# 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 B_{0}C_{0} segment in Figure 1b. If the spiral segment B_{0}C_{0} 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, r_{1}<r_{2}<_{r3 }and θ_{1}>θ_{2}>θ_{3}.

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, P_{0.7R} or P_{0.75R} can be recorded when abbreviated.

**B****lade section**

The section obtained by intersecting the cylindrical surface coaxial with the propeller and the blade is called the section of the blade, referred to as the blade section or blade section, as shown in Figure 1b. After the cylindrical surface is expanded into a plane, the shape of the blade section as shown in Figure 1c is obtained, which is similar to the wing section. Therefore, the method to characterize the geometric characteristics of the wing section can be used for the blade section, the shape of the blade section is usually a round-back section (a bow section) or an airfoil section, and there are also special fusiform and crescent sections, as shown in Figure 4. Generally speaking, the airfoil efficiency of the airfoil section is higher, but the cavitation performance is poor, and the bow section is the opposite. After the ordinary arcuate section is unfolded, the leaf surface is a straight line, the back of the leaf is a curve, and the thickest ends in the middle are quite pointed. The wing-shaped section has no certain shape after unfolding,

The leaf surface is roughly a straight line or curve, the back of the leaf is curved, the leading edge is blunt and the trailing edge is sharper, and its maximum thickness is close to the leading edge, about 25% to 40% of the chord length from the leading edge.

The chord length of the cut surface is generally divided into inner chord and outer chord. The straight line AB connecting the leading edge and the following edge of the cutting plane is called the inner chord, as shown in Figure 5, and the line segment BC shown in the figure is called the outer chord. For series-graph propellers, the outer chord is usually called the chord line, while for the theoretically designed propeller, the inner chord (nose tail line) is often the chord line, and the chord length and pitch are also defined according to the chord line taken. The chord length b shown in Figure 5 is a representation of the series of propellers.

The thickness of the tangent plane is expressed as the distance between the direction perpendicular to the chord line taken and the intersection of the tangent plane and the bottom. Its maximum thickness is called leaf thickness, and the ratio of t to the chord length b of the cut surface is called the relative thickness of the cut surface or the leaf thickness ratio δ=t/b. The midline or average line of the cut plane is called the arch line or the midline, and the maximum vertical distance from the arch line to the inner chord line is called the camber of the cut plane, expressed in f_{M}. The ratio of f_{M}< to the chord length b is called the camber ratio of the cut plane f=f_{M}/b (see Figure 5).