By Lauren Nagel
Published: 13-09-2021, Last updated: 16-11-2021
In 2020, there is a profound sense that we are reaching the future, the drone revolution, that has been anticipated for years. Prototypes are being tested that will see packages delivered by drone, humans flown across cities in eVTOL aircraft, and life made easier by swift and sustainable aerial vehicles.
In this article, we will explore:
- Drone definitions
- Fixed wing, rotary and powered-lift drones
- Power sources such as gas, electric and hybrid
- Types of drones such as helicopters, quadcopters, eVTOL, and more
1. Drone Definitions
The term “drone” has become standard for referring to any aerial vehicle controlled remotely or by onboard computers. The benefit of such a general term is that it makes it easy to discuss the industry without getting too technical, great for news, basic marketing, and explaining your interests to older relatives. The downside to such generality is that it can lead to confusion, as the average consumer likely pictures a modestly sized quadcopter when they hear the term, leading to a narrow understanding of the industry. Undoubtedly, the industry boasts a great and ever-growing diversity of aerial vehicles, one of the key points of this article. For clarification’s sake, here is a breakdown of common terms used in the drone industry.
Figure 1: Mavic Pro drone in action (Photo by Pedro Henrique Santos)
A ‘Drone’ is defined by the Merriam Webster dictionary as “An unmanned aircraft or ship guided by remote control or onboard computers”. It is used interchangeably with ‘Unmanned Aerial Vehicle’ (UAV), which shares the same definition. Saying the aircraft is ‘unmanned’ strictly refers to the fact that the pilot is not on board, as passenger-carrying drones are already proving quite feasible. An additional point to note is that ‘drone’ and ‘uav’ generally refer only to the aircraft itself, whereas the terms described below include all components that make the drone fly.
The International Civil Aviation Organization (ICAO), an institution dedicated to international air transport policy, employs the terms ‘Unmanned Aerial System (UAS)’ and ‘Remotely-Piloted Aircraft System’ (RPAS). UAS is defined as “an aircraft and its associated elements which are operated with no pilot on board”. The RPAS definition is more detailed and refers to “a remotely piloted aircraft, its associated remote pilot station(s), the required command and control links and any other components as specified in the type design”. The main difference between the two, according to the ICAO, is that in practice, UAS must receive special airspace accommodations and be kept away from other aircraft, whereas RPAS can be integrated into airspace alongside manned aircraft. The technical difference between the two is that RPAS can only be piloted from a remote pilot station (RPS) whereas UAS may be piloted from an RPS or a ground control station (GCS).
2. Fixed Wing vs. Rotary vs. Powered-lift Drones
Similar to manned aircraft, drones can be categorized based on the structure of their lift-producing surfaces.
Figure 2: Albatross fixed wing UAV
First off, as the name suggests,fixed wing drones have wings that do not move, they are bound to the body of the drone. They may have control surfaces that turn and rotate, such as ailerons and rudder, but the wings themselves are fixed. Figure 2 shows one such example, the Albatross UAV, which has just one forward thrusting propeller located behind the fuselage and wings. In this system, lift is generated by the forward thrust of the aircraft coupled with the aerodynamic shape of the wing. Pros of fixed wing drones are that their aerodynamic shape allows them to remain airborne for long periods of time, so they can cover large areas and are energy efficient. This is ideal for long-term missions that require the drone to be airborne for days or hours, such as surveillance and climate monitoring. The main cons are the steep learning curve for operators, the ample space required for take-off and landing, and the fact that they cannot hover in place.
Forrotary-wing drones, the rotor-blades / rotor-wings rotate around a central mast, forcing air downwards and creating the vertical lift required for the aircraft to become airborne. All vertical take-off and landing (VTOL) aircraft fall into this category, including small and large helicopters and multicopters. Rotary-wing drones may have a single rotor or even up to 8 rotors generating thrust. Pros of rotary-wing drones are that they are simpler to operate initially and can hover in place, allowing them to fulfill a wide variety of roles. Basic models are also relatively inexpensive, so small rotary drones are a great place to start for those wanting to get into the industry. The main cons relate to their short flight time, as generating upwardand forward thrust requires a lot of energy, thus restricting the range and endurance of the vehicle.
Further reading: Comparing UAV Power Systems
Powered-lift drones occupy a fascinating middle ground between fixed wing and rotary drones, using elements of both to complete a flight. Rotors are used for VTOL-style take-offs and landings, then once airborne, the aircraft transitions to forward, fixed-wing-style propulsion. As such, they benefit from the low space requirements of rotary drones, while also having the improved aerodynamics and efficiency of fixed-wings. There are several types of powered-lift drones, namely tiltrotors, tiltwings, and drones employing perpendicular sets of rotors. For tiltrotor drones, the rotors themselves rotate in order to transition from vertical to horizontal thrust during flight. The Skywalker X 8 VTOL is a great example, with three rotors seamlessly transitioning between copter and fixed wing mode. Slightly different in design, tiltwing aircraft undergo a more dramatic in-flight transition as the rotors are fixed to the wings themselves, so the entire wing undergoes a rotation during the conversion from lift-off to forward flight. Tiltrotors and tiltwings are also referred to as convertiplanes as they are converted from one configuration to another.
