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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  • T3
    Does the new code (that spins the motors at idle) prevent an out of control falling quad?
  • Interesting video and I like the idea of a flap system to slow the impact or fall time and maybe allow for a recovery.

    I also thought of something else... The comment "Nearly every tri/quad/hex/multicopter pilot has the impulse to chop the throttle while flying around." was to me a true statement. I have in a moment of panick, pulled my throttle back too far causing my Hexa to fall and crash before I can recover from my mistake. So I started to wonder why the APM/Pixhawk code doesn't have this particular fail safe coded in to allow pilots to chose it as an option (especially novel idea for Noobs like me). Is there a way to build this in to allow the APM/Pixhawks to measure altitude via baro, sonar, optics, or alt DR and then prohibit PWM/PPM minimums to be commanded while at an altitude above a operator entered minimums? Thoughts?

  • auto parachute

  • I like that you tried out the flap idea. Next step would be to make a quad glider ;)

    Like Ted said, maybe the secret to free falling is to point the craft upside down. Recovery should be easier.

  • I only watch his videos to see footage of Park City.  He's got loaded parents.  Going to 4000ft is a good way to put a hole in someone's roof, if wind shear takes hold of it.

  • This fellow has fun regularly. I enjoyed the video he did with teaching his girlfriend FPV with fixed wing. It was not winter yet and the view of his part of the country was beautiful.


  • Definitely provides an interesting perspective - worth buying a T shirt to see what he does next.

    I think using a snow covered landscape definitely let these "tests" go on longer than they would have otherwise.

  • 400ft AGL? I thought you said 400ft ACL! (Above cloud level) ;) 

  • 4000 feet eh? Im pretty sure he never exceeded 400ft...wink, wink.

  • T3

    You could turn on the ESC's "brake" function. This would dramatically reduce the backwards RPM the props achieve thus possibly making a restart easier. But also a MUCH faster fall. Or try descending inverted. 

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