After working on a quadplane for a few weeks, getting it flying and crashing due to pilot error - 3 times, I set it aside for a while and went back to my multi-rotors.
Left over from the quadplane experience were RTL operational thoughts.
I live and fly in a small-ish valley in New Mexico USA. From my “pad” to the south is all rising terrain. I only fly generally south and southwest due to neighbors. Directly west is a low ridge about 300ft above pad elevation that runs north south and is approximately 350-400 yards away.
The terrain south is gently sloping up and if I get out to “extreme” line of sight range with strobe lights attached, I might have to gain 500-1000 feet of altitude to maintain visual, radio and video, while keeping in mind the 400AGL rule.
I’ve reviewed the Copter WiKi concerning RTL with the above in mind and have setup my quad with RTL at the present altitude. This is to clear the west ridge, though then, if I understand the setting, the frame will return at some very high altitude putting me in the situation of having the frame very high for a vertical descent over the home point, with a possible low battery, and being above the 400AGL rule.
Further reading about RTL_CONE_SLOPE perked up my interest in how far away the parameter would work and while the frame climbs up to intercept the angle if below the angle(and distance from home), does it RTL at that altitude, still putting me in the high altitude return situation once near home.
The cone parameter brought me to the idea of using the cone angle and a radius from home, where the frame returns to the radius distance having climbed or descended to whatever altitude is calculated at the radius. Alternatively, the pilot would pick an altitude at the radius, like in my case, enough altitude to clear the west ridge and the cone angle is calculated from there - to be used a bit later.
The other piece of data needed would be the RTL_ALT (or equivenlent) above the home point. This would be the final altitude above the home point to start the vertical descent with the RTL_ALT_FINAL being “zero” for landing.
The 4 scenarios:
-if the frame is within the radius and below 350ft(my ridge clearance elevation), then it climbs up to the “cone” derived altitude before coming home.
-if the frame is within the radius and above the 350ft, it immediately starts a descent toward home to arrive at the 50ft home altitude(my selected altitude above the home point), using whatever descent rate and speed to accomplish it, keeping in mind the last 20ft or so of the vertical descent and is set by LAND_SPEED.
-if the frame is outside the radius and below the 350ft, it immediately climbs to 350ft, turns to home and when it hits the radius, begins a descent toward the home altitude
-if the frame is outside the radius and above the 350ft, it turns toward home and begins a descent based on arriving at the radius at 350ft, then completes the rest of the RTL normally.(Alternatively, if above the cone angle, returns in a descent directly home and arrives at the RTL_ALT above the home point)
I understand similar procedures have been used by other FCs, notably the EagleTree Vector.
My second thought on a terrain dictated RTL is some type of approach setup, not unlike an ILS instrument approach used by normal aircraft. Those approaches take into consideration terrain and obstacles. In my situation with terrain close by to the west, an approach could be setup with initial altitude and azimuths to the south to get the frame away from terrain into a clear area, then conduct the RTL final operation. This would be very similar to a pre-programmed AUTO mission. As an RTL, it would work for me, but many other pilots wouldn’t find it helpful in a low battery situation, especially if the frame was returning from the opposite side of the approach operation.
I also looked at “smart RTL”. I have also seen the messages about “the buffer being full, etc.”, so it doesn’t sound like SRTL would be helpful.
If anyone has thoughts on how to make RTL better for my situation, I would appreciate your input.