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

DJI FPV System & INAV – current situation

DJI FPV System

When DJI released updated firmware for their DJI FPV system with improved OSD, Betaflight users started cheering. The move to add OSD with elements like GPS position, the artificial horizon, etc is kind of puzzling. It works only with Betaflight flight controllers and a typical user of Betaflight does not really need it. After all, Betaflight concentrates on racing and freestyle on 5-inch mini-quads, not long-range.

On the other hand, users INAV, that concentrate on airplanes and long-range flights, can not use new DJI FPV OSD. Pilots immediately started to ask INAV developers to implement DJI FPV support. The problem is, that it's not working like that.

  • Support for Betaflight is built-in into DJI Air Unit, not another way around.
  • DJI FPV seems to be actively checking if the flight controller it talks to is Betaflight or not
    INAV and Betaflight support the same serial protocol: MSP. This means DJI FPV is capable of talking to INAV, it just refuses to do so
  • INAV and Betaflight use the same OSD positioning protocol using the same MSP frames. Still, DJI FPV refuses to talk to INAV
  • We have no idea what DJI Air Unit expects from a flight controller since it is the closed source!

All of that means that INAV developers can not fix something that is not within the code of INAV. For INAV support, DJI has to implement it. Not the other way around.

Pirx Seven – The ultimate custom 7-inch FPV frame, part 1

Do you know this old saying "If you want something done right, do it yourself"? I do and sometimes I just live according to it. Not always, I'm not crazy, but at least from time to time!

I fly 7-inch FPV drones a lot. To be honest, they are my favorite flying devices. There are a few good 7-inch frames, like TBS Source One for example. There is a difference between good and great tho. I do think that Source One is a good frame, there are a lot of things on it that do not make me very happy tho! The list might not be very long, but it is relevant:

  • props in the view
  • limited space to install split-like hybrid cameras
  • no very convinient place to install VTX
  • it's not that big inside as one might imagine…

Because of that, and because I think I know how to do it (I'm an engineer) I've devided to design and manufacture my of 7-inch FPV drone frame that would fix what I do not like in Source One.

This is how Pirx Seven was born!

Pirx Seven - 7-inch FPV drone frame

  • Dead Cat configuration
  • Plenty of space inside for everything you want! There is even a place in the back to fit the whole 30x30mm stack!
  • Place for Split-like hybrid camera in the front
  • Attachable mounts for antennas, GPS and other accessories
  • Clear, unobstructed view for HD camera like GoPro on the front deck

So far, I was able to mill a prototype unit to test it in flight. So far so good, I really have to say that I like what I designed.

Pirx Seven - 7-inch FPV drone frame

Fully assembled frame weight 205g with steel screws and aluminium standoffs.

Pirx Seven - 7-inch FPV drone frame

What next? Test flight and some changes. The full list consists of 12 points in total and basically means that I will have to redesign almost everything…

If you want to know why the name and what does Pirx stands for, here is the answer. It's a tribute to Polish writer Stanisław Lem and his book Tales of Pirx the Pilot.

Your first 7-inch FPV drone

I love 7-inch FPV drones and during the last year, they were my most used FPV gear. For freestyle, cinematic, cruising and long-range. I currently own 3 7-inch FPV drones and 4th one in the assembly process.
I think of myself as a 7-inch expert. I've wasted enough money and equipment to know what works and what does not work well with 7-inch propellers.

TBS Source One 7-inch FPV drone

If you are thinking about building your own 7-inch FPV drone, here are some tips that might save you a lot of time and a lot of money on wrongly chosen equipment.

The Frame

Of course, the frame is the most important part of a build. No matter if you will be building a freestyle or long-range rig, there is one thing that you have to know: Horizontally scaling 5-inch frame by making arms longer is a no-go. Longer arms require more stiffness and they also have to be thicker! The sweet spot is 6mm thick good quality carbon fiber. 5mm thick might work too but forget about 4mm or thinner arms. They will be too flexible and the whole quad will be in a state of a constant wobble!

Alfa Monster 7-inch FPV Frame

If you are looking for 7-inch frame recommendations, here are my choices: Team BlackSheep Source One with 7-inch arms, Alfa Monster and AlfaRC Fighter.

Propellers

Kind of obvious, right? The choice of good 7-inch propellers is not that big as one might imagine. Sure, you can buy some "inventions from 4 years ago for cheap" but believe me, they are not worth it. There are 3 types of modern 7-inch propellers that I find worth looking at in late 2019:

  • DAL T7056C – good for freestyle and thanks to aggressive pitch they are very dynamic. Can be used no longer flight, but this is not where they really shine.
  • HQProp Durable 7x4x3 V1S – a more aggressive version of 7×3.5×3 that in my opinion works much better than original. Slightly more pitch only helps during steady flight. They are rather for cruising than for freestyle.
  • GemFan FLASH 7042 – kind of good propeller, but don't get me wrong, you should try 7042 only if DAL or HQProp are too heavy for your motors. So if you have 2207 or 2306 you might try FLASH. If you have bigger motors, just go with DAL or HQProp.

Motors

Brotherhobby Avenger V2 2507

Probably the most often made mistake by the "7-inch virgins" are too small motors. Beginners think that putting the same motors that work great with 5-inch propellers on a 7-inch frame is a good idea. OK, maybe sometimes they lower the KV. This will not work. I know. I tried that by myself and only wasted some money and a lot of time and nerves.

What you need are big motors. In a size of 2506-2507. 2208 might work as well and 2508 is rather the max size that makes sense.

