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…

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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. Read More

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:

7 tips and tricks for smooth HD video on 7-inch quads

7 inch quads are great, but they are tricky. It’s much harder to make them fly smoothly than 5-inchers. It’s just more demanding in term of stiffness, propellers, vibrations, jello and so on. After almost one year of fighting with all those issues, here is my list of 7 tips and tricks how to get a smooth HD video footage on 7-inch quads. Bear in mind that almost all of those tricks apply for 5 and 6 inchers too. They are maybe not that important for great results there.

Put a gimbal on a 7-inch quadcopter drone

My 7-inch Source One got an interesting upgrade: instead of static GoPro Session bracket, I’ve put a gimbal on her. She might have got slightly heavier, but gimbal allows to take amazing shoots not. And it’s detachable! Sure, it looks awkward a little but gets the job done. The gimbal… the gimbal is crazy cheap Chinese clone of AlexMos hardware but works surprisingly well. Much better when it was carried by my 12-inch machine. I would not put a gimbal on 5-inch racing quad, but 7-inch Source One didn’t even notice the extra weight. And that’s something!

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!

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!

Betaflight 4.0 – improved dynamic filtering on 7 inch quad

The next major release of Betaflight will be 4.0 with new and improved Dynamic Filtering. For now, today I only did a quick test of Betaflight 4.0 running on my TBS Source One 7-inch FPV drone. No tuning, no playing with settings. I only flashed, set modes and radio and that’s all. I pretty much like how it feels. This quadcopter was mainly doing cruising, no freestyle, and it felt good to put those props into more demanding use. Yes, Betaflight 4.0 has a good chance to be just awesome…

Hardware

A note about the sound. It indeed sounds like a V8 engine. Bear in mind, that those are default settings and Gemfan FLASH 7042 props are light and do not like to have too much D gain.

7 inch quadcopters fly like shit? Well, kind of, yes…

So, you got yourself a 7-inch quadcopter drone, you try to tune it, and you finally discovered that something is bloody wrong? Whatever you do, no matter how long you tune the PID gains and filtering, you can not get rid of jello and some strange vibrations? Welcome to the club mate!

Apparently, scaling 5-inch frames to 7-inch ones in XY axes does not work! Longer arms equal lower modal frequencies. Bigger propellers equal lower vibration frequency. After combining both factors, we have frame oscillations that are happening on the hardware level. No way you can tune that thing out. To make thing worse, 2205 and 2206 (even 2207) motors does not have enough torque to quickly change 7-inch propeller rotation speed.

For the details, watch the video.