Selecting a drone for emergency response: airframe selection
An emergency service should avoid simply following the status quo of multi-rotors and really assess their drone-use cases before selecting their airframe. Ultimately, the choice of drone will dictate a drone team’s capabilities and the level of performance that can be achieved.
The drone model that an emergency service decides to purchase dictates what drone capabilities they have and the level of performance they can achieve, thus making drone selection an integral variable in establishing best practices.
The EENA / DJI Pilot Project Report highlighted the following as the minimum drone configuration for First Responders:
- Reliable platform with redundant systems, i.e. minimising single points of failure within the system
- Global Navigation Satellite Systems (e.g. GPS)
- Integrated camera systems, preferably modular, with live downstream capabilities in HD format
- Ability to fly in moderate winds and light rain
- Integrated SDK (Software Development Kit) so that specific apps can be written to help operators, for example, the DJI/DroneSAR Search and Rescue app, DroneDeploy for 2D and 3D mapping, and many more
Several organisations are claiming to offer ‘search-and-rescue drone options’, yet seem to be simply offering a thermal camera to be coupled with their consumer or commercial drone, without any or very little adaptation to the airframe. A ruggedised search-and-rescue (SAR) worthy payload has limited value if the airframe hasn’t been designed to the same standard.
An SAR drone needs to be more than just a consumer/commercial drone with a thermal camera attached to it; its SAR capabilities need to extend further than its payload. For this reason, this ‘Selecting a Drone for Emergency Response’ series of articles will analyse each main component of a drone system individually, starting with airframe selection.
Although the majority of emergency services seem to have converged on small multi-rotor drones as the default platform, other models should not be dismissed. The following sections will summarise and then discuss the two main categories of drones: fixed-wing (which look and act like a plane or jet) and multi-rotors (which act similar to a helicopter).
The main strengths of a fixed-wing drone:
- They generally fly at higher speeds than multi-rotors
- They are usually able to fly at higher altitudes
- They are a lot more energy efficient than a multi-rotor and thus will have a much longer flight duration (range)
The main weaknesses of a fixed-wing drone:
- Most are limited by the need to take-off and land laterally, so terrain will affect launch and recovery sites
- They cannot be used in restricted or congested areas because of their requirements to move through the air to generate lift
- Without a stabilised camera, video imagery from a fixed-wing drone can be difficult to interpret due to the constant airframe motion
- Fixed-wing drones are influenced by aerodynamics much more so than multi-rotors, thus attaching a payload that hasn’t been specifically designed for the drone can considerably disrupt its flight performance by changing its aerodynamic shape.
The main strengths of a multi-rotor drone:
- Multi-rotors, with three, four, six, eight, or more powered rotors, benefit from mechanical simplicity and redundancy in higher-number configurations
- They have the ability to launch from, manoeuvre in, and recover to very restricted terrain
- They can provide a steady video picture from a stationary, top-down perspective
- Aerodynamics influence a multi-rotor drone much less so than a fixed-wing model, thus retrofitting payloads to the drone can be achieved seamlessly, providing the aircraft’s centre of gravity and maximum take-off weight have been considered first.
The main weaknesses of a multi-rotor drone:
- Hovering requires more power than fixed-wing flight, so multi-rotor platforms will have shorter flight times, lower top speed and lower altitude limits than equivalently sized fixed-wing airframes.
Austin Fire Department’s Robotic Emergency Deployment (RED) Team and Wimberley Fire Department’s Unmanned Aircraft Team collaborated on a report, ‘Using Unmanned Aerial Systems During a Natural Disaster in Texas’, to reflect their lessons learnt from implementing drones at the 2015 Texas Memorial Day Floods, recently dubbed as a “millennial flood” due to its severity. Interestingly, a key recommendation that the report offered was: “Provide Incident Command with both fixed-wing and multi-rotor capabilities.” Both a fixed-wing and a multi-rotor drone were used to great effect throughout the Texas flooding event. The two report extracts quoted below indicate the rationale behind the selection of each drone based on the application it was to fulfil, along with the drawbacks to each selection.
