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Aviation History
1944
1944 - 2155.PDF
OCTOBER IOTH, 1944 FLIGHT 4?3 Contra- Rotating Airscrews A Brief Review of the PrepCnt Position and Fntvcre~-Qmlook DURlNSNthj^Jast th£fl^j^tTs~ aircraft designers have evolved ana p'foauced new high-performance machines which in each case have imposed increasingly stringent technical demands on the designer of the airscrew. These advanced technical requirements have sh»vn the necessity for very close collaboration between the respective designers of engine, aircraft and airscrew, as the airscrew designer could no longer be left in the position where a proposed new machine was committed in design before he was consulted. He is now brought into the scheme from the very earliest stages^-s not only do aerodynamic factors demand the most cifeful consideration, but he is in a position to give advice on reduction gear ratios and the mechanical co-ordination of the airscrew with the engine and aircraft. In considering the main design points at issue, the question of airscrew diameter is of primary importance, for it directly affects considerations for optimum efficiency over a variety of performance conditions ranging from take-off and climb to maximum speed. In each condition, medium and full supercharge ratings are involved, and with the •"employment of the two-speed two-stage supercharger the operation at extreme altitudes involves further stringent demands. Diameter and Blades The question of diameter also has a large bearing settling undercarriage height, whilst the number of blades comes into the picture from the aerodynamic standpoint as it affects the tail-unit design in ensuring satisfactory stability in flight. The purely mechanical side of airscrew design is con cerned with making sure that the pitch-change mechanism is capable of the quick response necessary to prevent violent fluctuations in engine speed during rapid manoeuvres, and this requirement has meant the closest study with engine designers whose department it is to provide adequate facilities for obtaining these high rates of pitch change. With the introduction of the very high-speed, high-., powered fighter ths problem of efficient power absorption" has been largely in the hands of the airscrew designer, for not <p,\ \: is adequate diameter opposed to efficient under carriage design, but is also limited by the permissible tip speed at the high-speed /high-altitude conditions. The problem of housing in the modern thin-section wings an undercarriage which allows ' reasonable airscrew diameter ls a further difficulty. Again, wings with improved low- The Rotol contra-rotating airscrew for aircraft of the 1,600 h.p.-2,ooo h.p. class with rated altitude of 28,000ft. upwards. drag sections, which are being used to attain still higher speeds, set a further limit within which the airscrew designer must work in striving to obtain optimum efficiency without incurring helical tip speeds which are too high for the thin wings. . These limitations have resulted in obtaining the nece>*rtry "blade area" for efficient power absorption by increasing the number of blades for a given diameter. So far the maximum number of blades used on a single hub has been five, and for two reasons this is considered to be the prac tical maximum, viz.: — (r) From aerodynamic considerations an increase in the number of blades in a single hub introduces loss ol efficiency due to the rotational disturbance created in front of the airscrew which results in decreased pro pulsive effect. (2) The hub mechanism becomes so bulky that spinner diameter becomes excessive. It is essential that the spinner fah» into the fuselage and the alternative wouhl be a lafger fuselage, which, of course, would not be. acceptable to the aircraft designer. Introduction of more than five blades demands the contra-rotating airscrew, which consists of two banks of three blades each, mounted co-axially and rotating in oppo site directions. Contra-rotation has the effect of nullifying the rotational slip-stream losses and, at the same time", reduces the hub size, so that the spinner and fuselage, or nacelle, dimensions can be kept down to efficient propor tions. The main advantages of contra-rotation, whilst possibly well known may, with advantage, be reiterated, viz.: — Improved airscrew efficiency at maximum speed. Increased climb performance at various altitudes. Cancellation of airscrew torqug. reaction. Cancellation of gyroscopic e#rtct on engine mountings. Retention of normal spinner diameter. The first two of these advantages have already been mentioned, but further stress should be made on climb performance. It has been the policy of the Air Staff to
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