Greenacres Electric primer

 

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greenacres model aero club electric primer march 2013 www.greenacresmac.co.uk

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electric flight primer gmac march 2013 by chris bott the following diagram shows a typical electric setup.

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components:battery these days mostly lithium polymer lipo are used these can be highly dangerous if not looked after properly some gmac modellers use a123 life which have advantages and disadvantages electronic speed controller esc as it says on the tin this controls the motor speed these often contain a circuit which supplies a steady 5v for receiver and servos this eliminates the need for a receiver battery so is called the battery eliminator circuit bec note not all esc s have this especially ones for higher powered setups brushless motor almost universally used these days the brushless motor tends to be simple and reliable if it isn t overloaded connectors these may have to be purchased separately and soldered to the cables good quality connectors are essential to avoid loss of power and overheating propellers propeller choice is critical with electrics changing prop size can either make or break literally an electric flight system apc-e electric are more efficient than standard ic props choosing components for your plane it seems there are new choices available every day for all the components in an electric setup there are so many variables that it can be very confusing to decide what to buy rules of thumb our usual starting point is to decide what the plane might weigh when complete including the motor battery esc etc etc when this is decided we use the 100watts per pound rule i.e if we have a 6lb model we look for a setup that will manage 600w or more of we are greedy watts are calculated by watts volts x amps so here are more choices if

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we want 600w then we can do that using 10v and 60a or 20v and 30a or any other combination our other general rule for using a123 batteries is to use one cell per 100w so here we would use a 6cell pack for the 600w setup how long will it fly battery capacity fuel tank size is specified in mah milli amp hours how long it lasts depends on the capacity and how fast we take the current out in amps so using less current will give us a longer flight one we have chosen the power required and the voltage or cell count we want to run at we can calculate the current we are likely to use remember watts volts x amps so therefore amps watts ÷ volts once we know the current we can pick a pack capacity that will give us our flight time note any backing off of throttle during flight will increase the flight time so this is very actively encouraged with electrics what s all this 3s2p nonsense batteries are made up of individual cells our normal receiver battery has four or five cells these are connected in series we would call this a 4s1p 4 series 1 parallel when we connect batteries in series the voltages add up but the capacity stays the same if we had eight cells connected like this we would call this a 4s2p 4 series 2 parallel when we connect batteries in parallel the voltages stay the same but the capacities add up note only identical cells should ever be connected like this c ratings all batteries are not equal some will allow current out much faster than others if we have a 2000mah 2ah it should in theory give us 2 amps for 1 hour or 4 amps for half an hour but there is a limit c is the current rating of a battery it tells us the maximum current that the battery will safely give if our 2ah battery here is a 10c discharge battery it will give us up to 20 amps

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and would last 1/10 of an hour at that rate 6 minutes if it were a 20c battery it would give us 40a and would last for 3 minutes flat out taking more current than the c rating can be dangerous and could easily destroy a lipo charging charging must always be done with a charger suitable for the battery type any other way can be very dangerous and lead to fierce fires in extreme cases the c rating can be applied to charging too virtually all lipo batteries should not be charged at a rate any higher than 1c i.e 2a for our 2000mah battery there are a few recent exceptions but charging faster than appropriate will reduce the life of a battery considerably speed controller esc if we have chosen the power we need and the battery we are going to use we now know the voltage of the battery and can calculate the current we might be using at wide open throttle watts=volts x amps so amps=watts ÷ volts the esc we need has to have voltage and current ratings higher than our calculated figures any lower and it is likely to burn out much higher is safer but will be more expensive and heavier we also need to decide whether to use an esc with a bec in it or not see diagram at the start motors brushless motor power rating we will only consider outrunner brushless motors here as they are the most common we need to find a motor with a power rating higher than the power we want to use again any lower and it could well burn out brushless motor kv rating brushless motors will always try to run at a particular speed that speed depends on the voltage of the battery the propeller load and the kv rating if we run a motor without a prop it s rpm should be it s kv rating multiplied by the battery voltage a 1200kv motor with a 12v battery should run at 14400rpm with no prop on it.

