## 3 Results and Discussion
### Comparison of Aerodynamic Loads Acting on the SC and DC Blades  
Detailed results of the CFD simulations and experimental tests relating to the aerodynamic loads acting on the single blade of the wind rotor, with respect to the wind attack angle \(\gamma\), are presented in Figures 18-20. Some differences between experimental and simulation data may result from simplifying assumptions made for the numerical analysis. These included the following: not considering surface condition in the finite element model, which can change the location of airflow separation, and consequently affect aerodynamic loads; adopting a 2D model for CFD calculations; assuming wind speed as an average value from the wind profile, and assuming average turbulence intensity, which can cause differences in the maximum aerodynamic forces.  
Figure 16: Results of CFD analysis for the SC blade (\(\gamma=40^{\circ}\)): (**a**) distribution of velocity magnitude; (**b**) distribution of pressure.  
Figure 17: Results of CFD analysis for the SC blade (\(\gamma=40^{\circ}\)): (**a**) distribution of velocity magnitude; (**b**) distribution of pressure.  
Comparing the results of aerodynamic drag \(P_{x}\), shown in Figure 18, it can be noticed that for the range of the wind attack angles \(\gamma=0^{\circ}\div 90^{\circ}\) (range I-II), the dual coherent blade generated significantly higher drag than the single coherent blade. The higher drag is advantageous for this angle range to obtain the higher propelling torque.  
In turn, considering the wind attack angle \(\gamma=90^{\circ}\div 135^{\circ}\) (range III), the drag obtained for DC blade was lower and therefore disadvantages for that angle range compared to the drag obtained for SC blade, because the blade moved downwind. Concerning the wind attack angle \(\gamma=135^{\circ}\div 180^{\circ}\) (range IV), the DC blade provided low and almost constant value of the drag, which was lower than 5 N. Low drag values in this range are preferred because the blade returns upwind to its starting position.  
Comparing the results of the lift force \(P_{y}\), shown in Figure 18, it can be noticed, that higher values occurred both for single and dual coherent blades, when considering the wind angles of attack \(\gamma=0^{\circ}\div 90^{\circ}\) (range I-II). In accordance with the diagram presented in Figure 4a and related \(x\), \(y\) coordinate system, higher values of the lift force \(P_{y}\), for in the range I-II have a positive effect on increasing the propelling torque. Considering the values of the wind angles of attack \(\gamma=90^{\circ}\div 180^{\circ}\) (range III-IV), lower \(P_{y}\) values in the range from 0 to around -7 N were obtained for the DC blade. In contrast for the SC blade, the values of the lift force \(P_{y}\) ranged from about \(-15\) N to \(-10\) N and had a better effect on increasing the  
Figure 19: Lift force \(P_{y}\) depending on the angle of the wind attack \(\gamma\).  
Figure 20: Aerodynamic torque \(M_{xy}\) depending on the angle of the wind attack \(\gamma\).  
starting torque. The effect of higher lift force \(P_{y}\) is advantageous for generating propelling torque, especially when the blade moves upwind, and the drag force does not occur.  
The small values of aerodynamic torque \(M_{xy}\) were obtained throughout the whole I-IV range, as shown in Figure 20. Aerodynamic torque has a minor impact on the torque generated by the entire turbine. The main influence on the total propelling torque is caused by \(P_{x}\) and \(P_{y}\) aerodynamic forces. Aerodynamic torque \(M_{xy}\) is generated by airflow around the airfoils. Although the presence of aerodynamic torque is beneficial, its importance for the overall turbine performance is relatively low because the drag force effect dominates.