On the KV size, the rule of thumb is that on 4S you should aim at 1500-1500KV and on 6S at 1200KV. Those values give a nice compromise between efficiency and raw power. Of course, this is only a rule of thumb and when needed you might choose higher KV. For example, my 7-inch freestyle drone uses 1700KV on 4S motors.

My recommendation is rather simple: for now, there is only one motor that comes is wide enough KV range and it's called BrotherHobby Avenger v2 2507.

Everything else

For the rest of the equipment, you can choose whatever you want. Any equipment will work and you do not have to match it to the propeller size. My choices are:

FrSky R9M vs R9M2019 teardown

I do not why, but FrSky does not want to keep things simple. Not only it's migrating from ACCST to ACCESS, it also makes a mess with FrSky R9M system by releasing new hardware that looks like old hardware, has a very similar name and can run ACCESS but only on some radios, not on the other. The nightmare…. Today let's compare the FrSky R9M transmitter module I own for last (almost) 2 years with a brand new FrSky R9M2019

3D Printed Impeller Fan aka Centrifugal Blower

I have a 3D printer and some CAD skills, as well as basic engineer training. I never studied fluid dynamics, but I also never studied running a website or how to be a father, but here I am.
Anyhow, I want to put an impeller fan (yes, not a turbo) on an airplane and use it as a propulsion system. After a few days with CAD software, I came up with a small 3D printed impeller fan that hopefully will be strong enough to make airplanes fly…
The centrifugal fan is power from a "standard" drone brushless motor and a standard RC ESC. It sounds terrific!

What happened to INAV horizon drift?

INAV suffers from horizon drift since day one. It's not that Betaflight does not suffer from it (both Betaflight and INAV originate from the same parent: Cleanflight). In Betaflight it just does not matter at all. In INAV it's a real-life problem tho.

So far, we somehow managed to mitigate horizon drift on airplanes but it's not really fixing the problem. It's there and it's irritating as hell.

Last month I published a post that describes my intent to use the external board as a secondary IMU for INAV. It is "of the shelf" Bosch BNO055. Today I finally put it into a test and to be honest, the results are very optimistic!

More about INAV horizon drift:

Matek VTX-1G3, VRX-1G3 and ANT-Y1200 ready for testing

The upcoming 1.2GHz FPV system from Matek called 1G3, where VTX-1G3 is the transmitter and VRX-1G3 is the transmitter and ANT-Y1200 is the antenna, are ready for some flights!

Matek VRX-1G3 connected to Fat Shark HDO FPV goggles

So far I have a couple of remarks:

  1. VRX-1G3 fits nicely inside a standard Far Shark receiver bay and you can use the standard bay cover too. SMA connector is in the correct place! The standard cover blocks access to the button that is used to change channels. You can not change channels from the goggles menu.2. ANT-Y1200 has a nice stiff coax cable and you can position it however you want
  2. ANT-Y1200 is much bigger than antennas used for 5.8GHz FPV and it's much harder to find a good place to position it on a model.4. VTX-1G3 is luckily small enough to fit in many places and does not overheat. Even when powered from 4S it stays relative cold (50C) at 28dBm of output power. I got the word it's because of much more efficient energy conversion that is possible with 1.2GHz band

Matek VTX-1G3 and ANT-Y1200 on a Flybot Flux

The best flight controllers for INAV – 2019 Q4 edition

Believe it or not, but choosing the right flight controller for your next airplane or a drone build is quite important. Yes, I know that some of you might say that hardware does not matter and your kwad will fly as good with the latest F7 flight controller as it would fly with Naze32. It is, not true. It would fly with Naze32, but do not even try to compare modern flight controllers with more advanced filtering, inputs, outputs, and peripherals.

Best flight controllers for airplanes

Matek F722-WING

Matek F722-WING

Matek F722-WING is the second generation of a big WING flight controllers started by a famous F405-WING. Comparing to the original, F722-WING offers more input/output options, including dedicated airspeed port, switchable camera inputs and switchable power supply for FPV installation.

FuriousFPV F35

FuriousFPV F35 is a well know and proven airplane flight controller brought to by by the FuriousFPV. Based on STM32F4 MCU provides plenty input/output options in a relatively small form-factor. Can be bought with matching GPS, Bluetooth and Airspeed modules. F35 utilizes plug & play idea, since most ports are broken out as JST-GH connectors.

Matek F411-WING SE

Matek F411-WING SE

When small size is required, F411-WING SE comes into play. Super small and lightweight but offers enought serial ports and outputs to satisfy most lighweight builds. Vertcal USB port makes it easily accessibke when installed inside alsmost any foam airplane.

Best flight controllers for drones

Kakute F7

Holybro Kakute F7

Holybro Kakute F7 managed to do what others failed. It fits powerful STM32F745, OSD, SD Card and enough serial ports for everyone in a form factor only slightly bigger than standard 36x36mm. The unique feature of the whole Kakute series is a gyroscope installed on a separate board connected with the main board with a thin ribbon cable and sponge. It helps to keep gyro noise under control. Made to use together with 4-in-1 ESCs.

Matek F722-SE

Matek F722-SE

Matek F722-SE is probably the best STM32F7 flight controller for drones on the market. Integrated OSD, power distribution board, output for 6 motors or 4 motors and 2 servos, integrated camera switcher and "power on a switch" function. INAV, Cleanflight, and Betaflight supported. Enough Serial ports of everything you need, including ESC telemetry.