propeller(redirected from Propellers)
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the most common type of ship propulsion device. It consists of a shaft and a bladed hub mounted on the shaft, with blades set with equal pitch and at some predetermined angle to the longitudinal axis of the shaft.
A distinction is made among propellers that are made in one piece, with blades that are cast or stamped together with the hub, with removable blades, and with rotating blades (so-called variable-pitch blades, whose pitch may be changed while the ship is in motion, resulting in a change in the speed and direction of the ship’s motion while maintaining constant direction and speed of rotation of the propeller).
The main geometric characteristics of propellers are the diameter D, which is twice the radial distance from the center of the shaft to the blade tip; the blade pitch H; the pitch ratio HID; the blade-area ratio, which is the ratio of total blade surface to the area of the disk of diameter D; the number of blades Z; and the shape of the cross sections of blades whose cylindrical surfaces are coaxial with the cylindrical surfaces of the propeller. The diameter of modern propellers ranges from 0.2–0.3 m in motorboats to as much as 10 m in large tankers. The pitch ratio is 0.4–2.0, and the blade-area ratio is 0.3–1.2. There are two to eight blades: propellers with three to five blades are the most common. Propellers are made from brass, bronze, cast iron, steel, and plastics.
The principle of operation of propellers is the same as that of airscrews. Modern methods of analysis are based on the vortex theory of N. E. Zhukovskii. However, the computations are complicated by the significantly greater width of blades and the development of cavitation. The propeller and the ship’s hull interact hydrodynamically; consequently, the power required to drive the propeller depends on the hull contours and the orientation of the propeller in relation to the hull.
REFERENCESLavrent’ev, V. M. Sudovye dvizhiteli. Moscow-Leningrad, 1949.
Basin, A. M., and I.Ia. Miniovich. Teoriia i raschet grebnykh vin-tov. Leningrad, 1963.
Katsman, F. M.. and G. M. Kudrevatyi. Konstruirovanie vinto-rulevykh kompleksov morskikh sudov. Leningrad, 1963.
I. IA. MINIOVICH
A hub-and-multiblade device for changing rotational power of an aircraft engine into thrust power for the purpose of propelling an aircraft through the air (see illustration). An air propeller operates in a relatively thin medium compared to a marine propeller, and is therefore characterized by a relatively large diameter and a fairly high rotational speed. It is usually mounted directly on the engine drive shaft in front of or behind the engine housing. See Propeller (marine craft)
Usually propellers have two, three, or four blades; for highspeed or high-powered airplanes, six or more blades are used. In some cases these propellers have an equal number of opposite rotating blades on the same shaft, and are known as dual-rotation propellers.
A propeller blade advances through the air along an approximate helical path which is the result of its forward and rotational velocity components. This action is similar to a screw being turned in a solid surface, except that in the case of the propeller a slippage occurs because air is a fluid. Because of the similarity to the action of a screw, a propeller is also known as an airscrew. To rotate the propeller blade, the engine exerts a torque force. This force is reacted on by the blade in terms of lift and drag force components produced by the blade sections in the opposite direction. As a result of the rational forces reacting on the air, a rotational velocity remains in the propeller wake with the same rotational direction as the propeller. This rotational velocity times the mass of the air is proportional to the power input. The sum of all the lift and drag components of the blade sections in the direction of flight are equal to the thrust produced. These forces react on the air, giving an axial velocity component opposite to the direction of flight. By the momentum theory, this velocity times the mass of the air going through the propeller is equal to the thrust.
A propeller blade must be designed to withstand very high centrifugal forces. The blade also must withstand the thrust force produced plus any vibratory forces generated, such as those due to uneven flow fields. To withstand the high stresses due to rotation, propeller blades have been made from a number of materials, including wood, aluminum, hollow steel, and plastic composites. The most common material used has been solid aluminum. However, the composite blade constructions are being used for new turboprop installations because of their very light weight and high strength characteristics.
For a small, low-power airplane, very simple, fixed-pitch, single-piece, two-blade propellers are used. The rotational speed of these propellers depends directly on the power input and forward speed of the airplane. Because of the fixed-blade angle of this type of propeller, it operates near peak efficiency only at one condition. To overcome the limitations of the simple fixed-pitch propeller, configurations that provide for variable blade angles are used. The blades of these propellers are retained in their hub so that they can be rotated about their centerline while the propeller rotates. For the normal range of operation, the blade angle varies from the low blade angle needed for takeoff to the high blade angle needed for the maximum speed of the airplane. See Aircraft propulsion, Airplane, Helicopter
Propeller (marine craft)
A component of a ship-propulsion power plant which converts engine torque into propulsive force or thrust, thus overcoming a ship's resistance to forward motion by creating a sternward accelerated column of water. Since 1860 the screw propeller has been the only propeller type used in ocean transport, mainly because of the evolution of the marine engine toward higher rotative speed.
The advantages of a screw propeller include light weight, flexibility of application, good efficiency at high rotative speed, and relative insensitivity to ship motion. The fundamental theory of screw propellers is applicable to all forms of marine propellers. In its present form a screw propeller consists of a streamlined hub attached outboard to a rotating engine shaft, on which are mounted two to seven blades. The blades are either solid with the hub, detachable, or movable. The screw propeller has the characteristic motion of a screw; it revolves about the axis along which it advances. The screw blades are approximately elliptical in outline.
One or more screw propellers are usually fitted as low as possible at the ship's stern to act as thrust-producing devices (see illustration). The low position of the propellers affords good protection and sufficient immersion during the pitching movements of the ship. The choice of the number of propellers to incorporate into a vessel design is based upon several factors. In general, a single-screw arrangement yields a higher propulsive efficiency than multiple screws, particularly when most of the propeller is operating in the boundary layer of the ship and can recover some of the energy loss. In addition, single-screw propulsion systems generally result in savings in machinery cost and weight in comparison to multiple-screw arrangements.
The formation and collapse of vapor-filled bubbles, or cavities, causes noise, vibration, and often rapid erosion of the propeller material, especially in fast, high-powered vessels. This phenomenon is known as cavitation. As long as the rotational and translational speeds of the propeller are not too high, the onset of cavitation can be delayed or limited to an acceptable amount by clever design of blade sections.
Supercavitating and superventilated propellers are designed to have fully developed blade cavities which spring from the leading edge of the blade, cover the entire back of the blade, and collapse well downstream of the blade trailing edge. The blade of such propellers has unique sections which usually are wedge-shaped with a sharp leading edge, blunt trailing edge, and concave face. Supercavitating propellers have cavities filled with water vapor and small amounts of gases dissolved in the fluid media. Superventilated propellers have cavities filled primarily with air from the water surface or gases other than water vapor from a gas supply system through the propeller shaft.
For ships which normally operate at widely varying speeds and propeller loadings (towboats, rescue vessels, trawlers, and ferryboats), the application of controllable-pitch (rotatable-blade) propellers permits the use of full engine power at rated rpm under all operational conditions, ensuring maximum thrust production, utmost flexibility, and maneuverability. Since these propellers are also reversible, they permit the use of nonre-versible machinery (gas turbines). See Marine engine, Marine machinery