Autorotation thoughts

Hi,

I've been flying my heli for a while but I came across a problem, which doesn't happen quite often but it may be important for safety. It's the power failure and autorotation.

Recently I had a major failure of my battery during the flight, the battery just suddenly died that my motor has stopped but the servos were still working thanks to the separate battery for servo and PX4. Luckily I was about to land my heli about 4m high so I heard that the motor had stopped.

I changed to stab mode, and tried to do my first autorotation in my life without any experience and I'm glad I was not panic. Although my autorotation was not well performed, the stab mode helped a lot. In the end I just had a slight hard landing, nothing was broken.

After this accident I was thinking, whether it's necessary to have some way (from our PX4/Pixhawk) to perform the autorotation or maybe a warning to the pilot when such failure happens. If I wasn't close enough to my heli I'll have no idea that the motor had failed and the heli will sure lose all its power before I realized what was going on.

If it's a large heli like 600 or 700, it will sure turn out to be a disaster. Afterall for a UAV a lot of things are controlled by the FC, we may not notice it until it's too late, and unlike the 3D pilots, our heli can be far away and we won't feel anything. So yeah, I kind of feel that we may need some safe mechanism through Ardupilot, to protect the heli, and maybe protect other people.

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          • Hi Rob,

            maybe it helps, a german speaking guy talking (and showing)  about realistic behaviour in autorotation: RC HELI FLUGSIMULATORENVERGLEICH

            I think all simulators are good enough to learn the stick moves, the difference is i.e. in how much you have to move the stick. For me some simulators are harder to fly because I have more difficulties to judge my distance to an obstacle. One should not forget that changing the physics of a model can make a real big difference.

            I can use AeroSimRC, FMS, Multiflight, NeXt, RealFlight and Reflex XTR. I don't want to start yet another endless discussion about benefits of the different sims, but if you are interested you can send me a message.

          • Go for Phoenix, its very simple and realistic if you pick the large models. I have quite a lot of experience flying 3D so if I can be at any help with this let me know.
          • Rob,  I am using Phoenix simulator.  Horizon hobby sells it and I think its good.  My 2 indoor heli's and my 550 when it was stock flew remarkably similar on the computer.  Phoenix lets you adjust characteristics of the airframe as well.  Headspeed, weight, and a lot of other things.  It has a large database of aircraft that is always being updated.  Now, when you do what this community does to stock aircraft,  I don't know how accurate it is.  It is a good tool for flying practice.  Nothing can replicate when you have a problem occur and you are trying to save a few thousand dollars of hardware.

            Regards,

            David R. Boulanger

  • Xin,  You are lucky it happened at such a low altitude.  If you have STAB mode set up like most of us probably do then you may not have enough negative pitch to perform an autorotation successfully. Its a good idea IMO to always have access to ACRO mode and initially set your swash to allow for plenty of negative pitch.  Maybe someday a good Autorotation algorithm will be in the code somehow.  I think it would be very complicated and require precise altitude measurements along with headspeed , descent rate, weight of the heli, blade length and so on.

    Regards,

    David R. Boulanger

  • Hi Rob,

    Great to know you already have ideas :)

    We are thinking about the same. I2C is the way to go indeed. What comes in my mind for motor RPM monitoring is a magnetic sensor near the motor, measuring the pulse frequency of the magnetic field. It should be a reliable way to measure, I've ever seen some similar sensorsin the university, measuring the metal gear RPM by measuring the pulse when the teeth pass. I'm not familiar with gas engines but it seems doable by measuring some pulse from the motor, like the spark plug.

    It feels difficult to make a good algorithm for autoration in UAV as the system looks very complicated, I guess the last step of landing could benefit from a sonar, like changing the descent rate when the sonar reports approaching to the ground.

    One thing I'm worried about is the DDVP tail, which shares the same power chain. If the battery fails, autorotation still won't work in this case unless there is a separate battery for it, just like that for the servos.

    • I already have the measurement part set up.  I'm using a hall effect sensor and magnet, actually from an EagleTree data logging system.  This part works very well.

      You can still fly a helicopter even after the tail drive fails.  As long as power to the main rotor is cut, there is no longer any torque trying to rotate the airframe. At this point, the helicopter flies similar to an airplane.  Aerodyanmics simply causes the tail to follow behind the airframe, as long as you have forward speed.  If you have no forward speed, then there is nothing to control the tail at all.  It won't spin, I guess it would just random-walk.

      David, how much negative pitch does it take?  I normally have about -3 or -4 degrees in Stab mode.

      I really should get a simulator and start practising this. 

      • Nice, that EagleTree hall sensor would do the job.

        I didn't realize the release of torque after cutting off the engine, now I can just keep the DDVP tail in the main power train with confidence. Looks like if the heli has no forward speed and the tail rotor stopped, the tail will still start to spin just a little due to the friction in the one-way bearing.

        Actually the autorotation was pretty fun, although my hands were already wet after landing :)

  • Xin, your point is very valid.  This is something that needs to be worked on.  It's on my list. :)

    The first problem is that Arducopter needs to be aware that the propulsion system has failed.  How?  Well, it's going to need at least one more sensor.  Best thing to have would be rotation speed of the main rotor.  I have actually written code to run on an Arduino Pro Mini that will work as a tachometer.  The intent is to connect the Pro Mini to the Autopilot through I2C, and it will give the autopilot information it needs.

    From there, we needed to decide how to handle it.  I think that what we would do is have a setting for minimum rotor speed.  If the speed falls below that, we can assume the motor has failed.  At that point, if in an auto mode, we would probably warn the operator.  If in an auto-mode, then we would try to enter some kind of auto-rotation.  At first would be nothing more than a pre-defined descent rate to maintain a minimum rotor speed.  It would still hit the ground, but not as hard as it would otherwise.  Then in the future, we could actually have it attempt a soft landing.

    This will become much more critical if using a gas engine which could flame-out.

    I'd like the sensor to eventually expand to be a full engine monitoring system.  It could have temperature sensors for example.  Also, I'd like to monitor both the rotor speed, and the motor speed.  This could give us early warning when the motor stops.

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