Severe Pitch Oscillations in VTOL Reverse Transition

ArduPlane
, ,
1

Hi everyone,

I’m currently flight testing a 6m wingspan hybrid VTOL aircraft (quadplane configuration) with an all-up weight of ~110 kg, and I’m facing issues specifically during reverse transition.
Firmware - 4.6.2

Problem Description

During reverse transition, I observe significant pitch oscillations, especially while trying to bleed airspeed before switching modes.

Flight Profile

  • Flying in FBWA mode

  • Reducing throttle to zero to bleed airspeed

  • Preparing to switch to QLOITER

Observations

  • As the airspeed drops to ~27 m/s (current Q_ASSIST_SPEED for reverse transition), the aircraft:

    • Suddenly pitches up

    • Enters noticeable pitch oscillations

  • These oscillations are quite pronounced and repeatable

Parameter Setup

  • Q_ASSIST_SPEED (forward transition): 23 m/s

  • Q_ASSIST_SPEED (reverse transition): 27 m/s

We intentionally set a higher assist speed for reverse transition because:

  • Reverse transition appeared smoother at 27 m/s compared to 23 m/s

Log Analysis

  • From the logs, I noticed that:

    • The desired pitch demand drops to ~ 0

    • The controller does not seem to command pitch corrections

    • This happens when the aircraft is below Q_ASSIST_SPEED (27 m/s) during reverse transition

Key Questions

  1. Is it expected behavior for the desired pitch to go to zero and not command corrections below Q_ASSIST_SPEED in this phase?

  2. Could this indicate:

    • A limitation or behavior of FBWA + QuadPlane assist logic, or

    • A tuning issue (e.g., pitch PID, TECS interaction, or transition parameters)?

  3. Are there recommended practices for:

    • Managing airspeed bleed before reverse transition?

    • Avoiding pitch oscillations during this phase?

Additional Context

  • The issue is more pronounced in FBWA during deceleration

  • The oscillation begins right around the assist speed threshold

  • I came across a similar issue reported by another user where the root cause was the use of a fixed-wing ESC on quad motors

  • However, in my case, I am not using fixed-wing ESCs for the quad motors, so that particular cause does not apply here

Log Link - https://drive.google.com/file/d/1W-kP8qYd2xqjc5ZxZrUcQadmU6ZMNzUH/view?usp=sharing

Any insights, suggestions, or similar experiences would be really helpful

Thanks in advance!

2

How about switching to Q-hover mode during deceleration?
How can you set up two different Q_assist_speed for transition and reverse transition?