I'm experimenting with a Skywalker X5 flying wing. It flies very well in stabilized and mission mode. Now I'm trying to understand the autoland planning in QGC and how it works in the air.
My playground is a large football pitch surrounded on 3 sides by trees and the other side by high voltage power transmission lines. So my landing space is a bit constrained. I need to do the pre-land loiter at about 40m and make as steep a descent as is feasible.
I see that PX4 warns the user via QGC if the distance from loiter to landing spot is too short, so I planned a landing that PX4 would accept and then I tested the mission.
Waypoint following and altitude control were very good. On transition to landing, the wing started a perfect 15 degree descent but was still 8 meters above the ground at the landing point, making a beeline for the trees at the edge of the field. I tried to abort and pull out of the landing but was too late and the plane slammed into a tree at 20 m/s.
Log is here: https://review.px4.io/plot_app?log=d030852f-da16-49b3-89b4-5d58700a8609
Amazingly, no serious damage. This is a great plane!
Anyway I would like to understand my mistakes. I was expecting the plane would land within 10m of the target spot and allowed for that, but in this case, I think the overshoot would have been about 60m.
From the logs, I notice that the estimated altitude at launch was -2m, then the estimated altitude on the ground after it fell from the tree was -4m. The difference is partly because I mistakenly armed the plane while holding it, preparing to hand launch. I guess if I had armed on the ground then the beginning and ending altitude would have been closer. Anyway, with the 2m altitude error at 15 degrees, I'd expect 8m or so of overshoot, not 60m.
For the landing parameters, I'm using the defaults for the Skywalker X5: 5 m flare height, -10m HVIRT, 15 degree descent.
So my questions would be:
- why does PX4 allow this plan, if there is not enough distance from the loiter to the landing spot?
- is the landing routine using altitude of 0 or the estimated altitude at launch (-2m)?
- shouldn't landing be at a lower speed than the cruising speed? When I see videos of flying wing autolandings online they seem very nice and gentle, not 80kph! But I didn't see any parameters to control this.
Sorry for all the noob questions and thanks for any help!