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Aviation History
1948
1948 - 0716.PDF
554 FLIGHT May INTAKE REPORT . . . spinner under test. This value of air intake efficiency is denned as the existing total head at the com- pressor entry above free stream static, divided by free stream dynamic head. It may be noted that all the ducted spinners show a considerable improvement in effi- ciency over the conventional type, thus bearing out the theoretical arguments. In the case of the long-chord spinner with circular blade root fairings, however, this improvement is less marked. At low values of tan <p, this type of spinner shows a very high percentage of dynamic head recovery, owing to the fact that the blade root fairings were set about 8 deg. coarser than for the short-chord spinner. Over normal operating conditions, however, values of 1.3 to 1.5 are experienced, so that the large gain of the long-chord spin- ner will not be experienced in practice. In the normal operating range, the short-chord spinner possesses a slight advantage, added to which it has the attraction of being 10 lb lighter. The curves for these two designs should not be compared too closely because of the 8 deg. difference in their fairing setting angles: nevertheless, a direct com- parison is possible by shifting the curves bodily with regard 0 8 O.7 0.6 _ - % —r. MM v ——. \ • 4 S 6 7 e- f O THBUST CINE INCIDENCE. DEGREES Fig. III. Performance curves for short-chord ducted spinner showing air intake efficiency against thrust line incidence for climbing conditions. to each other by an amount corresponding to 8 deg. Figure III indicates that the loss in recovery due to the incidence of the model is very slight up to angles of 12 deg. In the design of the Naiad, as represented by the model, the losses caused by the duct passages through the engine gear box were measured, and improvements in the duct design showed that the intake efficiency of the ducted spinner was increased to 94 per cent, whilst the efficiency of the conventional entry was also improved to 80 per A short-chord ducted spinner on the jf-sce/e Naiad model with which tunnel tests were performed. cent. It was not possible to reproduce the full-scale Rey- nolds Number which, under test conditions was about one-tenth full scale. A limited number of tests were, how- ever, carried out representing a Reynolds Number range of 2:1 to give the trend of scale effect on the two ducted spinners with blade root fairings. The indication was that, in both cases, the full-scale spinners should have a better performance than that given by the models, but this improvement would probably be less marked in the case of the long-chord spinner due to its lower t/c ratio. From these tests, it was considered that the short-chord ducted spinner would prove to be the most efficient, and designs, which were started in January, 1948, were com- pleted for spinners for the Naiad engine. Constant Speed Airscrews, Ltd., who worked in co-operation with Napiers on the original Sabre ducted .spinner trials, undertook manufacture, and several spinners, both for test-bed run- ning and flight development, have been delivered. A model of a contra-rotating ducted spinner has been manufactured, the general arrangement for which is shown in Figure II. Tests on this ducted spinner have been com- pleted and have shown that, whilst a considerable improve- ment over the conventional entry behind a contra-rotating airscrew is still evident at climbing speeds, that is, condi- tions corresponding to high values of entry velocity ratio, the gain is less marked at cruising conditions. The advantages to be derived from the use of ducted spinners were apparent in the early stages and, at the request of the Ministry of Supply, Napier's designs were made available to the airscrew industry. The adoption of ducted spinners for other makes of airscrew turbine has previously been arnounced in Flight. Fig. IV. Comparative curves showing percentage recovery of dynamic head at the compressor entry for the different types of spinner tested. CURVES SHOWING VARIATION OF INTAKE EFFICIENCY WITH TAN * MAXIMUM CLIMB 2OJQOOFT MEAN APPSOACH VELOCITY v CHOBO DUCTED ROOT MMNC* SHORT CHOBO OUCTTO «"!!•*» WITH »UU«-»OOT LOHO CHOW avcrco (PMNER WITH-itoarMEAN POWIONAL VELOCITYTTn D m averts •»•»••» wn-H__ • IUM-ROOT FAItWCl AIRSCREW BLADE -ROOT FAIRING IN SPINNER NOMML OMUWMO DEFINITION OF ANGLE 4 OS 16
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