What determines RTL circling speed?

Given a plane without an airspeed sensor, what determines the airspeed or groundspeed during an RTL?

Yesterday, a friend who is not accustomed to flying FPV was flying the plane, got a bit too far away, so I put the plane in RTL mode and let it return. The first 1,5 minute of the following video shows the RTL in action:

TRIM_THROTTLE is set to 40%, which the manages nicely during the straight descending section. However, once it enters the circling part of RTL, throttle is raised quite a bit, even hitting THR_MAX of 80%. Why is it doing that? Is that TECS_RLL2THR in action?

Link to log: Nextcloud

Disclaimer: I have not done a complete tune on the plane according to the ‘tuning in detail’ section of the documentation; it only received an autotune for now. I will do that once I am done modifying things.

The plane is reaching it’s maximum bank angle of 55degrees to maintain the RTL loiter radius. So what’s probably happening is the stall prevention is keeping the speed of the plane high so it doesn’t risk stalling. This isn’t bad or wrong, its simply the FC has to make some guesses about performance, especially when the tuning isn’t finished or you don’t have an airspeed sensor. I’m also going to make the guess that there was some wind when you were doing the flight because the bank angle of the RTL circle was changing by about 20 degrees as the plane went around. Not an issue, but it may exaggerate the high throttle settings.

-Don’t worry about it. The plane is doing what you told it to do. Maybe just not what you wanted…
-Increase the WP_LOITER_RADIUS or RTL_RADIUS. A larger circle will have less bank angle, therefore need less throttle.
-Finish the tuning, and the TECS tuning. The min airspeed of 9 and max of 40 is a wild range. Not saying it’s wrong, but that’s wide. The Stall prevention will scale it’s ideal speed in a turn so if you have these values set more realistically for the plane then you should be in a better situation. If those values (9 & 40) are valid then you should maybe increase the turning radius a bit more.

Thanks for the elaborate answer.

I did a quick test today with increased WP_LOITER_RADIUS and lower TRIM_THROTTLE.

That is already better.

Regarding the min/max airspeed: with the small Li-polymer pack and small propeller I used to maiden the plane (less weight = less impact when things go wrong…) 9m/s was a bit above stall, full throttle was 45m/s airspeed.
With the somewhat larger propeller and Li-ion pack stall is around 11m/s, full throttle is around 38m/s airspeed.

What would you enter for ARSPD_FBW_MIN/ARSPD_FBW_MAX given these numbers?