Starling 2 Max back-right motor clicking and reduced RPM — suspected magnet delamination

[Julian]

Hi ModalAI team,

Sales recommended that I post this issue here for technical review.

We identified a mechanical fault in the back-right motor of our ModalAI Starling 2 Max, mapped to ESC ID 3. The motor produces a repeatable clicking/rubbing sound during rotation, and visual inspection appears to show a displaced or delaminated internal magnet.

We have removed the aircraft from flight testing and are requesting guidance regarding motor replacement or RMA.

System details

• Aircraft: ModalAI Starling 2 Max
• Compute/autopilot: VOXL 2 / QRB5165
• ESC: ModalAI 4-in-1 ESC, M0129-6, board version 44
• ESC firmware: 39.25
• voxl-px4: 1.14.0-2.0.138
• voxl-esc: 1.6.2
• VOXL ESC tools binary: 1.10
• Affected motor: back-right
• Mapped ESC ID: 3

Mechanical observation

The back-right motor clicks or rubs at a repeatable point during rotation. The clicking became more pronounced during the first powered low-power test, so we stopped testing the suspect motor after that point.

The suspect motor and a known-good motor both measured approximately 0.5 ohm phase resistance. We understand that matching winding resistance does not rule out a mechanical or magnetic fault.

Close-up of the suspect back-right motor.

Props-off low-power comparison

This was not an ESC calibration or a thrust test. It was only a comparative motor-health check with:

• all propellers removed,
• the aircraft secured,
• voxl-px4 stopped,
• each motor tested individually,
• and the same initial low-power command applied to each motor.

Motor position	ESC ID	Average RPM at first valid low-power point
Back-left	0	1699.79 RPM
Front-left	1	1770.04 RPM
Front-right	2	1755.47 RPM
Back-right — suspect	3	1482.49 RPM

The good-motor median was approximately 1755.47 RPM. The back-right motor was therefore approximately 15.6% below the good-motor median at the same command.

The raw test log also records that the clicking worsened during this test, so we did not continue with higher-power points on the suspect motor. The other three motors completed additional low-power sweep points without the same mechanical symptom.

Image 2 — close-up frame sequence

Image 3 — props-off bench-test setup

Current assessment

At this point, we do not consider the back-right motor safe to fly.

The conclusion is based on the combination of:

• repeatable mechanical clicking/rubbing,
• apparent magnet displacement or delamination,
• reduced RPM at the same command,
• and worsening clicking during the powered test.

We plan to replace the motor before conducting any further hover testing.

Questions

Could the ModalAI team please advise:

1. Does this appear consistent with a delaminated motor magnet or another internal rotor fault?
2. Would you recommend replacing only the motor, or should we also inspect or test the ESC channel, wiring, and motor mount?
3. Is this appropriate for a replacement or RMA request?
4. Is there a specific low-power validation procedure you recommend after installing the replacement motor?
5. Would the raw VOXL ESC logs, mapping CSV, and full sweep data be useful for your review?

Supporting material

Attached or available:

• close-up video showing the clicking/rubbing,
• powered props-off bench-test video,
• close-up still images,
• ESC-to-motor mapping,
• raw VOXL ESC logs,
• comparison CSV,
• and a PDF engineering evidence report.

Thank you for reviewing this. We can upload any additional logs or hardware information that would help with the diagnosis.