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Aviation History
1933
1933 - 1285.PDF
FLIGHT, DECEMBER 21, 1933 (3) Correspondingly, in case of emergency, a lower grade of fuel can be used, with any given compression ratio, than with the poppet-valve engine. (4) Lower fuel and oil consumptions have been obtained than with the corresponding poppet-valve type. (5) Lower cost of manufacture than with the corre sponding -poppet-valve type. (6) Greater scope for large production with unskilled labour. (7) Materials are more easily obtained. <8) Lower maintenance costs. .. (9) It is more able to stand ground boosting, and more easily adaptable to higher rates of supercharge, than with the corresponding poppet-valve engine. (10) Cooling and cowling problems are simpler. (11) Deleterious effects from leaded fuels would appear to be* less apparent than with the poppet-valve engine. (12) Lower exhaust ring temperatures, combined with sleeve-valve operation, minimise fire risk. It is suggested that there is an important future for this type of valve operation for the air-cooled aero engine, and the complete absence of normal maintenance work makes it attractive for civil work. Obtaining the Maximum Thrust Efficiency from Available Brake Horse-Power It has been claimed for some time past that an improved take-off and cruising range could be obtained from existing aircraft power plant if some means of altering the propeller shaft gear ratio or changing the pitch of the propeller itself, could be effected at will by the pilot, while in flight. I realise that this problem does not strictly come within the purview of this paper, because it is aerodynamic rather than thermodynamic, but I propose touching on the mechanical side, as I hold the view that, if adequate reasons exist for either or both of these changes being introduced into the power transmission section of an air- Fig. 22 : Two views of the Bristol " Perseus " sleeve-valve engine. craft, they have been far too long coming into fruition, and that one of the most important direc tions in which they can be speeded up is by the aircraft de signer instructing the engine maker as to his requirements aero- dynamically, and then letting the engine maker endeavour to develop the device from a mechanical standpoint. The above remarks apply more aptly to Europe than to America, where a combination of aircraft and engine has been developed, which enables the efficient use of the controllable pitch propeller to be made, and where this device has actually been brought to fruition on the lines which I suggest are the only practical solution of the problem. The problem can be divided into two classes—(a) multi- speed gear, and (b) variable pitch propeller. (a) While, at one lime, I had hoped that the multi-speed gear would be worth while on such types of heavily loaded aircraft as flying boats, etc., where take-off is extremely critical, even though it might not be the ultimate solu tion, an investigation into the possible improvements in performance, with a multi-speed gear and supercharged engine, showed a disappointing increase in thrust horse power for take-off, and different gear ratios would be required for varying installations. (b) Propellers whose pitch may be altered while in flight may be divided into four main types, and, omitting the blades, it would appear, in the present state of develop ment, that the weight of the hub, complete with its con trollable mechanism, for an engine of 600 to 700 h.p. is of the order of 100 lb. The four main types are as follows: — (1) Automatic Type. In this type no pilot's control is provided, and the pitch of the blades may depend on either the speed or power transmitted, or on a combination of both. (2) Manual Controllable Type. This type is one in which movement of the pilot's con trol causes continuous alteration of the pitch. (3) Governed Pitch Type. This consists of a controllable pitch airscrew fitted with a suitable follow up mechanism. It enables the pilot's control to be graduated, and it is possible for his lever to be set immediately at a desired pitch, whereupon the follow up mechanism causes the blades to assume this pitch gradually and maintain it. (4) Governed Speed Type. Here the pilot's control permits adjustment to any engine speed within the operating range, this speed being subsequently maintained by some governing device. This, Fig. 23 : Bristol " Perseus engine installed in a Bristol 1293 Bulldog."
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