Hi,
I'm facing two issues with my Brain2 on a new build. I fly scale/sport style only (no 3D). The heli has about 15-20 flights so far and these issues have been present since the beginning. All Brain2 parameters are at default except where noted below.
Setup:
- XLPower Specter 700 V2 – WC Edition
- Main Blades: SAB S701
- Tail Blades: SAB S105
- Motor: XNova Lightning 4525/YY-530KV 10P
- ESC: Hobbywing Platinum Pro 180A HV V5
- Servos: Theta Razor – 3× C1 (cyclic) / 1× T1 (tail)
- RX: RadioMaster RP3-H
- TX: RadioMaster TX16S
- Head speed: 1950 RPM (ESC internal governor)
- Brain2 firmware: 3.4.275
Mechanical checks I've done:
- Swashplate is level — pitch measured at multiple rotor positions using the Brain2 Swashplate Setup page (with the software commanding the servos to the zero-pitch position) reads 0° ±0.15°.
- No tracking issues visible in hover — blades are running in the same plane.
- Heli is balanced (nose and tail weigh the same).
- Transmitter is confirmed sending center pulse with stick at neutral.
Issue 1 — Constant backward drift in hover:
The heli has a persistent tendency to pitch nose-up during hover. It's a small but noticeable drift — after 3–4 seconds of hands-off hover, the heli starts moving backward. This happens regardless of wind direction. I don't experience this behavior on other helis flown at the same location under the same conditions.
Issue 2 — Oscillations in forward flight (main concern):
I'm seeing two distinct oscillation behaviors during forward flight:
(a) Slow pitch oscillation ("porpoising"): During fast forward flight, the heli exhibits a slow, lazy nose up-and-down oscillation, similar to a dolphin swimming. This gets worse with increasing speed. To address this, I reduced both the elevator P-gain and I-gain by 5 points (from 60 to 55). This seems to have resolved the porpoising.
(b) Fast vibration at the tail boom: During moderate-speed forward flight — for example, coming out of a fast banked turn — the tail end of the heli vibrates rapidly. It's difficult to tell from the ground whether the movement is in the pitch axis (tail moving up and down) or in the yaw axis (tail moving side to side), since the vibrations are quick and the heli is far away. However, the movement appears more likely to be in the pitch axis — as if rapid, small elevator commands were being applied, causing the nose to oscillate up and down and the tail to follow in the opposite direction. This vibration got worse after I lowered the P and I gains. It's worth noting that the flights where it worsened were on a windy day, so that may be a factor, but the wind conditions were nothing unusual for a 700-class heli.
Logs:
I'm attaching the flight logs I have. Note that I changed some logged parameters between flights while trying to address the porpoising, so the earlier logs may reflect different gain settings. If it would help, I can perform additional flights this weekend with specific parameters logged — just let me know what you'd like to see.
Thanks in advance for the help. undefined
In the files you sent (thank you), we don’t notice anything unusual except that the default channel for the “Setup Selection” control has been changed from channel 5 to channel 9.
Please note that with the setup we recommended for the ELRS protocol (Packet Rate = 333Hz Full (16ch), Telem Ratio = 1:8, Switch Mode 16ch Rate/2, Dynamic Power set to Off), channel 5 no longer serves as “Arming” (since in RC helicopters, gyroscopes do not control propeller operation as they do in drones, but instead control the much less dangerous swashplate servos). Furthermore, with the new ELRS 4.0 firmware, they have finally made the use of channel 5 much more flexible even with other configurations.
As for the two reported issues, they are easily resolved using the same practices employed over the past 50 years, when tail gyroscopes already existed but flybarless controllers did not yet exist.
For problem 1: “the model drifts backward when trying to keep it hovering”:
shorten the two front links going to servos 3 and 1 by half or one turn by rotating the uniballs, and lengthen the rear link going to servo 2 by the same amount.
This does not alter the setup of the symmetry of the rotary servos (90-degree servo horns with zero pitch) but compensates for the different front and rear aerodynamic characteristics of the specific model you are using when pushed by the wind generated by the rotor against the canopy and tail boom, as well as any lateral imbalances in the model (rotor precession).
In case it is unclear, please note that the preliminary swashplate leveling procedure is not intended to keep the model in position during hovering, but rather serves solely to facilitate the measurement and adjustment of collective and cyclic pitch.
Once these adjustments have been made, you can then adjust the swashplate angle to achieve stable hovering.
For problem 2b (you state that problem 2a is now resolved): “tail oscillations when exiting a turn or during fast forward flight”:
Simply reduce the main tail gain by two or three percentage points on the transmitter.
BR