## 1 Introduction  
The wind energy required to power wind turbines is currently considered a renewable energy source that is completely free and inexhaustible. It is widely available, therefore delivering it to wind turbines does not require arranging a supply chain, as is the case with fossil fuels. The generation of wind energy does not release harmful carbon dioxide, sulfur oxides, or other pollutants into the atmosphere. It is a pure and cheap source of energy that, thanks to its virtually global reach, helps remote or hard-to-reach populations to access electricity. Nowadays, the share of renewable energy in the fuel market, especially wind and solar energies, continues to grow strongly. According to statistical data [1], renewable energy accounted for 13% of the total global power generation in 2022. On the other hand, in 2021 it grew by 17% and accounted for more than half of the increase in global electricity production over the past two years. Comparing the contribution of wind power capacity to the overall electric power produced in a country or region, the same report referred to 2021 indicates the following shares: 47.8% in Asia Pacific with 39.9% in China, 28.2% in Europe, 18.8% in total North America, and 13.1% in the rest of the world. Wind turbines with horizontal axis (HAWTs) [2, 3] are currently the most popular wind turbine design. Rotors with a horizontal axis convert the kinetic energy of the wind into mechanical work, using part of the power of the air stream to drive various types of machines and devices. If the powered machine is an electric generator, then the wind turbine or several wind turbines grouped together are called a wind power plant. Nowadays, the most technologically advanced wind turbines are those with a nacelle containing a rotor consisting of blades mounted radially in a hub, a gearbox and a shaft housed on bearings and connected to a generator. The nacelle is mounted pivotally on a tower or mast, along with a device to control the speed of the rotor, as well as a self-adjusting system for wind direction. The blades set on the shaft drive the wind turbine generator, and the gearbox regulates the speed of the generator depending on the speed of the rotor, which movement is forced by the wind force. Therefore, the efficiency of the rotor is mainly related to the shape of the blades [4]. The most common solution of HAWTs, for both high, medium, and low power, is the three-bladed rotor design with airfoil-shaped cross sections, as shown in Figure 1a. This shape provides the rotor blades with adequate aerodynamic properties and is directly related to the lifting force that is created when the pressure on the lower part of the airfoil is greater than on the upper part. In general, the wind rotors' aerodynamic efficiency is mainly affected by the shape of the blades and the speed ratio \(Z\) given by the following equation:  
\[Z=\frac{\omega R}{W} \tag{1}\]  
where \(\omega\) stands for the rotor angular velocity [rad/s], \(R\) is the rotor radius [m], and \(W\) stands for the wind speed [m/s]. HAWTs are high-speed turbines, which require the use of additional mechanisms to protect against reaching critical rotor speed. For example, the rotor axis or the blades change the angular position depending on the wind speed to reduce the drag force. The three-blade HAWT achieves the best use of the energy of the air stream, making this type the most popular [5]. The rapid development and great success of wind turbines with horizontal axis in the wind power industry has also contributed to the increasing development of wind turbines with vertical axis (VAWTs) [6]. An example of the VAWT turbine is shown in Figure 1b. Despite higher wind power utilization rate, wind turbines with vertical axis represent a minor share of installations in operation today. The reason for less interest in this type of structure is mainly the load occurring during rotor rotation. The vertical axis of the rotor is subjected to periodic bending that changes over time.