Let us first analyze the situation when the plate is vertical to the direction of the airflow, as shown in Figure 2. At this time, the plate receives the greatest resistance and the lift force is zero. When the plate is stationary, the resistance is large but does not work on the plate; when the plate moves under the resistance, the airflow works on the plate; if the direction of the plate motion is the same as the airflow, the airflow is zero relative to the plate speed, then the resistance is zero, the airflow did not work on the plate. In general, a flat plate subjected to resistance movement can obtain a large power when the velocity is 20% to 50% of the air flow velocity, and the resistance type wind turbine works by utilizing the resistance of the blade.

Below is an animation of the lift force and resistance generated by the airflow.

Aircraft and kite can rise to the air by relying on lift force, and lift force-type wind turbine rely on the lift force of the blades.

(4) Maximum thickness and maximum thickness position

The maximum distance of the upper and lower airfoil on the y coordinate is called the maximum thickness of the airfoil, referred to as the thickness, and is represented by t; the x coordinate of the maximum thickness point is called the maximum thickness position, which is represented by xt.

The angle between the tangent to the upper and lower arcs of the trailing edge of the airfoil is called the trailing edge angle and is represented by τ.

The curvature f of the symmetrical airfoil is 0, t1 = t2, and the upper and lower surfaces are symmetrical.

The working airfoil is subjected to the suction pressure of the lower airflow and the suction of the upper airflow. These forces can be expressed by a resultant force, which is the pressure center of the airfoil chord (the line connecting the leading edge and the trailing edge of the airfoil).

The asymmetric airfoil is operated at a non-stall state. At a large angle of attack, the pressure center is at a 1/4 blade chord position from the leading edge of the blade. At a small angle of attack, the pressure center moves backward along the chord length of the blade.

Where ρ - fluid density; V - characteristic velocity in the flow field; L - characteristic length; μ - viscosity of the fluid.
The viscosity of the fluid mainly changes with temperature, and the viscosity of the air increases with the increase of the temperature; while the liquid is opposite, the viscosity increases with decreasing temperature.  
Define ν as the kinematic viscosity of the fluid, ν = μ / ρ Then Re = vL / ν
Since the density ρ of the air decreases as the temperature rises, and the viscosity μ of the air increases as the temperature rises, the Reynolds number Re decreases significantly as the temperature rises.
When studying the aerodynamic characteristics of the airfoil, V takes the speed of the airfoil, and L takes the chord length of the airfoil to obtain the Reynolds number of the airfoil.
The Reynolds number has a great influence on the aerodynamic characteristics of the airfoil. The stall angle of attack of the general airfoil increases with the increase of the Reynolds number, and the maximum lift force coefficient also increases with the increase of the stall angle of attack. The drag coefficient will generally decrease. 

Figure 10 - The angle of attack exceeds the stall angle of attack and stalls

The critical angle at which the transition occurs is called the critical angle of attack or the stall angle of attack. The stall angle of attack for different airfoil stalls is also different. The normal airfoil is usually between 10 and 15 degrees. Generally, the thin airfoil stalls have a slightly smaller stall angle of attack. The type of stall angle of attack is larger; for the same airfoil, stall angle of attack of the stall is the Reynolds number and the smoothness of the fin when the fin is running.
For thin flat plate, the stall angle of attack is small, and the resistance is slightly larger. If the angle of attack is slightly larger, it will stall.

Wind energy and wind-power utilization coefficient

Since it is kinetic energy per second, E is also the power, called wind power. For example, an air flow with a wind speed of 6 m/s has a power of 129.6 W through 1 square meter.
When the wind passes through the wind rotor, it pushes the wind rotor to rotate, and transforms its kinetic energy into the energy of the wind rotor rotation. However, after the wind rotor works, the wind speed will not be zero, only the reduction, so the wind can only transfer part of the energy to Wind turbine, if the wind power flowing through the sweeping area of the wind turbine blade is E, and the power obtained by the wind turbine is set to P, the wind-power utilization coefficient is Cp

The wind-power utilization coefficient is also called the power coefficient. For more information about the wind-power utilization coefficient, see the Wind Energy and Wind Power chapters.

Wind energy is the velocity energy of air movement. When the wind passes through the wind rotor, it pushes the wind rotor to rotate, transforming its velocity energy into the energy of the wind rotor rotation. However, after the wind turbine wind rotor works, the wind speed will not be zero, only decrease. Therefore, the wind only transfers part of the energy to the wind rotor.

Then how much energy can the wind transfer to the wind rotor? In 1927, the German Bates theoretically calculated the maximum value of 59.3%. If the wind speed in front of the wind rotor is v, the wind speed through the wind rotor is calculated to be 2v/3. The wind speed away from the position by the wind rotor is 1v/3.

59.3% is called the Bates limit, which is the maximum value of wind-power utilization coefficient of wind turbine. At present, the wind-power utilization coefficient of high performance wind turbine is generally 40% to 45%.

The wind speed felt by the wind turbine blade is the resultant speed of the actual wind speed and the blade moving speed, which is called the relative wind speed. Figure 11 is a blade section of a wind turbine. When the blade moves, the relative wind speed experienced by the blade is w→, which is the resultant vector of the linear velocity (vector) u→ of the blade and the velocity (vector) v→ before the wind enters the rotor.

The angle between the relative wind speed and the blade string is the angle of attack α of the blade, see Figure 12. 

The ratio of the tip speed of the wind rotor blade to the wind speed is called the tip speed ratio. Figure 13 is the rotor of wind turbine, u is the tangential speed of the outer diameter of the rotating wind turbine, v is the speed before the wind enters the rotor, v is vertical to the plane of the wind rotor, and the tip speed ratio is λ

The tip speed ratio of the resistance wind turbine is generally 0.3 to 0.6, and the tip speed ratio of the lift force type wind turbine is generally 3 to 8.

In the lift force type wind turbine, the tip speed ratio directly reflects the angle between the relative wind speed and the direction of the blade movement, which is directly related to the angle of attack of the blade, and is an important parameter for analyzing the performance of the wind turbine.  

The ratio of the total area of the wind turbine blade to the area of the wind passing through the wind rotor (the swept area of the wind rotor) is called the solidity ratio (volume ratio), and is a reference data for the wind turbine.

Figure 14 is the horizontal axis wind rotor, S is the projected area of each blade against the wind, B is the number of blades, R is the radius of the wind rotor, and σ is the solidity ratio.

The multi-blade wind turbine has a high solid ratio and is suitable for wind turbine with low wind speed and low speed and high torque, and its efficiency is low. The low solidity ratio wind turbine with small and narrow blades is more used, which can be operated at a higher speed and has higher efficiency.

For more information on the solidity of wind rotor, see the chapter " Rotor Solidity " and the " The Solidity of the Vertical Axis Wind Turbine ".