Jürgen Greil comments on the May 2023 edition of the Royal Aeronautical Society’s Aerospace magazine, Part 2
Since people are usually commuting by themselves and most inner urban and last mile deliveries are smaller than one cubic metre, lighter than 200 kg and below 50 km per leg, FlyNow is focusing on a modular family of aircraft consisting of a cargo, and a single and twin seater version.
All the derivates share the same drivetrain with a battery at the bottom of the cabin and coaxial rotor propellers, each driven by one of the two four times redundant electric motors positioned in one housing, mounted on top of the cabin forming a tilting head mechanism via a gimbal joint.
This leads to the simplest form of controlling the six degrees of freedom necessary for safe control of an aircraft, in a three-dimensional space. Due to the positioning of the passenger(s) or cargo and the packaging of all the components and modules, in such a way that the centre of gravity is below the drivetrain axis, this allows very simple cabin topology which avoids almost all bending moments.
This in return reduces the required power and reduces the size of the drivetrain. Besides low weight, accelerating a large amount of air at moderate speeds is always more efficient than accelerating a small mass of air at high speeds to achieve the same level of thrust. This can be observed in engine applications, which usually have engines with very large fan diameters to move large volumes of air at moderate cruising speed.
The same applies to propellers and helicopter rotors: the larger the diameter and the lower the speed and disc loading, the higher the efficiency and the lower the noise emissions.
FlyNow’s modular family of cargo and passenger VTOLs, which consists of a large low-frequency rotor assembly, is the most efficient, quietest and, due to its safety-relevant topology, the most reliable in this field.
In 2019, Bugatti, Lamborghini, Ferrari, and Bentley had a sales volume of around 26,000 units, while Mercedes, BMW and Audi together had more than 6.2 million units, which corresponds to a 1: 238 ratio.
To realise a meaningful air traffic over cities, take-off and landing hubs are required near or on the most frequented places or buildings, the size of which results on the one hand from the size of the eVTOLs and their ‘stackability’ on the ground as well as their noise emissions.
Noise cannot be higher than the background noise common in cities today. If a pilot is needed to fly an eVTOL then there is obviously no business case since the labour cost plays an important role, because if 3D mobility is to significantly replace ground-based traffic, the number of pilots will not be available.
This does not mean that it is necessary to fly autonomously, since the effort in terms of sensors and software is much too expensive and time-consuming and makes the certification process at least delayed, if not impossible.
The significantly lower energy density of batteries can not be compensated by high efficiency electric motors, so there is no way around a drastic reduction in weight.
Transporting urgent goods, medical and life saving equipment and other important things help to implement new means of transport, mitigates technical and financial risks, and proves the benefits to the common public.