The third type describes drones that use perpendicular sets of rotors for lift-off and forward flight, neither set rotating or changing position. In this scenario, one set of rotors is operated only for take-off and landing while the perpendicular set is used only during portions of the flight requiring forward thrust. While seeming to make the best of both worlds, powered-lift drones are quite complicated to design, which explains why they are not more common. Designing a system that can transition between configurations while remaining stable is challenging, though once achieved, the result is a highly versatile aircraft.
3. Electric, Gas-powered, and Hybrid Drones
The majority of rotary drones on the market today are fueled by electric power from lithium-ion polymer (LiPo) batteries. LiPo batteries are rechargeable and lightweight, thus ideal from a design perspective. For example, Autel Robotics’ EVO Drone weighs only 1.9 lbs and has one of the longest flight times on the market for its type. Solar power has also been employed as a source of electricity, with solar cells mounted on the upper surface of the drone. This concept could turn the industry on its head as it would effectively eliminate flight time limitations as long as the sun keeps shining. That said, solar cells require significant surface area and have almost exclusively been used with fixed-wing models. One solar-powered quadcopter was built by an engineering team at the University of Singapore, but the design was composed of little more than the solar cells and rotors themselves.
Further reading: Electric Motor Manufacturers for eVTOL and Aviation
Figure 4: VA001 diesel-powered drone (Photo: vanillaunmanned.com)
At the other end of the spectrum, gas-powered drones have become major players for carrying out long-distance missions. While electric motors are more efficient than combustion motors, gasoline has about 50x the energy density of LiPo batteries, so maintains the energy advantage. There are several possible fuel options for gas-powered drones, such as unleaded gasoline, two-stroke motor oil, nitro-based gasoline and diesel. Most gas-powered drones are fixed wing since they can more easily accommodate larger engines. Their aerodynamic shape produces so much natural lift that little gas is needed to achieve flight times of many hours or even days. On a record breaking flight, Vanilla Unmanned’s diesel-powered VA001 drone not only stayed airborne for five full days, but when it landed, they found it had an additional three days worth of fuel on board. Thus far, gas motors have been uncommon in rotary drones as a heavier motor and fuel system is required, requiring a larger drone overall. Furthermore, they can be noisy and require ignition of the motor to start. That said, some models are in development, such as the Goliath quadcopter, which uses a single gas-powered motor to power four propellers.
A third category of drones has emerged that boasts not one, but two power-sources. Hybrid-powered drones are changing the game and can come in different configurations, such as gas/battery-powered or battery/solar-powered. The performance of these vehicles is unparalleled and they have been breaking all kinds of records with both rotary and fixed-wing aircraft. Skyfront’s Perimeter 8 octocopter is a great example, using electronic fuel injection to convert gasoline into electricity in-flight. The novel system provides the aircraft with a record 5 hours of no-load hovering flight time, 10 times longer than the best battery-powered drones on the market. Also to come is proof of concept for the PHASA-35, a high altitude long endurance (HALE) hybrid drone advertised to have a flight time of up to one year in the stratosphere. The drone operates on solar power during the day and battery power at night, with a 35 m wingspan to provide ample lift. The drone’s developers believe the drone could take over many of the current functions of satellites, such as delivery of telecommunications networks, weather monitoring, and surveillance.
Two other power sources to mention are hydrogen fuel cells and laser-recharging of batteries in-flight. Hydrogen fuel is a clean and energy-dense power source while laser recharging could enable drones to stay airborne indefinitely. Both concepts are currently experimental.
4. Types of Drones
Now that we’ve covered the ways that drones can be classified, let’s look at some specific types of drones available.
Figure 5: DJI Phantom 3 quadcopter equipped with a camera
Quadcopters, Hexacopters and Octocopters
Multi-rotor drones are perhaps the most recognizable form of UAV, complete with a compact body, multiple arms, and high RPM propellers. They come in a variety of sizes and are named for the number of rotors they have (quad = 4 rotors, hexa = 6, octo = 8). These drones can also take on a single or coaxial rotor configuration, which describes the layout of the rotors. Single rotor configurations have just one rotor in a given vertical section whereas coaxial configurations have two, either completely or partially overlapping. Many drones are piloted by visual line of sight (VLOS), whereby the pilot has the vehicle in view for the duration of the flight, and controls it based on observation. Other drones are piloted via first-person view or FPV, whereby the cameras on the drone deliver footage to a set of FPV goggles or a monitor so the pilot can fly the drone remotely from an onboard perspective.
Multi-rotor drones are a great way to get started in the industry as they are fairly easy to learn to fly, come in relatively inexpensive models, and can perform a wide range of tasks. That is not to say these drones are reserved for beginners, as top drone pilots spend years mastering complex aerobatic maneuvers and racing skills. One of the fastest racing quadcopters available, the Walkera F210, can perform precise maneuvers and reach speeds in excess of 50mph. No matter your needs, there truly is something out there for everyone.
