motor choice questions

Motors, speed controllers, batteries and other e-power stuff

motor choice questions

Postby S-rob » Thu Jun 07, 2012 7:45 am

Hi there
In general, generally speaking

Im pondering motor choices. Too many to choose from on Hobbyking.
correct me if Im wrong
For the same size motor, I can use higher KV motor with lower pitch prop, or lower KV with 'bigger' pitch prop prop.
Both could give about the same thrust (??)

so when do you go high KV & when do you go low KV on same size motor ?? or does it not matter ??

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Re: motor choice questions

Postby RCModelReviews » Thu Jun 07, 2012 8:14 am

There are many factors -- prop pitch and diameter, KV etc.

What plane are you thinking of.

Typically "fast" models do better with hi-KV motors driving high-pitch but small diameter props.

Slow models do much better with lower-KV motors driving larger diameter, lower pitch props.

The setup for a pylon racer are way-different to those for a 3D bird., just the facts.
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Re: motor choice questions

Postby RockinRobbins » Tue Jul 03, 2012 1:15 pm

This has really been up a long time without being answered properly. Yes, the choices can be bewildering, but there is a German man, Christian Persson, who produces a marvelous piece of free software called Drive Calculator. It's available for download at Make sure you download the newest database, available in a separate link there.

The only thing that comes close to this program is a commercial effort, MotoCalc, which extends its domain into predicting performance in various airframe types and telling you what a particular power combination might be good for. In practice, that is fluff that really doesn't add to your ability to match power system components.

When I was building my Slow Stick, none of the recommended power combinations was available, and being the impatient person I am, I went to Drive Calculator to find a motor/propeller/ESC/battery combination that would give me gobs of thrust with a low pitch speed of about 30 mph. I wanted unlimited vertical performance but didn't want to tear the wings off with all that power.

Drive Calculator guided me to a Turnigy 2217-20 860kv motor, rated at 22A (laugh heartily at that published rating!). But since Drive Calculator had data based on motor testing, not manufacturer published claims (big difference there--beware!) I knew that I could hook that motor up with a 1350A Rhino 3s battery pack and an APC 11x4.7 SF propeller to produce three times more thrust than the plane weighs and 20 minute flights at 18A, a safe power setting for that motor.

When I hooked it up, actual performance was so close to Drive Calculator's predicted numbers that any tiny difference could have made the difference. I can say that because I bought a Turnigy Super Brain 40A data logging ESC to verify performance. That ESC is just astoundingly good. It can change your life! :lol:

Drive Calculator looks at your combination of prop/motor/ESC/battery and gives you a confidence score on its predicted performance based on whether it is built from actually tested combinations of components or whether the results are calculated. That is unique and even the paid program MotoCalc does not have this aspect. Also, there are many, many testers, including Dr Kiwi, contributing the results of their tests to Drive Calculator. Updates of the database are frequently available and the program will tell you when they are available.

Check out the manual at to see whether this might help you choose a power system from the incredible variety available, especially at Hobby King. It decoded the morass for me!
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Re: motor choice questions

Postby S-rob » Wed Jul 04, 2012 9:00 am

Thanks for the reply

I found my answer here

for the benefit of others, cut & paste
MOTOR POWER CHOICE(base on reccomended AUW, or 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
Warbirds - 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

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 Lipoly 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 reccomendation for the model by model manufacturer and in conjunction with the size/weight data published with all our advertised Lipoly packs, 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)
500w up to 800w: 4s (This is the 0.40~0.46 glow equivalent range favoured 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 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, their are practical considerations, the most obvious is ground clearance. Please refer to motors such as the NTM range, which give you prop 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: Warbird/scale Model, 1lb AUW 120w motor, kv choice, either of the above, it is personal choice
Example 4: 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.
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Re: motor choice questions

Postby RockinRobbins » Thu Jul 05, 2012 1:12 am

Again that follows under the category of general advice and won't really help you pick a specific battery, ESC, motor and propeller for a plane. It's good to know but won't help you build your plane. You need more specific information than that. Good background info though!
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