Propeller

Propeller

• The propeller is like a rotating fan.   When the blades rotate, their inclined surfaces interact with water or air, pushing the water or air backwards.   According to Newton's third law, water or air will give the propeller an equal and opposite reaction force, which is the thrust that pushes the ship or aircraft forward.

• The propeller is constructed by a group of airfoils, and the working principle is similar to that of the wing.   The wing relies on the geometric changes of the airfoil and the angle of attack of the inflow to make the fluid flowing through the upper and lower parts of the airfoil have different speeds.   According to Bernoulli's principle, different speeds will cause different pressures on the upper and lower surfaces of the airfoil, thus generating lift.   The component of the airfoil lift in the forward direction is the thrust of the propeller.

Main types

• Fixed pitch propeller: The structure is relatively simple, and the blades are generally three or four, and up to seven.   It has the highest efficiency at a specific speed, but the pitch is fixed and cannot be adjusted according to the driving conditions.   The efficiency may be affected under different sailing conditions.   It is suitable for ships with high requirements for speed and stability.

• Adjustable pitch propeller: referred to as adjustable pitch propeller, which can adjust the pitch by rotating the blades through mechanical or hydraulic manipulation of the mechanism in the hub.   It can give full play to the main engine power and improve the propulsion efficiency.   The main engine rotation direction can be kept unchanged when the ship is reversing.   It has good adaptability to the change of blade load and is mostly used in tugboats, fishing boats and general transport ships.

• Duct propeller: A circular duct with a wing-shaped cross-section is installed on the outer edge of an ordinary propeller.   The duct can improve the propulsion efficiency, but the reversing performance is poor.   The fixed duct propeller will increase the turning diameter of the ship.   The rotatable duct can improve the turning performance of the ship and is mostly used for pushing ships.

• Tandem propeller: Two or three ordinary propellers are installed on the same shaft and rotate in the same direction at the same speed.   When the propeller diameter is limited, the blade area can be increased to absorb more power, which is beneficial for vibration reduction or avoiding cavitation, but the weight is large, the propeller shaft is long, and the layout and installation are difficult, so it is less used.

• Contra-rotating propeller: Two ordinary propellers are installed one after the other on the concentric inner and outer shafts, and rotate in opposite directions at the same speed.   It can reduce the rotational energy loss of the propeller wake, improve propulsion efficiency, and enhance the maneuverability of the ship, but the structure is complex and requires high installation and maintenance.

Geometric parameters

• Diameter: It is the core size of the propeller, which is related to the required power, rated speed, flight envelope, etc.

• Number of blades: It has a great impact on efficiency and weight.   Adding blades will increase the tip loss, but it can also reduce the blade load, so that the power absorbed by each blade decreases, and the change in aerodynamic efficiency varies with the overall propeller disk load.

• Twist distribution: In order to make the propeller work in the g-efficiency angle of attack range in each section, the torsion angle needs to be optimized according to the incoming flow conditions.   Propellers flying at high speeds need to be designed with a larger total torsion, while those flying at low speeds have a smaller total torsion.

• Airfoil: The airfoil shape of the blade section is a key aerodynamic design parameter, which determines the lift-to-drag ratio when the airflow flows around the propeller and directly affects the efficiency.

Material selection

• Metal materials: Common ones are copper alloys, aluminum alloys, and stainless steel.   Copper alloys have good corrosion resistance, strength and toughness;   aluminum alloys are light and high in strength, which can reduce the overall weight of aircraft or ships;   stainless steel has high strength and strong corrosion resistance, and is suitable for harsh environments.

• Composite materials: such as carbon fiber reinforced composite materials, which have high specific strength, high specific modulus, good fatigue resistance and corrosion resistance, and can be customized according to design requirements, but the cost is relatively high.