Many times at the field, people ask me what motor they should buy for their ARF airplane or simply their home made RC plane. The first thing they mention is ” can i use a 400 size motor?”

I am guessing most people mention the 400 size motors because that was one of the most common brushed motors used for most applications when brushless motors were not readily available 10  years back.

I personally dislike using motor size to find the right motor for my applications.  Few simple thing to keep in mind when you are looking for the right motor is the weight of the motor, the KV of the motor and the thrust of the motor /grams.

The KV of the motor ( KV stands for RPM/volt ) will pretty much help you determine the size of the prop needed for your plane. The higher the KV, the smaller the propeller, the lower the KV, the bigger the prop.
the weight of the motor is also very important, if you have a small scale plane that weighs around 250grams, you would not want to use a motor that weighs anything close to 250grams, e.g an optimal motor weight for a 250 gram rc plane would be somewhere around 20 to 35grams.

( If you have a 2000kV motor with a 7.4V lipo pack (assuming the  voltage is all outputted to the motor), you would potentially have 14,800rpm at the motor. )

Most motors you purchase should provide how much thrust it outputs per /grams. This varies due to the quality of the motor, some motors are built with quality magnets, quality stators and evenly wound copper wire around the stators. Most motors made in China will last 20 flights before it looses power, this is due to low quality material used in their motors.

Keep in mind, smaller outrunner motors that weigh around 10grams to 30 grams have a different KV to prop size reading than say motors that weigh 40 to 100 grams. Same goes with motors that weigh 150grams to 500 grams.

With some motors, if a lower voltage lipo is used, e.g a 2s lipo instead of a 3s,  the prop size may be upped by one inch.

Also motors that weigh from 5 grams to 40grams, always use GWS propellers, as they are light in weight, flexible and your motor will not have to work extra hard to spin the prop, hence saving your motors and batteries lifespan. motors from 45 to 150grams, you may use a slow fly APC prop.

Here are 3 different motor size groups:

Small
Weight: 25g
RPM: 1800kv
Max current: 10~12A
Pull: 320~450g
Prop: 6×3~9×4.7
ESC: 20A
Max Current: 14A
Battery Type: 2~3S Only
Suited for Mini indoor planes.

Turnigy 3020 Brushless Outrunner Motor 1800kv

 

Medium
Cells: 2~4S
Kv: 2100rpm/V
No load Current: 1.9A/10V
Max power: 500W
Weight: 67g
Recomended ESC: 2~4S/50A
Prop Size: 6×4  or 7×4
Turnigy 450 H2218 Brushless outrunner 2100KV

 

Large
Battery: 4~5 Cell
RPM: 550kv
Max current: 55A
Weight: 303g 

Test Data:
14.8v – 14×10 Prop – 46A – 2450g Thrust
14.8v – 15×8 prop – 50A –  2950g Thrust
18.5v – 12×6 prop – 36.5A – 2450g Thrust

Turnigy G46 Brushless Outrunner 550kv (.46 Glow)