## 1 Introduction  
In recent years, the demanding nature of civilian and military applications has brought significant attention to autonomous vehicles. Moreover, to provide a solution to distinct applications which cannot be handled using land or water, flying vehicles are the only choice. Furthermore, applications that require hovering and vertical take-off further limit solutions for autonomous vehicles and rotary flying vehicles, such as helicopters and quadrotors [1]. The quadrotor vehicle has more advantages compared with the helicopter, such that the presence of four rotors allows for load distribution and enables the quadrotor to have smaller rotor diameters, and hence smaller vehicle size, as compared with the single rotor helicopter. Furthermore, the smaller rotors store less kinetic energy per rotor and thus reduce the extent of damage in the event that the vehicle is involved in a crash. In addition, rotors can be protected by enclosing them within a frame. Unmanned quadrotors, with the capability of handling suspended payloads with a cable, can be used in situations where normal land vehicles either cannot reach or are not recommended to be used. Applications such as tracking moving targets, photographing for surveillance, rescue operations for natural disaster areas, discovering mines, or inspecting gas and oil pipelines that exist in remote or hazardous areas are better operated with aerial vehicles [2]. A vehicle with a suspended load system can be treated as a multi-body dynamical system. The pendulums motion of a suspended payload with a cable can limit the applicability by either slowing down the flying vehicle or can damage the load itself. Moreover, the pendulum oscillation may prevent an accurate alignment during the pickup and/or placement of the load. In reality, a violent suspended payload swing due to a wind gust or pilot input can cause an accident [3]. It is thus critical to limit the payload swing to improve safety and performance [4]. In recent years, the delivery of packages has become an attractive application for multi-rotor vehicles, especially quadrotors. Many companies started offering this service, such as Amazon [5]. The payload can be attached to the quadrotor with a rigid connection or it can be suspended underneath the quadrotor by a cable. The rigid connection is the most common because of its practicality and simplicity. However, it is not suitable for heavy and large size packages, and in some situations, it may not be possible to land the vehicle. Moreover, with this configuration, the inertia of the quadrotor will increase significantly, which will reduce its maneuverability [6]. With a suspended load, large packages can be carried with one or more quadrotors. This configuration allows delivering the package without the necessity to land the vehicle, which improves the mission efficiency. However, it will complicate the dynamics and the performance of the quadrotor, especially when controlling the position of the payload in windy scenarios. Therefore, many researchers have recently started to develop control systems for the quadrotor with suspended load to stabilize the vehicle and suppress the oscillations of the load, alongside attaining accurate pickup and positioning of the load. It is expensive and time-consuming to develop and test algorithms for autonomous vehicles in the actual world. Quadrotor simulations must be conducted prior to the UAV's actual flight to limit the danger of property damage caused by aircraft crashes, system failure, or controller malfunction. Different simulations, such as numerical simulations, software-in-the-loop simulation (SIL), hardware-in-the-loop (HIL) simulation, and actual tests on testbeds, can be used to avoid these problems from arising; moreover, these methods make it possible to test and build multirotor vehicles safely and cost-effectively.