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.