# A Survey of the Effects of Vehicle Configuration on Urban Air Mobility +
## 2 Aerial Urban Vehicle Configurations and Concepts
### Hybrid
#### 2.3.4 Tilt-Rotor  
Tilt-rotor aircraft are similar to multirotors in the takeoff phase, as reviewed in the previous sections; after the transition phase, they fly like fixed-wings in the cruise phase. In both main phases of flight, the rotors produce the required propulsion, and control surfaces are used to control the aircraft. The first prototypes of this configuration were built and flown by the likes of the Bell Helicopter Company and the Transcendental Aircraft Corporation in the 1950s. Nowadays, tilt-rotor aircraft are popular among designers because of their flight efficiency, stability, and simple structure compared to other hybrid configurations. However, tilt-rotors still have some complexities and challenges in their design [88]. There are few rotors and engines which are suitable for both vertical and horizontal flight which causes some limitations for tilt-rotor aircraft. These aircraft can be easily equipped with ducted or coaxial rotors. It is also easy to combine different multirotor configurations with conventional fixed-wing or flying-wing configurations to easily benefit from each configuration's advantages [15]. A recent study reveals the efficiency of the tilt-rotor configuration in comparison to other hybrid configurations. It is shown to have less price and operational expenses amongst hybrid configurations [75].  
One example in this category is Kitty hawk HVSD eVTOL aircraft which is a tilt-rotor hybrid aircraft which is designed to be quiet enough for urban applications. It is a lightweight, one-passenger aircraft with eight tilting rotors which has a range of about 90 km [89]. Another example of this configuration is the Lilium Jet, an all-electric vertical take-off and landing jet. Four different generations of Lilium prototypes have been designed and tested to provide up to seven-seater air taxi solutions. Lilium is a canard, tilt-rotor aircraft which employs a tilting set of ducted engines to provide the required propulsion. Along with fixed wings and canards, the engines are aerodynamically optimized to increase efficiency. Using up to 36 ducted fans, Lilium has a maximum range of 250 km. Using ducted fans provides it with a high payload capacity at a low footprint as well as a low noise level. This configuration makes the cruise flight highly efficient for Lilium. In order to increase the efficiency of the aircraft, a variable nozzle system is designed for this aircraft [90]. Figure 18 illustrates the tilting mechanism of the Lilium Jet in take-off and cruise modes. Other examples of tilt-rotor UAM have also been developed by JobyAviation [91], KittyHawk [89], Aston Martin [92] and Archer [93]. A summarization of the advantages and disadvantages of tilt-rotor configuration considering the studied topics in this section is provided in Table 12.  
This type of aircraft has tilt actuators, motors, and propellers that may experience failures. The most common failures are related to the motor and propeller, which can cause the aircraft to rotate in yaw, pitch, or roll. However, if the aircraft has a DEP system, the controller can adjust the thrust and RPM of other propulsion units to maintain stability and safety. Another common failure is damage to the actuators of the tilt mechanism. If the aircraft has a DEP system and separate actuators for each propulsion unit, the controller can disable the failed unit and actuator, and use the remaining ones to control the aircraft by adjusting thrust, RPM, and propulsion unit angle. In cruise mode, the controller may also use the elevator and aileron, if there are any, to stabilize and control the aircraft fo a safe landing. The worst-case scenario is when all propulsion units or actuators on one side fail, or the wing or canard becomes broken. In such a situation, the controller may be unable to stabilize the aircraft, and it may fall to the ground and crash [87; 94].