Ive flying FPV for around 2 Month with a RTF 3.5 Quadcopter.
Today I put my first self-built 3.5 inch quadcopter into operation.

Foxeer Mega 3.5 Deadcat
HGLRC F7 Aio
GEPRC 2105.5 - 2650Kv
6s Lipos
Runcam Nebula Pro
Betaflight 4.4


Except for the small problem that follows, the copter flies very well with Betaflight standard settings/PIDs + filters and my usual custom rates and much better than my pre-tuned RTF 3.5" copter.

The problem: despite the same rate on the roll and pitch axes, the copter does the 360° flip in the pitch axis a lot faster and overshoots to much. Roll axis 360° is perfect.
I used different batteries for the test flights. 650maH-6s / 1050maH-6s.
This problem exists with the 1050maH LIPO. Rarely or not at all with the 650maH.

For a similar setup, the frame manufacturer recommends to use for Lipos 650-1000maH.

Why is that? The higher weight of the 1050mah batteries? Or possibly the Deadcat design?
For me it feels like a problem with higher mass in conjunction with the Deadcat Frame
Should I just turn the rates down until the pitch axis feels the same or is this a PID tuning thing.

Thank you

It could be a balance issue, move the lipo to figure out.
Maybe mount it vertically or diagonal just to check a balance issue.
Oscar has some informations under center of gravity - cog

https://oscarliang.com/how-to-build-fpv-...#The-Frame


Is the reccomandation for 6s batteries or maybe for a 4s settup?
1000mah 6s is very big for small blades like 3,5" but you have big engines....

Do you get any other benefit by the 1000mah 6s battery?
I believe you doesnt get true benefits in flytime either?
Personal i wouldnt use such a battery on small 3,5inch rotors.
If its to heavy and the rotorresponse is to low, it might act wrong even on good cog. It needs a lot rpm changes to manage more than 300g on 3,5inch blades, that could be the issue: not able to follow your stick inputs.

Maybe post an image of your build?


To tune out an issue about heavy battery, iterm is mainly compensate that.
Higher pid values would be benefical, you can try to tune by the multiplier slider or iterm slider only.
Increase the values gently, touch the motors after short tests, it shouldnt heatup much more.

The recommandation is for 6s yes. I asked for that info directly at the manufacturer
The Foxeer Mega 3.5 Inch is a pretty Heavy Frame compared to other 3.5 Frames.(Without Lipo the complete build is 260g). But the 2105.5 - 2650KV Motors are strong and bigger on what you normal put on 3.5 inch.
Beside this problem, the quad flies with the 1050mah Lipo very good. Its doesn´t feel too heavy. Fast Response. Flighttime 5-6 Minutes, with the 650mah one its around 3 minutes.

"Drift wobbles" i gain

Did you any adjustements on pids or filters?

https://oscarliang.com/fpv-drone-tuning/
There would be a lot of information

I did 2 testflights with taking more care about the center of gravity and raised the masterslider carefully up until 1.3. The only thing i enabled, are settings like Dynamic Idle Value to 25%, Thrust Linearization 25% (48khz ESC), which did not help with the pitch problem (Expected that, but i helped with Low-Throttle-Stability, which was also i little bit off, from what iam used to)
My next step: i ordered two 850maH-6 Lipos to see if its gets better, those should be a good compromise between weight / flightime. the 850maH Lipos are around 55-60gr lighter compared to the 1050mah´s.
Next testflight i will also try your advice and play around carefully with Drift Wobbles settings, if the problem still existing with the lighter lipos.
Thank your for your advice until now.

Some tuning makes sense, 3.5inch is different to 5inch, the bf defaults does match common 5inche freestyle builds best.

As you have a deadcat, tune single axies would make sense, but that doesnt work well by sliders.

Set the multiplier to 1,3 might be a good direction.
It sounds like it is mainly a tune issue and adjust pids would solve it. As you are using different batteries, maybe a single pid profile for the 1000mah packs would bring the best out of both worlds.

Ok the problem is solved. I fly now with 850-mah 6s which provide enough flytime. I also raised the Master Slider until 1.2 and did some small adjustments to PID´s and lowered my Max Rate on pitch a little bit. Now both axis have the same turn-speed and no more overshooting.
Thank you for helping