When developing performance schedules, aircraft manufacturers make certain assumptions about the operating conditions of the helicopter and the pilot`s capabilities. It is believed that the helicopter is in good working order and the engine is able to develop its rated power. The pilot is expected to perform normal operations and have average flight capabilities [2]. If the angle of attack is reduced, the total lift decreases and the helicopter begins to sink (Fig. 10). The autorotation force in forward flight is generated in exactly the same pattern as when the helicopter descends vertically into the still air. However, due to forward speed, there is a loss of axial symmetry in the induced velocity and angles of attack above the rotor disc. This tends to change the distribution of the parts of the rotor disc that consume energy and absorb energy, as shown in Figure 16b. A small section near the root undergoes reverse flux; As a result, the size of the driven area on the retractable side is reduced [1].

This expression shows that the induced speed is explicitly dependent on the T/A disk load, which is an important parameter in helicopter design. This lateral displacement, in turn, must be corrected by the main rotor. The airflow, also known as downward washing, is lightly guided against the direction of travel, so the helicopter is kept in a stable hover. The main rotor of a helicopter interacts with the surrounding airflow to manipulate the main aerodynamic forces in the following ways: Perhaps the biggest advantage of helicopters is that they can both hover in one place and fly forward. The transition from hovering to forward flight is an extremely complex process from an aerodynamic and mechanical point of view. For simplicity, treat the rotor like a disc instead of examining aerodynamic conditions on single-rotor blades. In forward flight, as in an airplane, rolling motion is strongly associated with yaw movement. These two types of movement are therefore called a single “lateral” movement of the helicopter. The lateral movement of a single-rotor helicopter during forward flight is usually periodically stable.

While in low-speed modes, the relationship between roll and yaw movement is still weak and rolling motion, such as hovering, is unstable, the lateral movement of a single-rotor helicopter is unstable. In addition, when calculating the required power of the helicopter, it is also necessary to calculate the required power of the tail rotor. The power required by the tail rotor typically ranges from 3% to 5% of the main rotor power in normal flight and up to 20% of the main rotor power at the ends of the flight area [1]. It is calculated in a similar way to the power of the main rotor, with the thrust required set to the value required to compensate for the moment response of the main rotor to the hull. The use of vertical tail surfaces to generate lateral force in forward flight can help reduce the proportion of power required for the tail rotor, but at the cost of some increase in parasitic and induced resistance. The correlator is a device that connects the collective lever and the accelerator of the engine. When the collective lever increases, the power automatically increases and when it decreases, the power decreases. The correlator keeps the speed close to the desired value, but requires additional fine-tuning of the accelerator. The controller is a sensing device that senses the speed of the rotor and motor and makes the necessary adjustments to keep the rotor speed constant. In normal operation, once the rotor speed is set, the governor keeps the speed constant and no throttle adjustment is required.

The regulator is a typical device used in turbine helicopters and is also used in some helicopters equipped with piston engines [2]. Since the rotor blades have a relatively wide angle of attack due to the collective slope in forward flight and generate a corresponding high air resistance, the rotor speed drops rapidly without power. As a result, the necessary elevator is also lost, and the helicopter will soon crash. The helicopter belongs to the category of flying machines with the highest operational efficiency, as it does not require special take-off and landing sites with expensive logistics and supply equipment. For short and medium radii, the flight efficiency of helicopters is comparable to that of aircraft. It is capable of gliding, laterally, backward, forward and performing other desirable manoeuvres in the civil field such as sea and mountain rescue, police surveillance and firefighting; or military missions such as battlefield surveillance, troop transport, assault and anti-tank operations. So far, more than a million people have been rescued using helicopters. In recent years, the results of scientific research in many aviation disciplines have led to a sharp increase in the flight dynamics, control, navigation and lift capacity of helicopters. Figure 13 shows the net power required for a particular helicopter in direct flight. Due to the forward inclination, the air is no longer accelerated vertically downwards, but backwards (Fig. 16). As a result, the helicopter begins to move forward.

However, since the lift no longer acts vertically upwards, the pilot slightly increased the power during the booster phase in order to obtain the correct lift-to-weight ratio. Before talking about helicopter aerodynamics, we must first introduce some basic principles of aerodynamics. The most common helicopter configuration consists of a main rotor and a tail rotor at the rear of the fuselage (Figure 2a). A tandem rotor helicopter has two main rotors; one forward fuselage and one rear (Figure 2b). This type of configuration does not require a tail rotor because the main rotors are counter-rotating. It was proposed by Serbian Dragoljub Ivanovich in 1953. Parasitic performance, PP, is a loss of power resulting from viscous shear effects and flow separation (pressure resistance) on the hull, rotor hub, etc. Since helicopter fuselages are much less aerodynamic than their fixed-wing counterparts (equal weight), this source of resistance can be very important [1].

Parasitic power can be written as 1917: Stephan Petroczy (Austrian) built and flew a helicopter with a coaxial rotor; In this chapter, an analysis was performed to define the performance of the helicopter. It has been shown that performance characteristics can be derived using simple models as theories of momentum and blade elements. The influence of weight and height on the power required and the available power was presented. The effects of ground operation on the helicopter`s performance are discussed. The case when the engine stops in flight and the main rotor performs an autorotation is also presented. Some elemental analyses of stability properties were performed. The influence of the different parts of the helicopter on stability was taken into account. Finally, it was shown how the helicopter can be controlled.

If the angle of attack of all the blades of the collective pitch rotor is increased at the same time, the downflow through the rotor disc increases, more lift is generated, and the helicopter begins to rise vertically (Fig. 9).