3D Robotics

Experimenting with quads in free-fall

From Hackaday:

Nearly every tri/quad/hex/multicopter pilot has the impulse to chop the throttle while flying around. Most quadcopters are fixed pitch, which means that as power drops, so does control authority. When power is cut, they fall like stones. A quick throttle chop usually results in a few feet of lost altitude and a quickened pulse for the pilot. Cut power for much longer than that, and things can get really interesting.  [RcTestFlight] decided to study free fall in depth, and modified a test bed quadcopter just for this purpose.

First, a bit of a primer on free-falling quadcopters and their power systems.  Quadcopters always have two motors spinning clockwise, and two spinning counterclockwise. This configuration counters torque and allows for yaw control. Most large quads these days use sensorless brushless motors, which can be finicky about startup conditions. Brushless controllers are generally programmed to kick a motor into spinning in the proper direction. If a motor is spinning in reverse at several hundred RPM, things can get interesting. There will often be several seconds of stuttering before the motor starts up, if it starts at all. The controller MOSFETS can even be destroyed in cases like this.

When a quadcopter loses power, the motors slow down and thrust drops off. The quad begins to drop. As the falling quadcopter picks up speed, the propellers begin to spin (windmill) due to the air rushing up from below. If the quadcopter started its fall in a normal attitude, all four of  the propellers will rotate reverse of its normal direction.  The now spinning props will actually act as something of an air brake, slowing the fall of the quad. This is similar to a falling maple seed, or autorotation in a helicopter.  The spinning blades will also act as gyroscopes, which will add some level of stabilization to the falling quadcopter. Don’t get us wrong – the quadcopter can still be unstable as it falls, generally bobbing and weaving through the air. 

None of this is a guarantee that the quad won’t tip over onto its back – which will reverse the entire process.  Through all of this bobbing, weaving, and falling the flight controller has been along for the ride. Most flight controllers we’ve worked with have not been programmed with free fall in mind, so there is no guarantee that they will come back on-line when the throttle is rolled on. Thankfully many controllers are open source, so testing and changes are only a matter of risking your quadcopter.



[RcTestFlight] found that his FPV h-quad quad was relatively stable in free fall, so he began experimenting with falls from high altitude. He found that many tests don’t end well. One crash managed to bend his aluminum frame badly enough that he replaced two of the arms with more forgiving wood.  His quad originally had 9 inch props. In an attempt to go for a slower sink rate,  [RcTestFlight] switched to 14 inch props. The larger props also needed slower motors with more torque. After these changes, the quad definitely fell slower, however he found the fall was actually less stable than the 9 inch props. Self deploying foam drag flaps slowed things down even further, but the flaps themselves became an issue when a particularly violent fall ripped them all off.  After taking his quad up to 4142 feet, nearly losing his quad, and a number of rough crashes, [RcTestFlight] had has his fill of free fall. We’d love to see more testing, especially with collective pitch quadcopters. We’ll keep our own quads safe in powered flight though.


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  • I tried the same once with my arducopter FC, also from about 4000ft, but I couldn't get inverted. Arducopter keeps trying to upright the multicopter after passing through the vertical, in acro mode, with acro trainer off.

  • @Muhammed: Thanks! On that video we used mainly 60"(1.8m2) chute. At last scene where copter crashed with early prototype it was 120" (6m2). 70"(2.5m2) chute arrives any day and will be tested also as well as 120" real eject system hooked to copter. These two tests should take place on february.
    Test results and videos are going to be at project blog: http://diydrones.com/forum/topics/diy-parachute-deployment-mechanis...
  • Very nice research, I particularly liked the way it covers the general range of motors and props used on quads and H-quads of this size. This makes me rethink my dismissal of an emergency parachute that has it's own accelerometer trigger circuits and power source.... If I finish my self contained automatic 48-inch parachute system, would people be interested having me post it on the blogs?

  • T3

    Very Cool!  Great job, thanks for sharing.

  • Ya that's why I was thinking of "labeling" the switch something along the lines of "Flight/Land".  Land mode allows the throttle to be reduced fully.  Flight mode would basically stop reducing the throttle at minimum maneuverability.

  • @Henri: Very nice! That last shot was quite ironic :) 

    @Matt: Yes, you can do this in the dual rates. On my 9X with Er9x, I can set the endpoints for individual rate modes. You may, run into issues arming and disarming your multi-rotor this way, as the FC will only arm/disarm below a certain throttle value. You may have to switch out, arm/disarm, then switch back into this mode. 

  • Interesting testing and risky indeed!

    Have you guys ever wondered how APM could slow descent in case of loosing one motor? My copter experienced once 150m free fall and I noticed that descent time was longer than it should have been. Copter did more than 10 attempts to gain control but flipped again every time. I'm sure APM could slow descent significantly using remaining motors if it notices after one full flip that one motor is lost.

    I'm willing to test this kind of free fall control / emergency landing control with my parachute test copter if someone wants to try programming it.


  • Could you program a switch on the remote to alter the throttle minimum?  Rather than trying to make the FC do it, you could just make the transmitter throttle axis not be allowed below X while the switch is in "flight" mode.

  • +100.. Wow! Never seen such a video.

    Hats off to [RcTestFlight] !!

  • @Doug:

    If I'm reasonably low to the ground and get in a pinch, I just cut the throttle. I'd much rather break a few props than try to power out and end up breaking arms and bending shafts, or ultimately hit whatever I throttled down to avoid. 

    At least for me, it seems that while powering out of tight situations works alright with (certain) planes, it doesn't work too well with multi-rotors. 

    In my beginner experience: say I were flying a multi under a tree and ascending too close to a branch. I would instinctively cut the throttle to avoid hitting it. Once the multi starts falling, I'd throttle back up to avoid the ground. For me this used to end one of three ways: Either I'd throttle up too late and hit the ground, breaking a prop or two, or I'd overshoot the throttle and hit the tree, or I'd get into a big throttle oscillation trying to regain altitude control. Having a minimum throttle probably would not save me from hitting the ground, nor would it prevent me from getting into a sort of pilot induced oscillation (though it may have dampened it a bit). 

    Soon you'll get pretty good and be able to manage these situations with ease. It's all about training yourself to fully exploit your proportional control of the aircraft. 


    I've noticed that in a fast descent, having the throttle at 15~25% prevents my quad from flipping over or yawing uncontrollably (I currently have a KK2.1 in my quad.) The idling motors in the latest APM multi may help keep things upright (at least in a straight fall) while you regain control, but it won't save you from nailing the ground. It will however slow the fall, thus slightly lessen the impact of the crash.

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