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brushless motor current rating it is usually too much current that will destroy a motor so we need to choose one with a high enough current rating too propeller sizes when we put a propeller on our motor the prop loading slows the motor down the motor tries to reach its full speed and draws more current to do so it won t reach it s no prop speed but the propeller size directly affects the top speed and directly affects the current drawn we can run with a small prop and be well within all our system component specs if we put a larger prop on we get more power out but then we get closer to the breaking point of something so it is very important that we measure current voltage power and ajust the prop size to give us what we need while looking after our components most electric fliers will end up with a large selection of props just for testing so which motor here is the crunch without some experience this is very difficult once you have done a few you have some experience for reference take our p51 funfighters these are about a 3lb model so we want 300w les say 400 `cos we want them exciting so we chose 3s1p lipo batteries those are 12.6v but drop a bit in use so let s say 12v for 400w we will therefore be using 400 ÷ 12 33 amps so we want an over 400w motor that has a current rating over 33a but what kv to find this we had to do some testing previous experience suggested 800kv would be too low and 1600kv would be too high so we bought an 1100kv and a 1400kv motor because we could find those two with the ratings required above testing at club night in 2010 showed that :the 1100kv motor gave us 364w using a 10x10 apce prop at 7300 rpm the 1400kv motor gave us 415w using an 8x6 apce prop at 1200 rpm both pitch speeds are almost identical so model speeds should be about the same if pitch speeds are the same and assuming that a larger propeller is more efficient then the 1100kv motor looks the better bet as we seem to be getting the same speed with less watts therefore the flights should be longer we have shown then that various motors will do the job propeller size is very important both motors look the same and are the same size if we put the large propeller on the high kv motor it would almost certainly draw too much current and something would be damaged.

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test equipment the all important watt meter hopefully all the above shows that without some way of measuring what s going on we are always in danger of breaking something or getting poor results the answer is some sort of power meter there are many devices on the market at reasonable prices that will show watts amps volts mah used etc etc these meters are absolutely invaluable to protect your investment chargers there are many chargers on the market too make sure you have a charger that can be set to charge the battery type you have a balancing charger is essential with lipo s balancing lipo batteries can drift out of balance i.e one cell may have more or less charge than the others if this is allowed to get worse then either the battery will be damaged beyond repair or worse an uncontrollable fire could be caused batteries must be checked for balance and balanced regularly probably not at a flying session but certainly every few charges lipo dangers the safety stuff ­ just has to be included much has been written about the dangers of using lipo batteries the problems come about when batteries are not looked after if any cell drops below a certain voltage it is very likely to be made a dead cell this is easy to do and can be very expensive it can be done simply by leaving the battery connected to an esc this will flatten the battery over time more importantly any cell that is taken over a certain voltage can burst into flames once burning the chemicals inside generate oxygen which means the fire cannot be put out this can come about by allowing batteries to go out of balance or by charging with the wrong charger setting it is strongly recommended that lipo batteries are charged after taking them out of the model and that it is done on a non flammable surface away from flammable materials if this all sounds very alarmist remember that there are also dangers and strict recommendations for petrol and glow fuel you wouldn t keep petrol in an open container in front of a log fire would you lipo s are fine if looked after correctly.

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for gliders or vintage etc a lower wattage will suffice for a hot performance a higher wattage is required table shows current required for each wattage between 100w and 1kw from lipo batteries from 2s to 10s lipo voltages used are nominal discharge voltages these can vary depending on the quality of the battery and the current used and the current rating of the battery model weights in lb are shown alongside for 100w/lb which gives a good sport performance model 2s weight power lb w 7v 1 100 14.3 a 2 200 28.6 a 3 300 42.9 a 4 400 57.1 a 5 500 71.4 a 6 600 85.7 a 7 700 100 a 8 800 114 a 9 900 128 a 142 a 10 1000 4s 14 v 7.1 a 14.3 a 21.4 a 28.6 a 35.7 a 42.9 a 50.0 a 57.1 a 64.3 a 71.4 a voltage under load used for this table 3.5 5s 17.5 v 5.7 a 11.4 a 17.1 a 22.9 a 28.6 a 34.3 a 40.0 a 45.7 a 51.4 a 57.1 a 6s 21 v 4.8 a 9.5 a 14.3 a 19.0 a 23.8 a 28.6 a 33.3 a 38.1 a 42.9 a 47.6 a 7s 24.5 v 4.1 a 8.2 a 12.2 a 16.3 a 20.4 a 24.5 a 28.6 a 32.7 a 36.7 a 40.8 a 3s 10.5 v 9.5 a 19.0 a 28.6 a 38.1 a 47.6 a 57.1 a 66.7 a 76.2 a 85.7 a 95.2 a 8s 28 v 3.6 a 7.1 a 10.7 a 14.3 a 17.9 a 21.4 a 25.0 a 28.6 a 32.1 a 35.7 a 9s 31.5 v 3.2 a 6.3 a 9.5 a 12.7 a 15.9 a 19.0 a 22.2 a 25.4 a 28.6 a 31.7 a 10 s 35 v 2.9 a 5.7 a 8.6 a 11.4 a 14.3 a 17.1 a 20.0 a 22.9 a 25.7 a 28.6 a

