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Aviation History
1947
1947 - 1340.PDF
164 FLIGHT AUGUST 14TH, 1947 Airscrews for Gas Turbines of operating conditions. If the airscrew is to give a high efficiency at the top-speed condition, the reduction gear ratio must be so arranged that the airscrew tip speed, when compounded with the forward speed of the aircraft, does not exceed the speed of sound at the appropriate altitude. If this compounded tip speed exceeds the local speed of sound, a serious drop in airscrew efficiency occurs due to compressibility losses. While the selection of a reduction gear ratio to give top- speed efficiency is practicable, it also gives such low air- screw r.p.m. at take-off and climb as seriously to impair the efficiency under these latter conditions. A piston- engined aircraft at this speed definitely demands a two- speed reduction gear giving high r.p.m. for take-off and climb, and a correspondingly lower r.p.m. for top speed. Due to the added complication of weight of the two- speed gear, its introduction has naturally been delayed as long as possible and, in- stead, airscrew development has pro- ceeded on the lines of improving blade aerofoil efficiencies at higher forward speeds. Whilst a certain degree of suc- cess has been achieved, the two-speed gear is still left lurking in the back- ground. With the advent of the gas tur- bine it was found possible to give aa overspeed on the airscrew turbine r.p.m. for take-off and climb (a feature not vety practicable with a piston engine), the degree of which varies according to the particular type of turbine unit. It would appear that this feature has further staved off the introduction of the two- speed gear. A parallel requirement which would also demand a two-speed gear is the limi- tation, which is being laid down by air- line operators, on airscrew rotational tip speed in order to keep down the noise level. Here again, if the reduction gear ratio has to be deter- mined by the permissible airscrew r.p.m. for acceptable noise level under the continuous cruising condition, its low figure would impair the take-off and climb performance. Over- speeding the airscrew turbine r.p.m. for take-off and climb assists in meeting this problem although the salvation is not so pronounced since the cruising r.p.m. of turbines are a much higher percentage of the maximum r.p.m. than applies with a piston engine, and a compromise is invariably essential. The noise level remains high at take-off and climb with either two-speed reduction gear- ing or overspeeding, but it is fortunate that this condition of flight is of relatively short duration. Universal Features Four of the main features which will become universally adopted in airscrews in the near future are: — (a) Braking, either for use in reverse pitch as a landing brake for the heavier aircraft, or as a diving brake in wind- milling pitch for naval aircraft. (b) Synchronization. (c) Electric de-icing of airscrew blades. (d) Hollow steel blades. The use of the airscrew as a landing brake in negative pitch is quickly becoming universal. Development carried out has shown its effect to be equal to that of conventional wheel brakes, with the added advantage that it is much safer in operation on ice-bound runways. The dive-brake airscrew with blades in the windmilling pitch position has been shown to give very high drag values, e.g., 15,000 1b drag can be provided on an aircraft of 23,000 lb all-up weight. This braking feature of the airscrew is rapidly becoming an essential requirement in the later naval bomber specifications. Automatic synchronizing of airscrew/engine r.p.m. has successfully passed through its development stages and is i Fig. S. Plan form of conven- tional and swept-back types of blade. being planned for future multi-engined aircraft. In a typical system any engine can be selected as the '' master '' and the other engines made to follow. The system consists of a small alternator driven by and mounted on each engine,* and a "corrector motor" mounted on each air- screw constant-speed unit. The alternator on the engine chosen as master supplies alternating current to the wind- ings on one side of the corrector motors on the " follower " engines, the other windings of the corrector motors being supplied by the individual engine alternator. This results in the corrector'motor for each engine rotating appro- priately according to the difference in speed between its own engine alternator and that of the master engine, thereby applying the requisite pitch angle correction to any of the follower engines as dictated by the master. Electric de-icing of the blades is being catered for on all new airscrew designs, temporarily by an airscrew hub- mounted generator feeding the current to the airscrew ^ blades via sliajsings at the rear of the hub. The blade- * Sgy^ heating elements at the moment consist either of an electrical conducting rubber envelope completely covering the blade, or a partial element over the leading-edge extending approximately one-third of the chordal width on the pressure and suction faces. Development is proceeding to determine the more practical solution of these two methods. Steel Blades The hollow-steel blade offers greater possibilities by permitting the de-icing to be done internally, thus avoiding the necessity for external rubber heating elements with their attendant "adhe- sion-to-blade " troubles and also the dis- turbance caused to the aerofoil shape. Over and above the advantage to de- icing, the hollow-steel blade affords greater durability and can also be made lighter than its duralumin counterpart with diameters in excess of 13ft. The absence of torsional vibration in the turbine should remove the early troubles with fatigue failures and make possible still lighter construction. The entirely self-contained airscrew wherein the hydraulic operating system is integral, and the various accessories such as the constant-speed unit, reversing pump, synchronizer alternator and feathering pump are mounted on the airscrew, has been given very serious consideration in this country,, and more so in the United States of America. It is, at the moment, speculative as to whether or not this type will oust the latest editions of the orthodox type, since it further complicates the airscrew, which has already had to cope with the aforementioned additional features demanded by turbine engines. It is doubtful whether the self-contained hydraulic system is altogether desirable, since its limited oil capacity in the event of a sudden leakage and loss of control could have serious con- sequences to the airscrew and engine. Furthermore, the mounting of all the airscrew accessories on the hub demands an unduly large spinner completely to house these compo- j| nents, this being a distinct disadvantage when designing^ the small-diameter inner shell of the turbine-engine ductecJ, spinner. As it has so far been possible to redesign the orthodox type of airscrew to meet every requirement requested to date by the turbine engine, it remains problematical as to whether or not the greater complication of the self-con- tained airscrew will be shown to be worthwhile. Airscrew weights for turbine engines show some reduc- tion over their piston engine counterpart, largely by virtue of the absence of torsional vibratory stresses from the engine system. This saving in weight occurs both in the blade and hub structure ; in the former the limit of " thick- ness '' of the aerofoils is now wholly dictated by the steady Continued ' p. 172.)
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