Starting from what have been developed by Matt with this feature I decided to make a little step forward and started coding a “full touchdown” autorotation flight mode.
Propedeutic to the code development was to have a reliable and reasonably accurate simulated environment, so RealFlight9.5/SITL has been a key tool for the project .
The starting point was a flight mode capable of entering and maintaining autorotative aerodynamic state, the parts that needed to be added were the flare and subsequent touchdown phases.
Not only, I wanted it to be capable of safely getting to the ground from whatever initial altitude/speed condition (of course within reasonable limits of the “dead man curve”).
More specifically the autopilot should have been able to “understand” the entry energy conditions and take action on the flight controls accordingly (low altitude “hover autorotation” VS high altitude autorotation).
Last but not least, keep things as simple (…and lightweight-codewise) as possible!
The first major problem (not only for UAV helis…) is to have a precise reading of the altitude from ground underneath. An accurate distance from ground is mandatory for obtaining a succesful and correctly timed maneuver, even centimeters matter in the final phases for calculating the correct time to impact .
Furthermore the reading needs to be “fast”, considering that the average sink rate can reach speeds of up to 8 m/s.
So the choice fall on Lidar sensors here, since they proved to be accurate enough and capable of high refresh rates while keeping the costs down.
The test machine I used is an old T-reX 600 GF, equipped as follows (using “low” budget stuff, another target of the project):
- CUAV v5 nano autopilot;
- M8N GNSS;
- TFmini-S (12m) lidar rangefinder;
- 600mm main rotor blades;
- 1100kV Hacker motor;
- Hobbywing Flyfun 80V5 ESC (using internal Ardupilot governor);
- 6s 5000 to 8000 mAh battery
The heli has been setup with a low headspeed, 1300rpm, more convenient to emulate a typical UAV helicopter setup and thus further validate the model.
So far the above equipment has survived about 60 autorotation test flights, no spare parts needed yet!
Links to test flight videos: