# A Survey of the Effects of Vehicle Configuration on Urban Air Mobility +
## 2 Aerial Urban Vehicle Configurations and Concepts
### Vertical Take-Off and Landing (VTOL)
#### 2.2.1 Helicopter  
Helicopters are a type of rotorcraft in which both lift and thrust are generated through one or more horizontally spinning rotors, each rotor composed of articulated blades controlled by some kind of swashplate mechanism. In helicopters, pitch and roll motions can be controlled by using a Swashplate. Usually, a fan-tail or propeller can be applied to overcome the reaction torque resulting from the rotation of the propeller. Moreover, to overcome the reaction torque resulting from the rotation of the propeller, usually, a tail with a rotor or a fan is utilized to generate a counteracting torque. Subsequently, the pitch angle of this rotor could be adjusted for yaw control. Like other VTOLs, helicopters have remarkable capabilities to perform vertical take-off, landing, and hovering flight in complex and unknown environments at low altitudes and speeds. Because of these advantages, helicopters have a variety of military and civil applications. These aircraft have the most complicated control systems due to their complex swash-plate mechanism, gyroscopic effects, and stabilizing tail-rotor [15; 40]. Table 5 summarizes the advantages and disadvantages of helicopters. For the time being and until other configurations see more development, helicopters can probably be seen as the most successful type of commercial vehicle in UAM, they are common in many countries and are being used for many applications including transportation, inspection, survey, agriculture, search and rescue, and delivery [15; 40]. Amongst many active companies in this field we can mention Alpine Helicopters Inc., Air MethodsCorporation, Bristow Helicopters Limited, Erickson Incorporated, Carson Helicopters Inc., Gulf Helicopters Company, Heli Air Limited, PHI Inc., Paramount Business Jets, and Abu Dhabi Aviation. Recently Uber, the famous transportation company, has also started to offer helicopter transportation. So far, it is only available in New York City and can take passengers to the JFK airport in eight minutes [41]. As with any other type of aircraft, faults in helicopters can happen in many different ways. Generally classified, the two more configuration-specific types of fault in helicopters are: actuator faults (where a malfunction in a part of an actuator results in partial or complete loss of that actuator's performance) and component faults which refer to entire components like the tail or part of the rotors being damaged. As mentioned before, helicopters have complex mechanical and control systems. Additionally, due to these complexities and the relatively smaller of this type of aircraft, component redundancy is limited meaning that a comparatively larger number of faults in the system have the potential to be catastrophic. As a result, the first order of business for this type of aircraft should be to make its subsystems prone to as few faults as possible. While partial components and actuator failures can be detected by and to some extent compensated for by fault tolerant control methods, complete actuator, or component failures in the absence of redundant parts cannot be dealt with using the methods that are available for some other configurations such as fixed-wing aircraft [42]. About a third of accidents in helicopters between 1963 and 2007 were because of a fault in tail rotor failure. The total number of accidents in this configuration is also considerable [43]. Conventional helicopters excel in hover and low-speed flight due to their reliance on the main rotor for both lift and thrust. However, as forward speed increases, the rotor's efficiency diminishes due to asymmetrical airflow. The advancing rotor blade experiences higher airspeeds, leading to potential compressibility issues, while the retreating blade faces lower airspeeds, risking stall [44]. These factors significantly limit the helicopter's high-speed performance. Consequently, conventional helicopters face significant limitations when attempting to achieve high speeds and long ranges.