# 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
The rotor system, while essential for vertical lift and hover, becomes increasingly inefficient at higher speeds due to increased drag and power requirements. By reducing the rotor's lift contribution and eliminating its propulsive role, these limitations can be alleviated. To overcome these challenges, designers have turned to compounding. This involves incorporating a fixed wing to generate lift (lift compounding) and auxiliary propulsion systems (thrust compounding) to augment the rotor's thrust. Thrust compounding involves adding a propulsive device, such as a propeller or jet engine, to supplement the thrust generated by the main rotor of a helicopter. This configuration reduces rotor speed at higher speeds of forward flight, mitigating compressibility effects and improving overall efficiency. Additionally, thrust compounding enhances maneuverability and decreases the pitch attitude needed during acceleration, making compound helicopters versatile aircraft capable of both vertical take-off and landing as well as high-speed cruise. However, the added propulsor increases the aircraft's structural weight and system complexity [44]. Lift compounding enhances helicopter performance by adding a wing to share the lifting burden with the main rotor. \begin{table}
\begin{tabular}{c c} \hline \hline
**Advantages** & **Disadvantages** \\ \hline No need for a runway (VTOL) & Complex structure \\ Scalability & High sound emission \\ Hover capability & Vulnerability to faults \\ Operational environment flexibility & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Advantages and disadvantages of helicopters. This offloads the rotor, allowing it to focus more on propulsion, leading to increased speed and efficiency. As a result, compound helicopters can achieve higher cruise speeds while maintaining the hover capabilities of traditional helicopters, expanding their mission range and reducing fuel consumption. However, the addition of a wing increases structural weight, hover power demand, and system complexity [44]. A full compounding helicopter maximizes performance by combining lift and thrust compounding. Lift compounding offloads the main rotor with wings or similar lifting surfaces, while thrust compounding adds propellers or other propulsion systems to augment forward speed. This synergistic approach significantly enhances a helicopter's speed, range, and payload capacity, overcoming limitations faced by conventional helicopter designs. The Piasecki 16H-1 Pathfinder, shown in Figure 9a, is a prime example of a full compound helicopter. This innovative aircraft combines the best of both worlds: the vertical lift capabilities of a helicopter and the speed and maneuverability of a fixed-wing aircraft. To achieve this, engineers added a wing to reduce the load on the main rotor during forward flight and incorporated a unique \"Ring-Tail\" ducted propeller at the tail to provide thrust for high-speed operation. This design effectively addressed the limitations of traditional helicopters, resulting in a more efficient and versatile aircraft. Another example of a full compound helicopter is the Eurocopter X3 (X-Cubed), shown in Figure 9b. Developed by Airbus Helicopters, the X3 was a developmental, high-speed compound helicopter designed to demonstrate the feasibility of a helicopter exceeding 250 miles per hour. The X3 combined a traditional main rotor with two side-mounted propeller units to achieve higher speed. It is important to state that compound helicopters and gyrodynes may be mistakenly categorized together. While these two differ in their rotor power systems, a compound helicopter employs a continuously powered rotor for lift while additional lift and thrust are generated by fixed wings or propellers. In contrast, a gyrodyne's rotor is only powered for takeoff, landing, and low-speed flight. During the cruise, the rotor becomes unpowered and relies on airflow for rotation, similar to an autogyro. In this phase, the lift is shared between the autorotating rotor and the fixed wings.