Propeller

I

Introduction

Propeller, mechanical device that produces a force, or thrust, along the axis of rotation when rotated in a fluid (gas or liquid). Propellers may operate in either air or water, although a propeller designed for efficient operation in one of these media would be extremely inefficient in the other. Virtually all ships are equipped with propellers, and until the development of jet propulsion, virtually all aircraft, except gliders, were also propelled in the same way. Even now, the turbofan engine uses a special form of propeller, mounted in a duct. A propeller acts as a windmill when placed in a wind current.

The propeller is essentially a screw that, when turned, pulls itself through the air or water in the same way that a bolt pulls itself through a nut. Marine propellers are frequently termed screws, and aircraft propellers were once termed airscrews in Britain. Typical propellers consist of two, three, or four blades, each of which is a section of a helix, which is the geometric form of a screw thread. The distance that a propeller or propeller blade will move forwards when the propeller shaft is given one complete rotation, if there is no slippage, is called the geometric pitch; this corresponds to the pitch, or the distance between adjacent threads, of a simple screw. The distance that the propeller actually moves through the air or water in one rotation is called the effective pitch, and the difference between effective and geometric pitch is called slip. In general, an efficient propeller slips little, and the effective pitch, when operating under design conditions, is almost equal to the geometric pitch; the criterion of propeller efficiency is not slip, however, but the ratio of propulsive energy produced to energy consumed in rotating the propeller shaft. Aircraft propellers are often operated at efficiencies approaching 90 per cent, but marine propellers operate at lower efficiencies.

II

Aircraft Propellers

An aircraft propeller blade has a cross section that is aerodynamically similar to that of a wing, and, when driven through the air, creates lift and drag, perpendicular and parallel to the air velocity relative to a section of the blade (see Aerodynamics; Aeroplane). The forces created by the motion of the propeller can be resolved into components. One, thrust, is in the direction of flight. The other component, in the plane of rotation, represents the force that must be overcome by the torque, or turning force, of the driving engine. The complete motion of a blade element involves a combination of the forward velocity represented by the flight speed, and the peripheral velocity due to the rotation of the blade.

This simple “blade-element” concept of propeller action has been extensively refined by aerodynamicists in recent years. Another method of analysis of propeller action is based on the changes in momentum of the flow as it passes through the propeller disc. This approach was originally used by the British engineer and naval architect William Froude, but in general it is not as comprehensive as the blade-element theory.

For a given rotational speed, the resultant velocity at a blade element increases in magnitude as the forward speed is increased, while at the same time the angle of the resultant velocity vector with the plane of rotation is also increased. Thus, if the blade has a fixed pitch, a condition will eventually be reached at which the blade will produce little or no thrust. On the other hand, as the forward speed is decreased, the angle between the velocity vector and the blade will become so large as to cause the blade to stall, with a severe corresponding drop in the blade's efficiency.

In order to adapt a given propeller to aircraft with different flight characteristics, adjustable-pitch propellers are usually used, in which the blade can be rotated in the hub so as to alter the effective pitch. With a variable-pitch propeller the pitch or blade angle is controllable in flight so as to maintain operating conditions very close to the optimum. Propellers of this type are often operated at a constant rotational speed by means of either a hydraulic or electrical governing mechanism. Controllable-pitch propellers are usually capable of being feathered, that is, the blade angle can be set parallel to the flight direction, so as to prevent windmilling that could otherwise occur in the event of an engine failure. The capability of setting the blade with a negative pitch may also be included in the design so as to provide reverse thrust and aerodynamic braking action during landing.

Modern propeller blades may be made of solid aluminium alloy, of hollow steel, or of reinforced plastic materials. The propellers may be equipped with de-icing equipment. The propeller must be very precisely balanced, both statically and dynamically. If, for example, a 57-g (2-oz) weight were attached to the middle of one blade of a two-bladed propeller, and a 28.5-g (1-oz) weight were attached to the tip of the other blade, the propeller would be in static balance, that is, it would not rotate if the propeller shaft were placed on knife edges with the blades in any position; it would not, however, be in dynamic balance, and would vibrate if rotated at high speed.

The rotor of an autogiro or helicopter is essentially similar to an ordinary aircraft propeller in that it consists of several blades, each shaped like an aerofoil in cross section, and produces lift. The blades are not always twisted, but, like ordinary aircraft propeller blades, their pitch may be varied.

Experimental aircraft have used “unducted fan” propellers, which have a shape approaching that of ship propellers.

III

Ship Propellers

A ship propeller operates in much the same way as the aeroplane propeller. In the ship propeller, however, each blade is very broad (from leading to trailing edge) and very thin. The blades are usually built of copper alloys to resist corrosion. The speed of sound in water is much higher than the speed in air, and because of the high frictional resistance of water, the top speed never approaches the speed of sound. Although efficiencies as high as 77 per cent have been achieved with experimental propellers, most ship propellers operate at efficiencies of about 56 per cent. Clearance is also less of a problem on ship propellers, although the diameter and position of the propeller are limited by the loss in efficiency if the propeller blades come anywhere near the surface of the water. The principal problem of ship-propeller design and operation is cavitation, the formation of a vacuum along parts of the propeller blade, which leads to excessive slip, loss of efficiency, and pitting of the blades. It also causes excessive underwater noise, a serious disadvantage on submarines.

See also Fluid Mechanics.