Drone and RC airplane motors – everything you wanted to know but you were afraid to ask!

Today, let's have a nice talk about brushless electic motors we use in the RC hobby. Both on airplanes, as well as on drones. In this video we will discuss:

  • motor size
  • motor KV and rotation speed
  • KV versus KT as motor constants
  • stator and rotor
  • number of coils and poles
  • winding
  • phases
  • airflow and cooling
  • does a motor care about rotation direction
  • how to change the rotation direction of a motor
  • shaft and hub
  • mounting holes spacing
  • how not to fry your motor right away

How to choose motors for drones – especially 7-inch FPV quadcopters

One of the mistakes pilots makes when they build their first drone is they use too small motors. People think that if a motor works fine with 5-inch props, it will work with 7-inch props as well. After all, both have plenty of power and power is the key to everything.

How to choose motors for drone - 2207 motor with 5-inch propeller

It’s not true. Power is only half on the story. Power is an effect of motor torque and rotation speed. The bigger torque and the bigger rotation speed, the power is bigger as well.

It’s easy to guess that bigger propeller will be rotation slower than the bigger one. If the rotation speed is smaller and the motor has the same torque, the power might not grow after all. Also, the bigger propeller will have higher moment of inertia and will require more torque to change rotation speed, and generated thrust, as quickly as smaller ones.

Best motor for 7-inch propeller - 2507 Avenger v2

This is why, when choosing a motor for a drone, you have to concentrate both on KV of a motor and a size. To be honest, the correct size is even more important than the correct KV.

How to choose the motor size

Drone motor manufacturers do not like to share torque data. I have no idea why. It would be so cool to be able to compare torque between motors of a different brand. Assuming, of course, they would be realistic. If we can not compare the torque of two motors, we can compare the next best thing: stator volume.

For our needs, we can just assume that bigger stator volume equals higher stall torque of a motor. Not very precise, but good enough. Luckily we can compute stator volume from the motor size. Two first digits are a stator diameter, two last digits are stator height. We can also skip Pi, since it’s a constant. And then:

V = (diameter / 2)^2 * h

Stator volume of different motor sizes

Motor size Stator diameter [mm] Stator height [mm] Stator volume [mm^3]
2204 22 4 484
2205 22 5 605
2207 22 8 847
2208 22 8 968
2306 23 6 793
2406 24 6 864
2506 25 6 937
2507 25 7 1093
2508 25 8 1250

Desired stator volume for propeller sizes:

Propeller diameter Min required stator volume [mm^3]
5-inch 600
6-inch 700
7-inch 900

To check is a motor will be good for a propeller size just check if stator volume is bigger than min. volume for a prop size. That’s all. You can choose bigger volume but never lower. The lower volume will result in not enough torque and characteristical bobbing o pitch axis that is just impossible to tune out.

How to choose KV

In the case of motor KV, it’s a similar case. Too high KV will result in an absolute lack of efficiency and overpowered drone (which is not desirable in most of the cases). Too low KV will result in an underpowered drone that will not be able to perform any fast maneuvers and feel very sluggish on the yaw axis.

The sweet spot/reference KV can be easily computed from this equation based on propeller diameter and supply voltage. Of course, more S will result in lower KV.

KV = Pc / S

  • KV – motor KV
  • Pc – propeller coefficient
  • S – number of LiPo cells

Pc for different prop sizes:

Propeller diameter Pc
5-inch 9600
6-inch 6400
7-inch 7600

Recommended motors:

iFlight ION Raaange/ION Drive 2506 1250KV Motors

220x motors are the workhorse on the RC hobby right now when it comes down to mini quads. Sometimes they are joined by 230x too. But 2205 and 2206 brushless outrunners have a serious problem that limits their use with propellers bigger than 5 inches. They might still be fine with 6-inch props, but on 7-inches problems begins. What 220x lack is torque! They are just too small to have enough copper wire and diameter to deliver enough torque to drive much heavier 7-inch propellers.

Let’s welcome something much more powerful: ION Drive / ION Raaange 2506 1250KV!

EMAX RS2 2206 1600KV motors hands on

Think about an electric motor for your 5 (or maybe even 6) inch acro or FPV racing quadcopter. What do you see? Something in a 2205-2306 size range with a KV rating somewhere 2300KV and 2600KV. Sometimes even more. And it works great, as long as you wat 5-inch props and 4S batteries. You can choose and choose and choose between different models of different manufacturers. But what if you want to go 6S or 7 inches? You have a problem. Very limited choice of motors (and propellers as well).

EMAX RS2 2206 1600KV motor

Luckily, it starts to change and you can actually but lower KV 220x and 230x motors. I got my hands on EMAX RS2 2206 1600KV since I’m not very happy with TBS Endurance I’m using on my 7-inch endurance drone. They seem to be missing some torque and when I’ve found that new low-KV (1600KV is still a lot) EMAX motors, I did not waited long to order. I love RS2205S and RS2306 and hope RS2 will not disappoint me.

EMAX RS2 2206 1600KV new box

EMAX RS2 2206 1600KV motor in a box

EMAX RS2 2206 1600KV motor

EMAX RS2 2206 1600KV motor

EMAX RS2 2206 1600KV motor

What can I say about EMAX RS2 2206 1600KV before installing them on anything? (so far I got only 2, the second pair is on a way)

  1. They finally changed those plastic boxes to something smaller
  2. RS2 2206 1600KV look good for sure. Black and Red!
  3. No bottom cap since, well, you do not really need it after all
  4. 29g with wires
  5. 4mm shaft made from titanium (at least they say so)
  6. Bearings are 8x4x3 if I can measure
  7. They kept this nice screw in the bottom instead of a c-clip
  8. Mounting holes spacing has changed! Instead of a standard 16x19mm it is 16x16mm now! Check your frame. The great majority should accept 16x16mm but there might be some rare ones that will not!
  9. Single wire winding. Looks ok, TBS Endurance has a nicer winding tho!

How will EMAX RS2 2206 1600KV behave in the air driving 7-inch propellers? work? Good, I hope…

Flight Controller Soft Mount – is it worth it?

Is it worth to soft mount flight controller? Are gyro pads I recommended here better than rubber standoffs? I feel that they are better, but I do not have any solid data to prove it. So I can only tell, that I find them better and that is all. But, I can answer the question is it worth to soft mount flight controller at all. The answer is YES and here is a proof…

The problem of twitching motors

After I updated my 5" racer to stronger motors (EMAX RS2205S 2300KV over RS2205 2300KV) I noticed something very very irritating: twitching motors. When the throttle was in more less middle position or after rapid throttle change, twitching from motors was very audible. Quad was flyable, but performance was very low. I had to reduce yaw P gain by more than 50% not to see those twitches in FPV footage!

Blackbox log revealed the truth: from time to time, the strong vibration was getting into yaw gyro traces. I also localized the source of those twitches: stronger motors with more torque and higher radial acceleration were able to shake the whole machine when the frequency was close to the resonance frequency of the frame.

hard mount twitch example

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