Electronics and Propulsion System in RC Flying - Part 9 - A guide to electric flight.

9. A rough guide to electric flight.
We will consolidate things we discussed.
  • An under-powered model is a disaster waiting to happen.
  • Over-powering is fine but the penalty is additional weight.
  • A good model is one that is balanced in terms of power, flying weight and build quality.
We will formulate the things we covered in to a rough guide to choosing the electric power train needed for various model types.
  • This guide is as the title says, a ROUGH guide and offers some basics to choose a power combination for your model.
  • It is not a definitive 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
  • This is based on recommended AUW, or Flying Weight of your model.
  • Vintage types and many non-aerobatic indoor flyers - 50w~70w per 1lb  (454gms).
  • Trainers, gliders and high wing scale - 70w~100w per 1lb .
  • Sport flyer with general aerobatic performance - 100w per 1lb.
  • War birds - 120w~150w per 1lb.
  • Multi engine 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 VOLTAGE CHOICE
  • We have to work out what voltage we need to use, generally, to keep Lipo's in good order.
  • Try and keep max amps to around 50~60% of the C rating.
  • 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.
  • Exceeding this will only reduce the service life of your lipo, but you can fly.
  • For low powered models, choose 20c packs,  for general flying choose 20c~25c packs, for high performance models 30c + packs.
  • Up to 50w: 1s~2s.
  • Up to 100w: 2s~3s.
  • 100w Up to 500w: 3s (This is the practical upper limit for 3s Lipo's, so basically, models of 5lb AUW – 2.2kg).
  • 500w up to 800w: 4s (This is the 0.40~0.46 IC engine (or glow engine) equivalent range favored by many club flyers).
  • 800w up to 1000w: 5s.
  • 900w up to 1500w: 6s (this is the 0.60~0.90 ic equivalent range).
  • 8s~10s packs are for very large and generally specialized models.
MOTOR CHOICE - KV or RPM per volt
  • The simple method is to treat low kv motors as 4 stroke engine equivalents and mid-high kv motors as 2 stroke engine equivalents. ( Jut think about your bikes, 4 stroke and 2 stroke, difference in acceleration, power and fuel consumption).
  • If you are not used to IC engine then I can give you some examples of the approach to take, this is an important choice as you can literally choose how your model flies.
  • Please refer the data sheets of motors such as the NTM or Turnigy range, which give you propeller data as well as power, dimension and weight data.
  • Example 1: 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.
  • Example2:  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.
  • Example 3: 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. 
ESC SELECTION
  • You have decided on your motor, so look at the MAX AMPS figure given by the motor manufacturer in the data section and generally add 25% to it to have a safe margin.
  • 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, if you are using a 4s Lipo, that the ESC is rated for 4s voltage.
  • Next, check if it has programming functions you desire, if you are flying a glider for instance, you require a brake facility so that the prop stops when soaring un-powered, allowing the prop to fold by not wind milling.
  • Also look at BEC rating, the BEC supplies power to radio receiver and in turn to  servo's without the need for a separate receiver battery, this should be checked carefully as per the number of servo's you are using in your model.
  • If the servo count is over 4 or above, normally 4 on basic models these days, then consider purchasing an ESC with a high AMP rated SBEC, or a separate UBEC, OPTO type ESC's (they have no BEC, keeping the ESC separate from RX supply) are recommended for large models that require a separate receiver power supply. 
Some use full tools in Hobby King site for Motor,Lippo and Servo selection.
  • With the help of these tools you can find out correct Motor, Lippo or Servo as per your need and also can do comparison..