and aerobatic as the original. Here are a few thoughts and suggestions for you to consider if you're thinking about trying electric models for yourself.
LiPo batteries come in all shapes and sizes, with a price range to suit. Each cell has a nominal voltage of 3.6 volts, more than double that of "standard" dry cells. I would recommend choosing just one battery size (number of cells and capacity) for all your electric models so that you don't have to match battery to model at the field. I have chosen a 3-cell 2200 mAh battery as my standard. Costing about £20 each and weighing just 6 ounces it provides enough power to run my Mini-watt 4 for over 10 minutes of sparkling performance. Most IC powered models carry at least 6 ounces of fuel, and a receiver battery which you don't need in an electric 'plane. (the electric speed controller has an output to power the radio control receiver). Buy a dedicated balanced-charger which monitors and charges each cell separately and you should have no concerns about the widely reported fire risks associated with mis-charging LiPo batteries.Motors & propellers.The one disadvantage of LiPo batteries is that they cannot be re-charged quickly. They must not be charged at a higher current than their "1C" rating. A 2200mAh battery should therefore be charged at 2.2 amps (or less) for an hour (or more). The solution to this problem is to own several batteries all of which are brought to the field fully charged, and then re-charged in sequence as the flying session progresses. This is another good reason for choosing a standard battery for all your electric aircraft.
Model Size and Weight.The jargon here is totally different from that of internal combustion engines. Instead of "cubic inches" we have "watts" and "kV". Remember the conversion factor 746 watts/Hp to work out that my preferred 200 watt brushless out-runner motor has a power similar to a 0.29cu.in glo-motor (0.27 bhp). Also remember that Watts = Volts x Amps to work out that my 200 watt motor will take about 18 amps from my 3-cell battery. If I ran the motor at full power all the time my 2200mAh battery would be exhausted in about 7 minutes. Of course the motor is only used at full power at take off and during certain manoeuvres. It is therefore frequently running at or below half throttle when the current consumption is reduced to 10 amps or less. Because of this I usually get flight times of well over 10 minutes.
An electric motor is much more sensitive to propeller choice. This is where the "kV" figure comes in. This is more accurately "revs per volt" so a 1000kV motor is designed to run at 10,000 rpm from a 10 volt supply. You must choose the correct diameter and pitch propeller to achieve this. Motors usually come with a guide for choosing the correct propeller but you should also buy (or borrow from a friend) a power meter so that you know exactly how much power your motor and propeller consume. My motor is 1000kV and drives either an 8" x 6" or 9" x 4.5" propeller at close to 10,000rpm. With either of these I'm looking at close to 1.5lbs of thrust. A higher kV motor would need a smaller diameter, finer pitch propeller to run at higher revs for its optimum power level.
The Radio - 35MHz or 2.4GHz?My favourite "conventional" model is my trusty Wot-4. This, and several other of my planes, are powered by 0.46cu.in engines, have wing spans of about 60 inches and weigh from 4 to 8 lbs. For my money these are too big to be configured in electric versions. You would need more powerful electric motors and, yes, these are readily available. But you would also need bigger and much more expensive batteries. These would also be heavier. I have decided to standardise on an affordable battery size, weight and cost and then to choose a motor power to give a reasonable flight time. This battery/motor combination will give a thrust of about 1.5lbs suitable for a model of anywhere up to 3 lbs weight. Lightly built models in the 36" to 48" wingspan range should be ideal. My Mini Watt 4 has a wingspan of 38" and weighs 1lb 9 oz.
One problem I had with earlier attempts using brushed motors was electrical interference from the motor brushes with my 35Mhz radio. This got worse with my attempts to get more power by using more cells in the NiMh batteries. I have bought a 2.4GHz Futaba radio for my current foray into electric flight and the advertisers message of "reliable and interference free control" seems to be borne out.
John Warner, October 2009