Further reading: Drone Design Calculations and Assumptions
A once niche segment of the drone industry, demand for helicopter drones is growing, with the market projected to be worth $11 billion USD by 2027. Operating on the same principle as manned helicopters, heli drones use a single or coaxial rotor to generate vertical lift, partnered with a tail rotor to counter torque. Like multirotor drones, a perk of helicopters is that they can take-off and land virtually anywhere. That said, drone helicopters are unique in that many are powered by gas, except for very small models that are powered by batteries. There are exceptions to this rule, of course, such as AV’s VAPOR helicopter, which is all electric but manages a payload of up to 10 lbs.
A significant societal benefit of helicopter drones is their potential to replace humans in dangerous missions. Manned helicopters play a big role in operations such as search and rescue, disaster relief, fire fighting support, etc., jobs that involve flying in challenging terrain with potentially poor flight conditions. Unmanned helicopters could change this by delivering supplies to inaccessible areas, flying in smoky or windy conditions, and even evacuating humans from hazardous situations. All this could be accomplished without having to send a human crew into the field. Designed precisely for this purpose, Laflamme Aero’s LX300 helicopter can fly in severe weather and has a payload of up to 90 kg, more than enough to carry the average adult. With these specs, it won’t be long before helicopter drones become widely used for saving lives.
The electric VTOL aircraft (eVTOL) category encompasses all electrically powered VTOL vehicles, but for the purposes of this article, we will focus on a large and highly competitive segment of the industry: urban air mobility (UAM). This new mode of transportation has the potential to reduce transportation-related emissions and cut down on commute times for a fair segment of the population. As such, there is an ongoing race to see who can design and manufacture a safe and efficient eVTOL vehicle for transporting people around urban areas, with companies such as Bell, Embraer and Overair leading the pack. Uber Air has worked with several of these companies to design their eVTOL fleet, which they plan to launch as early as 2023 in cities like Dallas, Los Angeles and Melbourne. If all goes well, many major cities could have eVTOL taxi programs by the end of the decade.
Figure 7: Bell Nexus 4EX eVTOL aircraft
Within the eVTOL category there are purely electric vehicles and also hybrid eVTOL vehicles, which most commonly use a combination of battery power and gas power. Hybrid aircraft are important players in the UAM movement as battery technology is still evolving to support extended flights independently. The Bell Nexus 4EX for example is built to be able to run on battery power or hybrid gas-electric power, offering flexibility and greater flight time between charges.
That said, a major driver in the industry is the mandate to offer more sustainable transportation solutions, so many companies are still focusing their development efforts on going fully electric. eVTOL vehicles may become competitive with hybrids sooner than expected, as aircraft like EHang’s AAV (autonomous aerial vehicle) boast flight times in excess of 35 minutes with payloads supporting up to four people. eVTOL vehicles can also be hybrid in nature in that they are designed for pilot-optional flight, meaning they can be operated manually onboard, piloted remotely, or pre-programmed for a flight. This versatility will come in handy as society gets used to the idea of inner-city travel via drone, appreciating a certain amount of redundancy.
High Altitude Long Endurance (HALE)
HALE drones are aptly named for their purpose of flying high altitude, long endurance missions. To give you an idea of what this means, ‘high altitude’ drones are expected to fly above 60,000 ft and ‘long endurance’ flights are expected to be days, months or even years long. Many HALE drones are intended for military use, designed for long-term surveillance, intelligence and reconnaissance operations. Others may be used as sub-orbital or pseudo-satellites, performing functions such as weather monitoring and telecommunications network delivery.
Figure 8: RQ-4 Global Hawk (Photo from the U.S. Air Force website)
The most widely used HALE aircraft to date is perhaps the Northrop Grumman RQ-4 Global Hawk, powered by a Rolls-Royce AE 3007 turbofan engine and most notably used by the U.S. Air Force (USAF). Capable of flying up to 60,000 ft for over 32 hours, it has mainly been employed for the military purposes described above. Another step up is Boeing’s hydrogen fuel-powered Phantom Eye drone, which has the ability to fly up to 65,000 ft and stay at altitude for four days. Currently carrying an experimental status granted by the USAF, the aircraft is not commercially used but can fly beyond protected airspace in order to complete testing. A number of other aircraft in development are advertised as having the ability to fly for months or years at a time, ideal for weather monitoring and telecommunications delivery. No such aircraft are in operation yet, but several companies claim to be getting close using solar-based technology, such as AeroVironment’s high altitude platform station (HAPS) and Airbus’s Zephyr UAV.
We hope that this article has provided you with a greater understanding of the diversity of drones available and the impressive magnitude of the industry. The field still faces many challenges such as obtaining government certifications, overcoming design limitations, and building trust with consumers. With much development underway, it won’t be long before these aircraft become bigger and better, offering a more permanent service to society.
Further reading: The Future of Wind Tunnel Testing for Drones