Motor failure does not have to crash your quadcopter

Responsible unmanned aircraft pilots (you and I, at least) have plenty on our minds when it comes to managing risk, from airspace and obstacle avoidance to not hurting pets or people. But most of us probably don't give much thought to mechanical failures in flight, because there's really not much we can do about it.

It has long been accepted by pilots and the industry that a four-rotor drone will fall out of the sky uncontrolled if any one of those motors fails. That makes each motor what aeronautical engineers call a potential single-point failure. Each motor is critical; the aircraft simply cannot fly without it. The vulnerability multiplies when you factor the electronic speed controller and propeller attached to each motor, and multiply those single-point failures by the number of mission-critical components involved.

Quadcopters may seem simple to fly, and they are, but only because a computer is handling some really complicated work every second the aircraft is in the air. Every maneuver you command with a thumb stick gets translated by that flight control computer into instructions to maintain speed, accelerate, or decelerate each motor independently. Every maneuver requires coordination of all these moving parts.

The rotors are usually sped up or slowed down in pairs, be that adjacent or diagonal pairs of motors depending on the maneuver you're commanding. To climb straight up, all four motors speed up together, but to make that a spiral or turning climb, the speeds of diagonal pairs are deliberately mismatched, allowing torque to get the job done. (Just imagine how hard it would be to do all of that yourself with just two hands if your controller had four sticks!)

Let's not get too bogged down in physics, though. The main thing to understand is that the vast majority of quadcopters on the market today share this basic design feature: Losing a motor will cause the aircraft to tumble out of control, because the flight control computer is programmed to work the motors in pairs, and if any participating motor checks out for any reason, Sir Isaac Newton takes over.

Modern drones are reasonably reliable, though only the manufacturers have enough data to really know just how reliable. So far, they have not been inclined to share. We do know that in-flight failures happen, and for a variety of reasons. A motor failure that sends your drone spinning out of control could obviously ruin the day by damaging property or hurting people.

This has long been an advantage of bigger drones with more motors. Flight control systems can compensate for losing one of eight motors, or even one of six, with relative ease. Most pilots, even manned pilots, will agree that you can't have too many motors from a redundancy standpoint, but let's stick to the practical: Most of us fly quadcopters.

Software saves the ship

Can a quadcopter become a tricopter, or even a duo-copter (if there is such a thing) without taking a tumble? Verity Studios of Zurich, Switzerland, had to figure that one out along the way to making a name for itself flying drones in numbers in the midst of public performances and events. They created dazzling and beautiful indoor and outdoor automated drone presentations for major touring acts such as CCTV New Year’s Gala, Drake’s 2018 tour, and even rock band Metallica among many others.

Company founder Raffaello D’Andrea said Verity created software that can achieve the seemingly impossible—allow a controlled descent and landing of a quadcopter after one or even two motors fail. He said this Failsafe computer code package could work with virtually any make or model of drone flying today. Much of the inspiration to create Failsafe came from a desire to improve flight safety after becoming aware of some very serious injuries to people on the ground struck by an out-of-control drone.

Verity Studios worked the Failsafe algorithm into flight control systems used in live performances where a failure would absolutely not be acceptable. To name one example, eight Failsafe-enabled quadcopters were flown on Broadway for performances of Paramour by Cirque du Soleil. The drones flew above the performers in front of audiences of 2,000 people, and completed more than 7,000 autonomous flights. Failsafe triggered only once during that run, due to a faulty battery, and the affected aircraft landed safely. There were no accidents or serious incidents throughout the show’s entire run.

Failsafe has some obvious advantages over parachutes. It requires no extra hardware or payload, and it functions automatically. The weight of any parachute reduces flight time, and reduces how much other stuff the drone can carry. Failsafe, being software, requires no such tradeoffs.

But here's the rub: While the Failsafe software is designed to work with virtually any drone, it has to be incorporated into the manufacturer's software, and that hasn't happened yet. The company's marketing department said discussions are underway with major drone manufacturers that could lead to Failsafe implementation via a routine software update.

While we wait for that, the possibility that a quadcopter might take a tumble should always be somewhere on our minds.