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Aviation History
1943
1943 - 2259.PDF
SEPTEMBER I6TH, 1943 FLIGHT 3» Airscrews A Review of the Present Position and Future Outlook of Constant'Speed Airscrews By L. G. FAIRHURST, A.M.I.Mech.E. Chief Engineer, Rotol Airscrews, Ltd.) TIE following article is mainlyintended to review the presentposition and future develop- ment of the constant-speed airscrew, which has been found an essential fitment to all aircraft used in the present war, and also to review the demands of the later aircraft and future progress. Method of Operation In order to keep in mind during the reading of this article the function- ing of a constant-speed airscrew, a brief description of its operation may be valuable to those who are not in constant touch with its use. The constant-speed airscrew is con- trolled by an engine-driven governor, which holds the engine at any desired r.p.m. by controlling the^-pitch of the blades. The desije>eK^r.p..m. is ob- tained by the pilot's control lever, which in effect compresses or relieves the main spring in the governor unit according to the desired r.p.m. at which the governor is required to operate. Once set, the r.p.m. will remain con- stant, and the most efficient combinations of r.p.m. and boost for the different operational conditions of flight can be obtained. Additional to the constant-speeding feature, airscrews for multi-engined machines have facilities for "feathering" whereby the -blades can be moved into the position where their leading edges are "edge on" to the air stream, and in which position the airscrew does not rotate. This feature prevents any further damage by preventing a v.'indmilling airscrew "churning up"' the engine. In the feathered position tjptrdrag on the aircraft is so much reduced that height can be maintained on a single engine for a much longer period than would otherwise be the case. The 3-blader Position As almost all aircraft are now designed around the variable-pitch airscrew, it becomes essential to obtain the maximum possible efficiency out of the airscrew, as the performance of a machine can be very easily marred by an inefficient airscrew blade. The leading British fighters are all equipped with 35-deg.-range constant-speed airscrews, while the bombers have the extended range to permit feathering. The 35-deg.-r^ifge has been proved sufficient to cover all con- ditions of constant-speed operation of both fighter and bomber airscrews. On later airscrews for flying boats, the range of blade angle is now being extended to permit running in negative pitch for manoeuvrability on the water. How far this "reversing" feature will be extended for use as an air-brake for night fighters and dive-bombers, or a landing brake for bombers, is at the moment unknown, but development will, no doubt, go ahead on these lines. Until comparatively recently the majority of airscrews in use were of the three-bladed type, as engine powers of 1,200 h.p. at 20,000ft. rated altitude could be conveniently handled by fcKree-bladers, providing there was no serious restriction/fjn the diameter by reason of a short under- carriage, or in the case of bombers by fuselage/tip clearance. As all fighter and bomber aircraft used up to the present Mr. L. G. Fairhurst, author of this article. in the wa^Eve rated kltitudes around 2o,oootJ|Fthe three blader has sufficed to £Q0rer the various operational con- demanded. In certain special stances it has been found necessary to use blades with wider chords than normal in order to obtain increased take-off and climb performance, but this usually reacts in a reduction in top speed. The Introduction of 4-bladers With the introduction of two-stage superchargers, having the effect of maintaining the engine power at much higher rated altitudes, combined with higher top speeds of aircraft,, it has become essential to introduce iour- bladed airscrews. This step has become necessary on account of the limitation on diameter which is im- posed, due to it being necessary for optimum airscrew efficiency to keep the helical speed of the airscrew tip within a certain percentage of the speed ot sound at the particular altitude. Airscrew diameter is also additionally limited in certain installa- tions by reason of undercarriage length, and in the case" of bombers, as pointed out above, fuselage clearance to the airscrew tips. The extension of the usefulness of the three-blader has been considered by increasing the blade chordal width in order to provide the requisite increase in blade area, but this, as pointed out earlier in the article, loses efficiency at top speed, which can be ill afforded on modern fighters and bombers. Operating at an increased angle of attack on the airscrew blade would also extend the usefulness of the three-blader, but under modern conditions of opera- tion this angle would reach such a high value that the blade sections would develop high drag and considerably reduce airscrew efficiency. In view of these limitations it is necessary to introduce a fourth blade, the addition of which gives the airscrew sufficient blade area or "solidity " to handle the power at the limited diameter under conditions of reduced air density, allowing the blades to operate at an efficient angle of attack and using normal chordal widths of blade sections. <. To assist in understanding this point an actual example can be quoted of an 11ft. airscrew on an aircraft of 350 m.p.h. with a rated altitude of 20,000ft. The helical speed of the airscrew tip under these conditions is 94 per cent, of the speed of sound. If the rated altitude of the machine is increased to 28,000ft., this percentage becomes 97. At relative tip speeds of this order a considerable reduction in efficiency occurs, due to compressibility losses, i.e., in- creased drag of the aerofoil section of the airscrew blade due to formation of shock waves. To get back to the original figure the airscrew diameter has to be reduced to delay the formation of shock waves, but this reduces the blade area to such an extent that it becomes insufficient effi- ciently to absorb the engine power. The obvious alter- native, therefore, is the addition of a fourth blade at the reduced diameter. If the airscrew rotational speed were reduced, still maintaining the original diameter, this would delay the formation of shock waves, but a loss in efficiency would occur on climb and cruising.
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