RTL from high elev. angle may kill your plane

ArduPlane Plane 3.3
1

I’m trying to understand why AP tries to get above home point and only then starts descent if RTL was fired at relatively high angle of elevation related to home point.

Here are two simulated RTL flights:

  1. Normal RTL.
    AP applied very little throttle during this descent.
    [attachment=1]Screen-Shot-2015-07-14-at-4.43.32-PM.jpg[/attachment]

  2. RTL from high elevation angle:
    AP user max throttle before it reached point above Home Point and even gained some altitude during that phase. Then Throttle went to zero and it started circular descent.
    [attachment=0]Screen-Shot-2015-07-14-at-4.44.42-PM.jpg[/attachment]

All settings are equal between these two RTLs.

2nd approach appears to be very dangerous, because wind at high altitude is generally way higher, than below. So AP might try to fight against headwind at full throttle and kill battery before it can even reach point of descent. This actually happened to be 2 days ago. Thanks to radio beacon, plane was found, but if it tried to glide from where it was - it could easily land at home point.

2

Quick update:
After playing with simulator, I’ve discovered, that Increasing TECS_SINK_MAX parameter, increases chances, that APM will go into a proper glide/dive in “2nd” situation above.

So if you want to be able to RTL from high altitude w/o killing batteries, try tweaking TECS_SINK_MAX to a maximum value, that your plane can handle.

Also, …PITCH_MIN parameter affects how plane can dive, but from my experience, default -2500 is fine. I will probably use something like -3500 for my bixler.

3

You really need to follow the TEC tuning guide for auto modes to work correctly.
plane.ardupilot.com/wiki/flying/ … ing-guide/