Here is a rough guide to choosing the correct electric motor / ESC / battery needed for various model types, bear in mind that over-powering is fine but the penalty is additional weight, and a good model is one that is balanced in terms of power, flying weight and build quality. This guide is as the title says, a ROUGH guide and offers a basis from which to choose a power train for your model, it is not intended to be a definative guide but will help to get you into the air with performance that will make your introduction to electric flight enjoyable and reliable.
MOTOR POWER CHOICE based on recommended AUW (All up Flying Weight) of model choice
Vintage types and many non-aerobatic indoor flyers -
50w~70w per 1lb
Trainers, gliders and high wing scale -
70w~100w per 1lb
Sport flyer with general aerobatic performance -
100w per 1lb
120w~150w per 1lb
Multi engined models -
100w per 1lb (thrust from Multiple props gives in effect, more than 100w per 1lb performance)
EDF Jets -
150w~200w per 1lb
3D, F3A and high performance Models -
150w~200w per 1lb
LIPO BATTERY VOLTAGE CHOICE
Based on the above, we now need to work out what voltage we are going to need to use, generally, to keep Lipo's in good order, try and keep max amps to around 50~60% of the capacity/C rating of the Lipo Pack, for example, if you purchase a 2200mAh 20c pack, then it is rated for 44A constant discharge, so keep the max amps at around 20A~25A IF possible, it isn't always! Choose the capacity of pack based on recommendation for the model by model manufacturer.
For low powered models, choose 20c packs.
For general flying choose 20c~25c packs.
For high performance models 30c + packs.
Up to 50w:
up to 100w:
100w Up to 500w
: 3s (This is the practical upper limit for 3s Lipo's, so basically, models of 5lb AUW)
500w up to 800w:
4s (This is the 0.40~0.46 glow equivalent range favoured by many club flyers)
800w up to 1000w:
900w up to 1500w:
6s (this is the 0.60~0.90 ic equivalent range)
8s~12s packs are for very large and generally specialised models.
MOTOR CHOICE - KV or RPM per volt
Which actually means, what prop size! If you are used to IC, the simple analogy is to treat low kv motors as 4 stroke engine equivalents and mid-high kv motors as 2 stroke engine equivalents, if you are not used to IC then we can give you some examples of the approach to take, this is an important choice as you can literally choose how your model flies, however, there are practical considerations, the most obvious being ground clearance.
Trainer/Sport Model, 1lb AUW, we want 100w motor (3s 20c Lipoly) mid kv for general flying, probably around 1200kv~1400kv, so around 8" prop
3D/F3A Model, 1lb AUW, we want 150w motor (3s 20c~30c Lipoly) low kv, 1000kv or under, spinning 10~11" prop, highly efficient at low throttle openings giving lot's of prop wash over control surfaces at all times, high thrust for low rpm and low amps draw at higher throttle openings.
Warbird/scale Model, 1lb AUW 120w motor, kv choice, either of the above, it is personal choice
High Speed Delta type model, 1lb AUW, 200w motor (3s 25c~30c Lipoly) 2200kv~3200kv motor, 5"~6" Prop, high speed/low torque, low thrust at low throttle openings, high speed from high rpm at full throttle.
FINALLY, ESC CHOICE
You have decided on your motor, so look at the MAX AMPS figure given by the motor manufacturer and generally add 25% headroom, so, if a motor is rated to 15A, then choose at least an 18A ESC, better still a 20A and so on. Next make sure that the ESC voltage is compatible, in other words, if you are using a 4s Lipo, that the ESC is rated for 4s voltage. Next, check if it has functions you desire, if you are flying a glider for instance, you will want a brake facillity so that the prop stops when soaring un-powered, allowing the prop to fold by not windmilling, we strongly advise purchasing a programme card to make programming the ESC easier.
Also look at BEC rating, the BEC supplies radio reciever power for servo's without the need for a seperate reciever battery, however, the can be limited in the number of servo's they are capable of powering, if the servo count is over 4, as it is on many models these days, then consider purchasing an ESC with a high AMP rated SBEC, or a seperate UBEC.
OPTO type ESC's (they have no BEC, keeping the ESC seperate from RX supply) are recommended for large models that require a seperate receiver power supply, they are also safer in high powered, large models as they will not arm until the RX is switched on.
Rough guide to Electric Flight
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2 May 2011
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