“The Wimberley aviation unit found fixed-wing unmanned aircraft launching, recovery, and visual observation somewhat difficult due to high tree lines, rocky river bottoms, steep cliffs, vegetative undergrowth, and at times, a densely residential population. However, the range flown and time involved dictated this choice. The Spectra has long flight endurance and using its autopilot provides precise imagery over large areas (1.77” or 4.5cm resolution at + or – 10cm accuracy).” (Kessler et al., 2015)
“Multi-rotor aircraft was favoured at times over the fixed-wing for its vertical take-off and landing capability. Early in the flood response, a DJI Inspire quadcopter was used to collect imagery for the primary search. The ability to get on scene, take high-resolution imagery, and then move to the next location in minimal time made the Inspire a great tool for low altitude information gathering (100’-200’ above ground level). Multi-rotor platforms can provide a quick frame of reference and influence decision-making that might normally place responders in areas of danger.” (Kessler et al., 2015)
With these comments in mind, both types of airframe clearly offer great advantages to Emergency Service personnel, whilst also having separate performance limitations to consider. Hence, hybrid drones are rapidly emerging to combine the strengths of both fixed-wing and multi-rotor drones, thus reducing the inherent weaknesses of each. Hybrids usually have a vertical take-off and landing (VTOL) and then transition into fixed-wing flight after launching, combining a multi-rotor’s ease of launch and landing with a fixed-wing’s longer flight duration. It should be noted that, at present, hybrid drones are at a lower level of technology maturation and market penetration than solely fixed-wing or multi-rotor type airframes, and thus currently come at a greater cost when comparable in quality. That said, based on the pattern of most technologies, we can assume as hybrid airframes mature and grow in popularity, the cost will likely decrease over time.
Similarly, another approach that could merge the benefits of both drone categories is through the concept of “swarm” technology. Put simply, swarm technology gives a drone the ability to work in conjunction with multiple other drones in the same space by constantly knowing the location of all drones in the surrounding area, thus allowing the drones to avoid each other autonomously. To contextualise, swarm technology allowed Intel’s drone display team to fly over 1,200 drones above PyeongChang for the Opening Ceremony of the Winter Olympic Games; the illuminated drones worked together to cleverly form shapes, such as a snowboarder and the Olympic rings, across the night sky. Into the future, as swarm technology matures, it may be possible to simultaneously reap the benefits of operating both fixed-wing and multi-rotor drone models; an Emergency Service would have the ability to deploy both kinds into the same search area for different roles. A fixed-wing drone could be used to cover large search areas quickly, with the multi-rotors examining all significant finds detected by the fixed-wing drone. This technology is unfortunately not matured enough to be feasible at present, but the concept of swarm technology is receiving great interest from the research community, as well as the tech giant, Intel – which is currently pioneering this area of drone technology, so perhaps it could emerge as a viable option for the First Response sector in the not-so-distant future.
As the technology evolves, the weaknesses of both choices will reduce, and is already occurring – for example, the duration of a multi-rotor is constantly rising as battery technology improves – but their operational style will remain the same: fixed-wing drones are far better for wide-area operational environments, whereas multi-rotors are best for local-area operational environments. Realistically, an emergency service could need both. However, given emergency services’ clear preference towards small multi-rotor drones, one can assume the ability to hover and ease of launch/recovery must outweigh the requirement of long endurance. Having said that, generally speaking multi-rotors can be lower in cost than fixed-wings of comparable quality, which is likely to be a key deciding factor for an emergency service.
Additionally, the current legal parameter in the United Kingdom of 500m as a maximum flight distance, prevents drone operators from utilising the full benefits of a fixed-wing’s range – although there are exemptions in place for emergency services if there is immediate risk to life (see General Exemption E 4506). All of these factors – and more – have lead multi-rotors to be the popular ‘go-to’ choice. Nonetheless, an emergency service should avoid simply following the status quo of multi-rotors and should really assess their drone use cases before selecting their airframe; perhaps their local operational area may suit a fixed-wing drone instead.
1 Kessler, C., Robinson, G., 2015. Using Unmanned Aerial Systems During a Natural Disaster in Texas. Texas: Austin Fire Department – Robotic Emergency Deployment (RED) Team & Wimberley Fire Department – Unmanned Aircraft Operations.