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:

How to choose drone motors

One of the mistakes beginner (but not only) pilots and drone builders make is that 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. It's not true. In this longish video, I will tell you how to choose motors for your drone: 5-inch, 6-inch, and 7-inch. We will talk about torque, power, a moment of inertia, what happens when you put a too big propeller on a motor.

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.

7″ FPV frames – window shopping on Aliexpress

7-inch FPV quadcopter drones are getting more popular and because of this, Chinese manufacturers are finally putting new frames on the market. While even 6 months ago the choice of 7-inch frames was very limited, now you have at least a half-dozen choices when you want to buy cheap from China. Let's go to Aliexpress and do some window shopping (not licking 😉 )…

Frames from this video:

This is my 7-inch TBS Source One

This is my 7-inch version of TBS Source One quadcopter frame. So far, so good, Source One is really tough and stiff frame so all the problems with resonance and oscillations in flight should be over. And looks like they are over.

It might sound like a good old V8 engine but flies smooth. And after all, that was my ultimate goal. So let’s say “bye, bye” to wobbly-wobbly iFlight XL7 v2 and say “hello” to 7-inch Source One! 6mm thick arms are the way to go!