Drone impact from a safety perspective

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Drone impact from a safety perspective

On-the-job-reflection

After 15 years in marketing and sales I still marvel at the inner workings of the market, and with a background in engineering (Space & Physics) I see how different the market reacts from how my engineering pedigree tells me it should. While the transition from one viewpoint to another for me came naturally, I still every now and then revert to the viewpoint of a technician, a hardcore engineer. You know, the engineer’s logic regarding what makes a good drone versus what advertising can achieve just by impressing how cool something is?

The reason I took up with SmartPlanes, some 2 ½ years ago, was that they did not only build what I thought was a cool product, – it was also said to be the strongest and one of the most enduring fixed wing drones on the market. Still, while a good product enticed me to start working for SmartPlanes, it is a bit of a struggle to balance my natural sales & marketing aptitude with all those years of training as an engineer. Sales & marketing is all good and fine but the long-term strategy to maintain the position as the strongest fixed wing drone on the market is where it gets interesting. We are not as driven by market trends as many of my old employers were, it is more of a continuous attempt to secure that we bring the right values to our customers, e.g. operational availability.

Drone impact, the physics

There are mainly two aspects of physics that affect the outcome of a drone impact, the energy of motion (kinetic energy) and the conservation of energy.

The Swedish Transport Agency was early in defining regulations and classes for drone usage in Sweden, and the current regulation states “For Category 1A, the maximum starting weight of the aircraft shall be less than or equal to 1.5 kg and that the maximum kinetic energy shall be 150 J. If any of these parameters is exceeded, an application for Category 1B shall be made instead. To qualify for category 1B, the maximum starting weight of the aircraft shall be less than or equal to 7 kg and the maximum kinetic energy shall be 1000 J.”. It is interesting to notice that the Swedish Transport Agency choose kinetic energy as a definition when so many other regulatory bodies all around the world choose weight alone, and even if kinetic energy is a bit erroneous it was the smartest thing to do (and way more accurate then weight alone).

Kinetic energy, or the energy of motion, describes almost all of the energy that is available to affect an impact (there are other examples, like e.g. heat, but these are of less consequence) and is dependent on the speed of motion. However, kinetic energy does not equate to the impact energy. Why? Well, there is a small thing called the law of conservation of energy.

Before we go into explaining further we also have to introduce two other concepts, the elastic and non-elastic collision. From Wikipedia we can read that “An elastic collision is an encounter between two bodies in which the total kinetic energy of the two bodies after the encounter is equal to their total kinetic energy before the encounter. Perfectly elastic collisions occur only if there is no net conversion of kinetic energy into other forms (such as heat or noise) and therefore they do not normally occur in reality.”.

If we assume that a drone impact was to transfer all its kinetic energy into an object with which it impacts (elastic collision), we are at the same time assuming that there is no sound, heat, disintegration or crumpling. Moreover, we are assuming that the UAV stops dead at the point of impact, not spinning away as a whole or in pieces, since this would imply residual kinetic energy in the form of motional and rotational energy.

One conclusion from this is that whatever the impact energy may be, it is always going to be less than that of the drone’s kinetic energy.

Fixed wing vs. multirotor

There are, apart from the obvious, differences to consider. A rouge fixed wing drone, whether with or without running propeller, often does not pick up significant speed by diving straight down, and is instead often found gliding to the ground thanks to its inherent aerodynamical capabilities, while a rouge multirotor, especially with the propeller stopped, will drop straight down amassing significant kinetic energy. This energy depends on what altitude it was at when it turned rouge, as all its potential energy ( = mass * gravity * height ) is then converted to kinetic energy.

Injury mitigation

Taking all of the above into consideration, what can we as a manufacturer of drones do to mitigate adverse impact consequences when they happen, i.e. not including avoidance technology?

Here is where we introduce yet another concept, impact energy per area. It is very likely that only a partial area of the drone will transfer most of the initial energy, so the characteristics of that area becomes important. Just as a knife edge will inflict damage with very little force, a football using the same force is likely to be harmless.

With built in crumple zones, rounded and softened edges, and built in features that makes the drone break apart in a controlled manner, thus dispersing the energy, we can achieve a non-elastic collision. Removing any sharp objects, such as propellers, from the natural direction of movement, we can further mitigate adverse effects from an impact.

Regulation

We now see different regulatory bodies submitting or having submitted new rules for the usage of drones. There is a clear understanding that impact energy is one of the main considerations and that regulations will include rules aimed at mitigating adverse effects from potential impact situations.

The European Aviation Safety Agency (EASA) has published a proposal to regulate the operation of small drones in Europe, where the latest development is a Notice of proposed Amendment (NPA) regarding Unmanned aircraft system operations in the open and specific category and its Impact Assessment. All interested parties are welcome to comment this proposal from 12 May until 12 August 2017.

The FAA issued a Small Unmanned Aircraft Rule (Part 107) in June 2016. Since then the FAA’s own Center of Excellence has issued conclusions which may/should result in revisions to Part 107.

Final Report for the FAA UAS Center of Excellence Task A4: UAS Ground Collision Severity Evaluation Revision 2 was issued by the University of Alabama in Huntsville (UAH), the University of Kansas (KU), Mississippi State University (MSU) and Embry-Riddle Aeronautical University (ERAU). The CoE Task A4 concluded:

The A4 team has reviewed the available research and techniques used to address blunt force trauma, penetration injuries and laceration injuries that present the most significant threats to the non-participating public and crews operating sUAS platforms. The most significant of these characteristics related to ground collision severity are:

  1. The impact KE and impact orientation based upon a specific vehicle is the most significant metric for evaluating blunt force trauma injuries. Blunt force trauma is the most likely cause of fatalities due to UAS collisions for mUAS and sUAS with the exception of single rotor helicopters whose blade mass and blade speed present a lethal impact threat. Impact KE can be easily estimated and measured, based on vehicle velocity, during testing.
  2. The energy density parameter is the best metric for evaluating the possibility of penetration injuries caused by sharp edges or small impact areas in the vehicle design. This parameter is very challenging to measure during testing.
  3. Rotor diameter is the metric for severity of injury from rotors and propellers to define when blade guards or other protective measures are required to prevent laceration injuries (which is the most likely type of injury to occur). Single rotor helicopter configurations present a potentially lethal threat to the throat and head area due to the blade mass and speed of larger single rotor helicopters. Rotor diameter is easily measured.

Conclusion

Safety is an important consideration now and in the future, and while both European and American regulatory agencies currently have decided on a weight classification for UAVs they are simultaneously investigating sub categories based on impact energy. Impact energy not only being about weight and speed, but also structure and functionality, such as crumple zones etc. These subcategories will most likely be based on the Abbreviated Injury Scale (AIS) where an Expanded Polypropylene (EPP) fixed wing UAV is more likely to achieve a higher safety rating than that of a multi-rotor.

Roger Ohlund, CMO SmartPlanes

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