Just saw this thread. The SkySurfer is a good plane for your project. I have a similar plane, the AXN Clouds Fly. If you need more room, or load capacity, for more equipment you could move to the Super SkySurfer, wingspan of 2400mm. Bannana Hobby has it for $219 RTF (Ready To Fly).
For stability control you normally use all three axis, Yaw, Pitch and Roll control. For auto-leveling you normally only need Pitch and Roll. I would recommend that you use auto-level in your collision avoidance design. If your plane tries to avoid an obstacle while using Pitch, Roll and Yaw it may put the plane into a spiral. Very hard to recover from. Using a auto-level style control with only Pitch and Roll would be a safer method to tie into your collision avoidance.
For collision avoidance you are looking at sonar. What is its max range?
The SkySurfer cruises at about 35MPH (capable of 60+ mph (70+f/s)) or about 30f/s (9.1m/s). The question is; will the sonar range be good enough to allow a reaction before you strike the obstacle? This will be determined by how much control surface throw your system can use and how fast it can apply full throw. Most servos are about 60 degrees in .12 seconds at 6V or about .14 seconds at 4v8. Most BEC's (Battery Eliminator Circuits) in your ESC (Electronic Speed Control) only provide 5V. So the actual servo speed is closer to .13 seconds. Also there is the time needed to for the control surface/s movement to take effect. That depends upon surface size and the models wing loading. I do not have any idea what those will be for any given plane.
It will be interesting to see how your software deals with these issues!
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