# Page:EB1922 - Volume 30.djvu/55

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AERONAUTICS

The Airscrew. The Rankine-Froude theorems on propulsion by the sternward projection of a stream of the surrounding fluid by the use of a screw-propeller, or other means, are well known. These state that the highest efficiency is attained by the projection of the greatest amount of fluid at the lowest speed, and indicate the use of propellers of the greatest practicable diameter. The only waste considered is the kinetic energy imparted to the fluid. An upper limit of efficiency is thus determined in terms of the diameter and the thrust of the propeller and the speed of motion. The design of marine screws proceeds mainly upon empirical lines based upon experience. The early airscrews were designed by a similar process of trial and error.

F. W. Lanchester (Aerodynamics, 1892), regarding the airscrew blade as a twisted aeroplane wing rotating about one tip as it ad- vances through the air, assumed that the total reaction may be ob- tained by integrating the forces which would act upon elements at successive radii if these were elements of a complete wing. This method of treatment, which was also advanced by Drzewiecki, has provided the basis of airscrew design. As first applied, the theory was incomplete, chiefly because it ignored the fact that the blades in following each other act on disturbed air. For example, if the number of blades be increased, the theory indicates no fall in the efficiency, and reactions directly proportional to the number of blades, which experiment showed to be untrue. Moreover, the effi- ciency so calculated might exceed that given by the Rankine-Froude theorems. It was therefore sought to combine the two aspects of the action of the airscrew in one theory, and the further theorem of Froude that the stream has reached half the final velocity at the propeller disc appeared to provide a means of estimating the degree of disturbance of the air in which the blade acts. It is generally agreed that the original theory is over-corrected by this modification. The blade element under consideration is itself partly causing the acceleration of the stream, and this acceleration is the total and not merely the initial disturbance of flow in the neighbourhood of the element. Figures for the reaction on the elements were obtained by testing a small wing of the same section in a wind produced artifi- cially in a " wind tunnel." This wing produces a disturbance of flow equivalent in an airscrew to an acceleration.

It was found in practice that the assumption of an arbitrary ac- celeration less than one-half of the final acceleration made it pos- sible by the use of the theory of Lanchester to predict the aerody- namic performance of an airscrew with a valuable accuracy. The com- bined theory leads to two important conclusions, completely verified by experience. Firstly, the efficiency increases with increasing ratio of the pitch at which the screw operates to its diameter up to an optimum value seldom employed in practice. Secondly, for given thrust and speed the diameter must be so large that it acts upon a sufficient mass of air per unit of time to attain a satisfactory effi- ciency. The latter brings the theory into conformity with the law of Rankine and Froude. The former in practice brings the airscrew designer into conflict with the designer of aeroplane motors. Higher crankshaft speeds are required to produce a light-weight internal-combustion engine than are demanded by this condition for high airscrew efficiency. This has resulted in a large number of aeroplane engines being arranged to drive the airscrew through a reduction gear. The point at which gearing becomes desirable in practice is not easily determined. It depends upon a number of factors. Among these are a small loss of energy in the gears, added weight and cost, various practical reasons for dispensing with addi- tional mechanism if this is not of sufficient value and the adverse effects of the greater torque of the slower running airscrew upon the control of the aeroplane, which must be offset against the gain in airscrew efficiency. In this question is also involved the considera- tion of the strength of the airscrew to resist the stresses due to ro- tation. This imposes a limit upon diameter, decreasing as the speed of rotation is increased, which may result in a further reduction of efficiency for the high-speed airscrew.

During the war large aeroplanes were built for which single en- gines of the required power were not available. In so far as two en- gines were sufficient, these were placed on either side of the main body of the aeroplane, each driving a separate airscrew. It became necessary ultimately to install four engines in a few aeroplanes and these were placed in pairs driving two pairs of airscrews in tandem. The design of the rear propeller in this arrangement involves an estimate of the rate at which air is supplied to it by the screw in front. With the same limitation of diameter the efficiency of pro- pulsion attainable is approximately the same as if the two engines were coupled and drove a single airscrew of the same diameter, but is less than would be obtained by the use of four separate systems of propulsion. The tandem system is preferred for reasons of com- pactness and the difficulties of control attendant upon the use of a number of lines of thrust.

The aeroplane propeller, unlike the propeller of ship or airship, is required to transmit the full power of the engine at different speeds of flight, both when the craft is flying level at full speed, and when it is flying slow in order to climb. The airscrew cannot be designed to discharge both functions in the most efficient manner possible in each case. This was of little consequence in the early days of flight when the range of flying speed was small ; but as the range was in- creased, some attention was paid to the design of airscrews of vari-

able pitch. These have been experimented with, notably at the Royal Aircraft Establishment, with some success; but they have not been used so far in service. If any device for preventing the loss of engine power with increasing height by an initial compression of the charge to ground-level density should come into use, the variable airscrew would become necessary. Such devices are, however, still in an experimental stage.