# Surface pitch and blade section of the propeller ### surface pitch of the propeller

The blade surface of the propeller blade is a part of the helical surface (see Figure 1a), so the intersection of any cylindrical surface that is coaxial with the propeller and the blade surface is a segment of the helix, such as the B0C0 segment in Figure 1b. If the spiral segment B0C0 is extended around the axis, the axial distance between its two ends is equal to the pitch P of the spiral. If the blade surface of the propeller is a part of the helical surface of equal pitch, then P is called the surface pitch of the propeller. The ratio of the surface pitch P to the diameter D, P/D, is called the pitch ratio. After the cylindrical surface is developed into a plane, the pitch triangle is obtained as shown in Figure 1c.

Assuming that the radius of the above cylindrical surface is r, the length of the base of the pitch triangle after expansion is 2πr, and the angle θ between the node line and the base line is the pitch angle at the radius r, which can be determined according to the following formula:

The size of the pitch angle θ at a certain radius r of the propeller indicates the degree of inclination of the blade surface at that position. The pitch angles at different radii are unequal, and the smaller r is, the larger the pitch angle θ is. Figure 2a shows the situation where three coaxial cylindrical surfaces with different radii intersect with the blades of the equal-pitch propeller, and the expanded pitch triangle is shown in Figure 2b. Obviously, r1<r2<r3 and θ123.

If the surface pitch at each radius of the propeller blade surface is not equal, it is called a variable pitch propeller, and the expansion of the helix at different radii is shown in Figure 3. For this type of propeller, the surface pitch at a radius of 0.7R or 0.75R (R is the radius of the propeller tip) is often used to represent the pitch of the propeller. In order to indicate its measurement method, P0.7R or P0.75R can be recorded when abbreviated.