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this table shows flight duration of various popular sized lipo batteries at various different currents flight duration will be increased by using lower throttle settings during part or all of the flight alongside duration is the c rating being used at this current if flown at wide open throttle duration should be reduced by a percentage so that say 15 of battery capacity is remaining after landing 5a mi nut es c 10 a 15 a 20 a 25 a 30 a 35 a 40 a 45 a mi nut es c 50 a mi nut es c mi battery minminminminminminnut mah utes c utes c utes c utes c utes c utes c es c 500 6.0 10.0 3.0 20.0 2.0 30.0 1.5 40.0 1.2 50.0 1.0 60.0 0.9 70.0 0.8 80.0 0.7 90.0 0.6 100.0 800 9.6 6.3 4.8 12.5 3.2 18.8 2.4 25.0 1.9 31.3 1.6 37.5 1.4 43.8 1.2 50.0 1.1 56.3 1.0 62.5 1000 12.0 5.0 6.0 10.0 4.0 15.0 3.0 20.0 2.4 25.0 2.0 30.0 1.7 35.0 1.5 40.0 1.3 45.0 1.2 50.0 1300 15.6 3.8 7.8 7.7 5.2 11.5 3.9 15.4 3.1 19.2 2.6 23.1 2.2 26.9 2.0 30.8 1.7 34.6 1.6 38.5 1800 21.6 2.8 10.8 5.6 7.2 8.3 5.4 11.1 4.3 13.9 3.6 16.7 3.1 19.4 2.7 22.2 2.4 25.0 2.2 27.8 2200 26.4 2.3 13.2 4.5 8.8 6.8 6.6 9.1 5.3 11.4 4.4 13.6 3.8 15.9 3.3 18.2 2.9 20.5 2.6 22.7 3300 39.6 1.5 19.8 3.0 13.2 4.5 9.9 6.1 7.9 7.6 6.6 9.1 5.7 10.6 5.0 12.1 4.4 13.6 4.0 15.2 4500 54.0 1.1 27.0 2.2 18.0 3.3 13.5 4.4 10.8 5.6 9.0 6.7 7.7 7.8 6.8 8.9 6.0 10.0 5.4 11.1 5000 60.0 1.0 30.0 2.0 20.0 3.0 15.0 4.0 12.0 5.0 10.0 6.0 8.6 7.0 7.5 8.0 6.7 9.0 6.0 10.0

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table showing propeller pitch speed for various pitch propellers at different rpms 5000 propeller pitch prop pitch speed in inches mph 4 19 5 24 6 28 7 33 8 38 9 43 10 47 11 52 12 57 13 62 14 66 15 71 16 76 23 28 34 40 45 51 57 63 68 74 80 85 91 27 33 40 46 53 60 66 73 80 86 93 99 106 30 38 45 53 61 68 76 83 91 98 106 114 121 34 43 51 60 68 77 85 94 102 111 119 128 136 38 47 57 66 76 85 95 104 114 123 133 142 152 rpm 6000 7000 8000 9000 10000 11000 12000 42 52 63 73 83 94 104 115 125 135 146 156 167 45 57 68 80 91 102 114 125 136 148 159 170 182

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note actual rpm will be less due to the load imparted by a propeller this table shows the theoretical rpm reached by various kv rated motors using different lipo batteries motor kv rpm 66500 63000 59500 56000 52500 49000 45500 42000 38500 35000 31500 28000 24500 21000 17500 14000 10500 7000 10 s 35 v 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 2s 7v rpm 13300 12600 11900 11200 10500 9800 9100 8400 7700 7000 6300 5600 4900 4200 3500 2800 2100 1400 3s 10.5 v rpm 19950 18900 17850 16800 15750 14700 13650 12600 11550 10500 9450 8400 7350 6300 5250 4200 3150 2100 4s 14 v rpm 26600 25200 23800 22400 21000 19600 18200 16800 15400 14000 12600 11200 9800 8400 7000 5600 4200 2800 5s 17.5 v rpm 33250 31500 29750 28000 26250 24500 22750 21000 19250 17500 15750 14000 12250 10500 8750 7000 5250 3500 6s 21 v rpm 39900 37800 35700 33600 31500 29400 27300 25200 23100 21000 18900 16800 14700 12600 10500 8400 6300 4200 7s 24.5 v rpm 46550 44100 41650 39200 36750 34300 31850 29400 26950 24500 22050 19600 17150 14700 12250 9800 7350 4900 8s 28 v rpm 53200 50400 47600 44800 42000 39200 36400 33600 30800 28000 25200 22400 19600 16800 14000 11200 8400 5600 9s 31.5 v rpm 59850 56700 53550 50400 47250 44100 40950 37800 34650 31500 28350 25200 22050 18900 15750 12600 9450 6300

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committee contact details:up to date email addresses will always be available on the website http greenacresmac.co.uk see the website introduction page telephone contact reg lewis chairman 01922 625354 07702 488212 written by chris bott greenacres model aero club march 2013 disclaimer no responsibility will be accepted for anything arising from the use of this booklet.

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