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Looking at the drone industry today, there is an obvious trend towards multi-rotors being used for increasingly diverse types of workload. We also see how multi-rotors represent a significant share of the market.
Different types of drones – what is the difference?
Just to clarify the term drones, technically there is no difference between drones and/or multi-rotors, helicopters and fixed wings. That is because they are not mutually exclusive things, a drone could be in the form of a multi-rotor, helicopter or a fixed wing. A drone is just a popular term for any Unmanned Aerial Vehicle, UAV. So, by popular terminology, any vehicle that is controlled via a remote or by pre-programmed computer software is a drone.
It just happens to be so that the multi-rotor is what the average person thinks of when they hear the term drone. The stricter definition, since the term drone is more of a pop culture word, is UAV and UAS. An umanned aerial vehicle (UAV) is a remote or software piloted flying aircraft capable of reuse. The term UAS includes the drone and any ground control and remote systems that are used to control the flight of the vehicle, including software.
A fixed wing aircraft uses the engine(s) to keep it moving forward, and the airflow over the wings generate lift. The wings are often optimized to maximize lift at cruise speed. The engine(s) is/are there to overcome the drag of the airplane and maintain speed. This drag is both due to the form drag of the plane and the generation of lift from the wings. The latter will change depending on climb rate.
In a rotary aircraft, the rotors not only provide the lift to keep the aircraft in the air, but also the thrust to propel it forward. The rotor itself is moving at different speeds over the radius (same as with the fixed wing), but also different speeds through the air on the left and right side of the rotor in forward flight. Since there is only one optimum speed, most of the rotor is working at less than optimum speeds, lowering the lift-to-drag ratio overall.
Also, when a helicopter or multi-rotor enters a hover, it is using a lot of power to provide lift, while a fixed wing aircraft can enter a holding pattern while using much less power to offset the drag and maintaining lift.
Drones and energy-efficiency
Looking at the traditional aviation industry will provide some clues with regards to energy-efficiency. For instance, helicopters use more fuel than airplanes (everything else being equal).
When helicopters are traveling slow, they need more energy to hold their weight. When traveling fast (according to their standards), the rotor drag is very high. And while power required drops significantly as airspeed increases (once translational lift is gained, induced drag drops with speed), it is still far beyond that of a winged aircraft.
The multi-rotor, in addition to being much more complex to control and steer without the assistance of onboard software, has a design that is even less energy efficient. Moving larger amount of air at a lower speed is easier than moving less air at faster speeds. This means that helicopter blades can spin at a much lower speed than e.g. multi-rotor blades and still produce more lift.
Multi-rotor blades are generally smaller than helicopter blades and spin at higher speeds. They also require energy to slow the blades during aerial manoeuvres. Helicopters don’t have to exhaust energy slowing the blades as adjusting the pitch controls all aerial manoeuvres. Helicopters are currently capable of longer flight times on the same amount of energy versus a multi-rotor.
Conventional aircraft have much less drag at a given weight and speed than a helicopter, and far less so than a multi-rotor.
Merging the benefits of fixed-wing UAVs with the multi-rotor’s ability to hover is a new category of hybrids, which can also do Vertical Take Off and Landings (VTOL). There are various types under development, of which many configurations were tried for manned aircrafts, but they proved complex and difficult to fly. With the arrival of modern autopilots, accelerometers and gyros, solving the complexities, we now see more and more solutions emerging on the market. While more energy efficient than true rotary aircrafts, they still pose a compromise in comparison with fixed wing UAVs.
Then why is the multi-rotor so popular? We can see that it attracts hobbyists, as well as manufacturers, even though both history and physics prove it to be an inefficient way of flying. My guess is that one of the answers can be found in its simplicity. Developing a multi-rotor is far less complex than developing a fixed wing, which requires a deep understanding of aerodynamics, or a helicopter, which requires a significantly more complex mechanical solution. The multi-rotor is certainly much more complex to control and steer without the assistance of onboard software, but this software can today be found ready-made from numerous sources, and to achieve a functional platform is more a question of integration and tuning than actual development. Another aspect is that it can hover and do VTOL.
The question is also, what will need to change before we see a drone industry that align with the proportional use of fixed wing vs. rotary aircraft within manned aviation? Once we are allowed to start flying far Beyond Visual Line-of-Sight (BVLOS), and once we start looking at transporting goods or even people, the multi-rotor will no longer live up to the expected range / weight / energy consumption, at least not enough so to provide a viable solution. Physics / mechanics on such a basic level do not lie, neither do empirical testing over time.
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