Working Principle Of Marine Propeller

The propeller can be considered as a wing that rotates and moves forward. The air flow through each section of the blade is composed of the forward speed along the axis of rotation and the tangential speed generated by rotation. Take a minimum section at the propeller radius r1 and r2 (r1 When the air flows through each section of the blade, it generates aerodynamic force and resistance Δ D and lift Δ 50. The total aerodynamic force after synthesis is Δ R。 Δ R The component force along the flight direction is the pull force Δ T. The force opposite to the rotation direction of the propeller Δ P prevents the propeller from turning. Add the pull force of each segment on the whole blade and the force to prevent rotation to form the pull force of the propeller and the torque to prevent rotation of the propeller [2].
It is necessary to make each section of the propeller work at the angle of attack with large lift-drag ratio in order to obtain larger pull force and smaller drag moment, which means higher efficiency. When the propeller is working. The axial velocity does not change with the radius, while the tangential velocity changes with the radius. Therefore, the air flow angle is small near the blade tip and at the larger radius, and the corresponding blade angle should also be small. However, close to the blade root, where the radius is smaller, the air flow angle is larger, and the corresponding blade angle should also be larger. The blade angle of the propeller should gradually increase from the tip to the root according to a certain rule. So it is more accurate to say that the propeller is a twisted wing.
The airflow angle actually reflects the ratio of forward speed and tangent speed. For a certain section of a propeller, the angle of attack of the section changes with the ratio. As the angle of attack changes, the pull force and drag moment also change. The inlet ratio "J" is used to reflect the airflow angle at the propeller tip, J=V/nD. Where D - propeller diameter. Theory and test prove that the propeller pull force (T), the power (P) and efficiency required to overcome the propeller resistance moment（ η) It can be calculated by the following formula:

η= J·Ct/Cp
Where: Ct - tensile coefficient; Cp - power factor; ρ- Air density; N - propeller speed; D - propeller diameter. Among them, Ct and Cp depend on the geometric parameters of the propeller, and their values vary with J for each propeller. The characteristic curve shows the relationship of the propeller's pull coefficient, power coefficient and efficiency with the advance ratio. It is one of the main bases for designing and selecting propellers and calculating aircraft performance.