We spend a lot of time here at Hackaday talking about drone incidents and today we’re looking into the hazard of operating in areas where people are present. Accidents happen, and a whether it’s a catastrophic failure or just a dead battery pack, the chance of a multi-rotor aircraft crashing down onto people below is a real and persistent hazard. For amateur fliers, operating over crowds of people is simply banned, but there are cases where professionally-piloted dones are flying near crowds of people and other safety measures need to be considered.
We saw a skier narrowly missed by a falling camera drone in 2015, and a couple weeks back there was news of a postal drone trial in Switzerland being halted after a parachute system failed. When a multirotor somehow fails while in flight it represents a multi-kilogram flying weapon equipped with spinning blades, how does it make it to the ground in as safe a manner as possible? Does it fall in uncontrolled flight, or does it activate a failsafe technology and retain some form of control as it descends?
Is One In Three Thousand An Acceptable Crash Rate?
With customers demanding faster delivery and low or no-cost delivery the industry is scrambling to develop delivery systems that undercut competitors. The Swiss postal service — appropriately named Swiss Post — are testing autonomous multirotors operated by the drone delivery startup Matternet. The design includes a parachute that can be deployed to slow an uncontrolled fall. But spinning rotors and string-like items don’t mix well. In their testing, a failure occurred almost immediately following deployment when the line securing the parachute to the machine caught in a rotor and broke/
In 3000 flights they had experienced only one other failure, when a machine deployed its parachute successfully and descended into Lake Zurich. Swiss Post are now insisting on some upgrades to the system including two lines to tether the parachute instead of just one, and metal reinforcement for the parachute lines. It’s unclear whether Matternet have developed their own parachute system or whether they used one off-the-shelf, but the incident provides a rare public examination of the technology.
Save the Drone: Parachutes and Lost Propeller Operation
There are multiple manufacturers offering parachute systems for existing multirotors, either triggered through the machine’s firmware or through their own on-board sensors. Just one example is SkyCat, whose spring-loaded parachute canon is demonstrated in this video. It’s interesting to browse their various sites and observe the different marketing tacks for the different communities, for professional systems the emphasis is on safety when flying over crowds while for the enthusiast flier the emphasis is on protecting the valuable machine from damage.
Of course, a parachute is not the only game in town when it comes to multirotor safety. There are systems that can keep a multirotor flying without a propeller. Clever algorithms in the flight controller can detect when a motor fails or a propeller is lost, and use the remaining motors to induce a spin and bring the craft to ground with some stability.
Normally, multirotor aircraft rely on the force from one rotor to keep that side of the aircraft level. Losing one propeller upsets a delicate balance between all rotors, but equilibrium can be regained in an emergency situation by using extreme yaw so that the force of the remaining propellers is be averaged over all sides of the aircraft. A team at ETH Zurich pioneered this technique. It has been used to great effect by the drone show provider Verity Studios who have produced a series of videos showing it in action. Even when the machine is assaulted in flight with a piece of timber and loses a rotor it regains level flight under some control and is able to descend safely.
We Have To Take This Technology Seriously
At the professional level, these technologies are essential for craft flying in proximity to an audience. Unfortunately these systems are absent from the hobby side of things. We are unaware of any open source or otherwise free drone firmwares that contain something like the ETZ algorithms, and we can’t find any parachute failsafe systems that we might have featured in the past.
Perhaps this doesn’t matter for a casual user with a small drone and an eye to safe flying rules, but should it be something that we think about? Those serious in the hobby are operating thousands of dollars worth of equipment worth protecting from a sudden and unexpected meeting with the cold hard earth. And we exist as never before in a hostile environment where the drone is the subject of a questionable moral panic, and anything to make flight safer and mitigate fear can only be a good thing. As always the comments are open.