# A Survey of the Effects of Vehicle Configuration on Urban Air Mobility +
## 2 Aerial Urban Vehicle Configurations and Concepts
### Hybrid
#### 2.3.1 Dual Systems  
A dual systems aircraft commonly uses a separate power system during each of the take-off and cruise phases. In each phase, one system is deactivated. Generally, these vehicles are comprised of a multirotor and fixed-wing configurations. Dual systems have a complex transition phase and they encounter challenges during the transition phase as the unnecessary use of a deactivated system in each phase imposes an extra burden on the system (in the form of extra dead weight) and reduces the efficiency [15]. A recent study reveals the higher energy consumption of dual-system aircraft in comparison to multi-rotor and tilt-wing configurations in short, medium, and long transportation. It seems that the efficiency of this configuration decreases for longer travels [74]. A recent study by [PERSON] and [PERSON] reveals the comparatively higher price and operational costs of dual systems in comparison with tilt-wing and tilt-rotor configurations [75]. One example is Ascendance Flight Technology's Atea. This aircraft uses the electrical multirotor configuration embedded in a fixed-wing configuration with two extra propulsion systems for horizontal flight. Atea has a range of about 400 km and is also suitable for regional flights. It is also designed to be green and quiet [76]. Wisk is another similar air taxi. It uses a single-rotor fixed-wing configuration along with twelve electrical rotors to provide vertical take-off and landing. It has a speed of 160 km/h and a range of 40 km [77]. The Hyundai Motor Company in cooperation with Uber, are also working on a similar dual-system electric hybrid air taxi. This air taxi is designed to have a speed of 290 km/h and a range of 100 km [78]. Tetra aviation's Mk-5 personal eVTOL is another example of dual system hybrid aircraft. It uses two wings equipped with 32 electric motors for vertical take-off and another horizontal rotor to help in cruise mode. This single-seat fully electric aircraft can travel up to 160 km with a maximum speed of 160 km/h [79]. Figure 15 illustrates some examples of the dual-system hybrid UAM solutions. Considering the studied topics in this section one can summarize the advantages and disadvantages of dual-system aircraft as in Table 9.  
The fault conditions in this configuration are combinations of the faults in the two configurations used for VTOL and HTOL modes. During VTOL mode, the most dangerous failure is a single or multiple propulsion failure. If the aircraft utilizes a DEP system (more \begin{table}
\begin{tabular}{c c} \hline \hline
**Advantages** & **Disadvantages** \\ \hline Stability & Low robustness \\ Hover flight without fuel consumption & High price \\ VTOL & Size \\ High cargo capacity & Low speed \\ High efficiency & Very complicated landing procedure \\ Low noise & Sensitivity to the wind conditions \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Advantages and disadvantages of lighter-than-air aircraft. than 6 or 8 propulsion units) and loses one or two propulsion units, the controller can handle the situation fully or partially depending on the number of rotors. If the controller fails to handle the situation by increasing the thrust and RPM of the other rotors, the aircraft can crash immediately. However, like multirotors, in the worst situation, stability and safe landing is expected especially if the cruise propulsion is still accessible. In transition mode, the pusher/puller propulsion increases the aircraft's speed until it reaches the cruise speed and uses the fixed-wing mode. If the pusher/puller propulsion fails, the aircraft can use its vertical propulsion to stabilize and control it and then take off immediately without any crash problems [80]. In cruise mode, if any propulsion, such as vertical or pusher/puller, fails, the aircraft can use its elevator, aileron, and rudder to stabilize it and use gliding mode for flight and landing. If the aircraft does not have any surface control, it can use a vertical propulsion unit to stabilize and glide and land like a conventional aircraft.