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Aviation History
1939
1939 - 2019.PDF
SUPPLEMENT TO 24 JUNE 29, 1939 FLIGHT 664& THE AIRCRAFT ENGINEER Fig. 3. Complete Onazote blade root fairing, and interior and exterior views of two halves prior to fitment to blade. at the end of October, 1938. The tests showed that after running for a short time at 1,800 r.p.m. the outer end of the fairing had moved outwards by rVin. and the inner end had not moved at all. After two minutes at 2,250 r.p.m. the outer end was observed to have moved outwards a further tVin., the inner end again showing no signs of movement. After a lapse of three days the airscrew hub was spun for 30 minutes at 2,250 r.p.m. and at the end of this time it was observed that the fairing had moved out wards, as a whole, by o.o8in. The r.p.m. at which these first synthetic resin fairings were spun were well above the 1,390 r.p.m. for which they were designed—in fact, the centrifugal load was 260 per cent, of that which would have been obtained when oper ated in service. Two major disadvantages of this type of fairing, however, existed, both arising from the density of the material used : the weight added to the airscrew was higher than desirable or necessary and the centrifugal force on each fairing was so great that serious unbalance might result from the failure of one of them. Other disadvan tages concerned the method of application of the fairing when considered from the point of view of quantity pro duction. This work was not carried to the flight test stage, not because it was unsuccessful but rather because it appeared less promising than another similar line of development. Expanded Rubber Fairings Since 1935 studies have been directed towards the use of very light materials as a suitable moulding for blade fair ings. Materials considered worthy of investigation were synthetic ones containing a large proportion of gas spaces, such as expanded rubber or blown metal. It was decided that it would be unwise to undertake work with metals containing air spaces, because those which were available had the strength characteristics only of cast material and were much heavier than synthetic materials. It is hoped that some day synthetic resins of this general character will be available, but at the time this work was done none existed. Rubbers containing a large proportion of gas spaces had been in use in this and other countries for a number of years. These materials are readily obtainable in densities of the order of 8 to 12 lb./cu. ft., that is to say, specific gravities of about 0.15. Therefore in June, 1938, we ap proached the Expanded Rubber Company of Croydon for help in the solution of the problem, and during the past year they have given us excellent co-operation. The material made by this company which was most suited to our use was called Onazote. To vulcanise Ona zote on to a metal requires temperatures of a magnitude which impairs the corrosion resistance characteristics of the material used for the standard blades. Fig. 2 shows the duralumin mould which we made for these early fairings. The mould, of course, surrounds the blade, leaving a space for the fairing. Experiments were carried out with a fairing moulded in one piece on the duralumin blade. In October, 1938, this fairing was spun on the electric spinning rig at Farnborough. After a short period at 1,600 and 1,800 r.p.m. no defect or movement of the fairing could be detected, but after running for one minute at 2,000 r.p.m. the fairing burst. This showed that the wedging action of the fairing on the blade was too great for the strength of the material and that it was necessary to prevent wedging by taking at least part of the load in shear. It was therefore decided to investigate the possi bilities of making the fairing in pieces and cementing it to the blades. Extensive basic tests were carried out with about 20 dif ferent kinds of cement. The characteristics included were high shear strength; resiliency which would resist deteriora tion under vibration; satisfactory resistance to oil, petrol and water; and maintenance of physical characteristics at all temperatures likely to be encountered. The static tests included the pulling of specimens at different angles of shear and at different rates. Actually some of the most interesting results of these tests were discovered as the result of very slow rates of pull. Specimens were loaded and left for periods of many days and the creep recorded. Ageing and weathering tests of the cements were carried out over periods of several months. Attachment of Fairing These tests took a long time to carry out and work was continued with the complete fairings. In one procedure used for fitting the fairing to the blade, the fairing is per forated with holes at intervals of every 4 sq. in. to facili tate the drying of the cement, and in a similar method this was done by applying the fairing in strips approximately 3m. wide. Other methods included moulding the fairing in halves, and various means were tried of obtaining good contact with the blade, drying off the cements. In Jan uary of this year a fairing moulded in halves, cemented to the blade and covered with four bindings of 2jin. aircraft tape, was spun at 1,500 r.p.m. for five minutes, no move ment of the fairing being observed. An increase of 50 r.p.m. showed a movement outwards ^in. after 10 minutes. At a further increase of 50 r.p.m. no movement could be detected, but after 10 minutes at 1,650 the fairing burst, due to the '' creep'' of the cement joint. This cement was of a type which remained tacky during its life, and by this time the static tests had disclosed other cements which were much more satisfactory. Shortly afterwards a hub with stub blades incorporating (a) a fairing vulcanised on to a sand-blasted blade root, (b) Fig. 4. Three-blade dummy hub, incorporating Onazote root fairings, assembled for spin test.
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