Free Falling Quadcopter Experiments End With Splat

quad-freefall

Don’t get too attached to the great picture up above, as the quad shooting it was in a death plunge when the frame was snapped. There’s just something tempting about free fall. 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.

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A Collective Pitch Quadcopter

StingrayCY(1)

Quadcopters aren’t a new thing, but for all the advances in multi-rotor craft, they all still fall into the paradigm of, ‘stick a prop on a motor and repeat three more times. [Curtis Youngblood], one of the top RC heli pilots in the world, came up with a very cool drive system for a quad, requiring only one motor and granting each blade collective pitch that allows for absolutely insane acrobatic ability.

There’s only one motor inside the Stingray 500, as [Curtis] calls his new toy. It’s at the rear of this quad’s H-frame, attached to a shaft running down the spine with a pair of pulleys. All four rotors are driven by this spinning shaft.

Because [Curtis] is an acrobatic pilot, he needed a way to control his ‘copter in more than one direction. To do this, he added four servos on each arm of the quad, giving each rotor collective pitch, just like the tail rotor of a real helicopter. The result is a quadcopter that can fly upside-down with the greatest of ease, perform barrel rolls, and all the other maneuver a true 3D RC ‘copter can do.

The awesome guys at Flite Test had [Curtis] visit their hangar and had him do an awesome demo flight. You can check out that video below.

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