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!
I took my 7-inch SourceOne for a spin with a gimbal onboard on a probably first really warm day this year. The results are encouraging, gimbal really helps to get very nice and smooth HD video footage…
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.
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…
I have a theory that 7-inch FPV quadcopters fly “below expectations” due to frame resonance, I did not expect, so many pilots agree with it. We had nice talks in comments, I’ve learned a lot about stiffness and moment of inertia (that you Pierre-Louis). Many of you shared some of their tricks and ideas on how to solve that. Let’s start testing some ideas how to fix that!
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.
Hey, don't leave yet, there is more!
Do you know that there is a YouTube channel with awesome, drone and FPV related video? Why don't you give it a try?