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.

How to choose motors for drone - 2507 motor with 7-inch propeller

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:

How to setup INAV 2.2 on 7-inch quads – PID tuning

INAV 2.2 is with us and it brings a series of cool improvements that should make all owners of mini quads very happy. Those are:

  • Improved Airmode that is more suited for small and powerful drones
  • Iterm Relax that allows getting rid of bounceback at the end of flips and rolls
  • D-Boost allows having cold motors and a lot of Dgain when doing fast maneuvers. It's an equivalent of D_Min from Betaflight
  • Configurable gyro stage 1 filter type: PT1 or BiQuad
  • Faster D-term filtering
  • and a few other goodies
    In this video, I will show you how to set up INAV 2.2 for 7-inch quads to get the best flying experience possible.

Threaded Inserts instead of 3D printing threads

You can print threads with a 3D printer. As long as the thread is big enough. Like M10. What about smaller sizes like M3 or M2? You can just print 2.9mm-3mm hole and tap it by force with a screw but on a longer run this sucks too. Recently I have "discovered" brass threaded inserts that you can get in hundredths for cheap and just install them with a soldering iron in 3D printed parts.

Armattan Marmotte Schmarmotte

This is my second adventure with Armattan frames. The first one was Rooster. And it would be great if not moving arms. I yielded again and got me an Armattan Marmotte this time. I recorded this video before flying it. When I write those words I already know how it flies. It flies good, but I still have to tune and update motors to something more modern and not beaten up. In other words: Armattan Marmotte is great in the air but some of the design elements are extremely bad!

DJI Digital FPV System – are we there yet?

DJI did it and introduced a digital FPV system that actually makes sense. There are goggle, radios, video transmitters and it not only works but also does not costs a fortune. Really, DJI Digital FPV is relatively cheap when you have 1 or 2 drones. When you have more, it gets problematic. Also, they do not show it, but where do you put it on a 5-inch drone? None of my frames would accept a camera and transmitter inside.

FPV drone with a pusher prop | DIY Pentacopter!

Here it is Penta Engine Nitro Incredible Ship – P.E.N.I.S. Or shorter: Willie. OK, let's be serious for a moment. It is a pentacopter. A normal 5-inch FPV drone based on Martian II frame with the 5th motor attached in the back. When I flip a switch, the 5th motor starts and it flies almost like an airplane. It's a VTOL. The idea is not mine and a few years ago there was something called Foxtech Screamer 250.

Matek F765-WING – 12 PWM outputs and 8A BEC for your airplane

Matek F765-WING is the most powerful (probably) flight controller on the market. OK, technicaly it's not on the market yet since I only have one of the preproduction samples, but it is a powerful design.

Matek F765-WING will be compatible with INAV, Betaflight and Ardupilot. It is based on STM32F765 MCU, has 8 UARTs, 12 PWM outputs, I2C and 6 ADC channels for Vbat, Current, RSSI, AirSpeed. On top of that it holds a 8A BEC that can power all the servos you will ever have on your RC airplane model.