FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 1780.PDF
16 December 1955 As already related, the turbine rotor blades were fully shroudedbut were otherwise similar in general design to those employed m the Nenc. Early Avon trials established a tendency for theblades to crack at the root, and a partial cure was effected bv welding the roots of adjacent blades together in pairs Neverdieless it was clear that a better design could be evolved, and this materialized as an extended-root form of blade. The revised blade was not only an attempt to eliminate tur-bine blade failure but was also introduced to allow the disc to run at a significantly reduced temperature. From the viewpointof blade mechanical strength, a test programme of photo-elastic analysis and electro-magnetic vibratory rigs was used to find aroot design devoid of severe stress concentrations. As in the case of the compressor blading, it was found possible to reproducethe failures experienced, and suitable blades were eventually developed. At first each blade was made with a root of the type shownin the left-hand detail sketch opposite. From the front, such a root formed a complete gas seal, the whole flow being forced toleave between the rotor blading and so perform useful work. Nimonic alloys are poor conductors of heat, and the temperatureof turbine rotor discs is therefore largely a function of the temperature of the gas or air flowing over them. This isemphasized by the high gas velocity and correspondingly high transfer ratio. By lifting the blades well clear of the disc it wasfound practicable to maintain the whole disc at a comparatively low temperature. This was a notable advance, for it meant thatthe weighty austenitic steels could be rejected in favour of ferritic steels, which are much stronger and consequently lighter for agiven job. Nevertheless, under vibrating conditions it was found thatexcessive stresses were set up at the edges of the webs of the box-like blade roots. The solution was to remove these websentirely, leaving a simple, solid root with a fir-tree at its lower extremity. When inserted in the disc adjacent roots were incontact only at the disc and at the platform carrying the blade; the gap in the intervening portion was blanked off by a series ofsteel plates which also acted as locking members for the blades. This form of construction, which is still used, is shown in theother detail sketch. Development of the RA.2 continued through 1949. Early in the year a semi-production batch was made, and in May a pair of engines was sent to the English Electric Company for installa- tion in the first Canberra. Later in the year another batch was made, and some of these were sent to Canada in January of 1950 for installation in the prototype Avro Canada CF-100 fighter. Meanwhile, flight time was gradually building up, the first pair of flight engines logging 100 engine-hours between them in the Lancastrian by October 1948; and another pair put on 200 more engine-hours in the same aircraft by February 1949. By this time the results of the various tests had told Rolls-Royce just what sort of engine they had. Altogether it was obviously going to be an outstanding turbojet; and already it was earmarked as the power unit of more new British military aircraft than all other engines together. Nevertheless, it was not yet quite as good as it could be. Evidently the large single-stage turbine was overloaded, and the next major step in design was therefore the issue, on October 28th, 1948, of drawings for a two-stage turbine. The new turbine incorporated all the advances made with the RA.2 and introduced further improvements. One particularly remark- able fact is that, owing to the use of ferritic steel for the disc, the new two-stage turbine weighed less than did the original, massive single-stage unit. The two-stage turbine was extensively rig-tested, the results indicating a significantly better efficiency over die whole operating range. With the new turbine fitted the Avon became the RA.3, and the rating returned to the original design figure of 6,500 lb. The first RA.3 ran on April 4th, 1949; subsequent progress was markedly superior to the pace of the first year of Avon develop- ment, for the basic engine was by this time a proven unit. By July a 25-hr test had been run; this was followed by a 150:nr test in September and the flight of one RA.3 in the Lancastrian the following month. By September 1950, this engine had logged 50 flying hours, but greater time had been recorded by engines 55 and 56 which had been airborne in Meteor 4 RA 491 since April 1950. Many readers will recall the memorable display by Harvey Heyworth in this aircraft at the 1949 S.B.A.C. display, when both Avons were extinguished together and re-lit alter an upward roll; and at the 1950 display the extra urge of the RA.3 was most apparent. . . ., 78 In May 1951, Vickers-Armstrongs received engines 75 to /» for the prototype Model 660 bomber, precursor of the Valiant. Before these were delivered a full type-test had been run, in November 1950. , . , ,. Thus was the way cleared for quantity production, and the RA.3 engine received the appellation Avon Mk 1, and tesena number 103. Tooling-up for production occupied much of 15W ••"• •••"• ••'••' " . . _ «» and the first half of 1950, and Bamoldswick assisted by makingthe first five engines to production standard. The first of these was finished in March 1950. The first production engine of themain batch from Derby followed in June. During the latter half of 1950, deliveries of engines of RA.3rating steadily mounted. Intensive development proceeded on both the aircraft and its powerplant, and full service clearance pre-ceded the formation of the first Canberra squadron in June 1951. This was No. 101 Bomber Squadron, based at Binbrook, Lines,and it was the first British unit to be equipped with aircraft powered by axial gas turbines. Meanwhile, development of the engine had been accelerated.In November 1949, a project was completed which marked a significant advance on all previous Avons, for it would haveemployed a 13-stage compressor and a cannular (or tubo- annular) combustion system. However, it was not built, andserved as an interim study leading to odier engines. From the point of view of production, the next Avon was theRA.7. This can be described in general terms as a cleaned-up and rationalized RA.3, with an increased capacity and a rating of7,500 lb. Other major changes introduced in this engine were strengthening of me structure to meet the requirements of high-speed (transonic) flight, deletion of the compressor/turbine bal- ance piston, development of new starters and provision of fullanti-icing. Avon RA.3s were modified to use high-energy ignition, which was a basic feature of the RA.7. In the RA.7 (which dates from 1950) high-pressure air fromthe delivery end of the compressor is allowed to leak past a restrictor to the front of each turbine disc, where it providesboth cooling and a rearwards axial load supplementing that due to the turbine rotor blading and approximately balancing out theforward pull of the compressor. A slight forward pull remains The Avon compressor rotor shaft is a beautiful piece of engineering. On it are mounted the discs for the twelve stages of rotor blades, the discs and intermediate spacers being pinched up against the stiff double-coned rear disc seen here at the left-hand end of the rotor. Further left (towards the rear of the engine) is the drive shaft terminating at the centre coupling. to be taken by the centre bearing. It has been established thatthe deletion of the original balance piston makes for smoother running, and this is accordingly being done as a retroactive modi-fication on RA.3-type Avons. In the development of revised forms of starter Rolls-Roycehave co-operated with British Thomson-Houston. The first series of Avons were started by a large Rotax electric motormounted directly on die nose, under the bullet fairing, and driving directly on to the compressor. For some military engines a morerapid acceleration to idling speed was necessary, and independ- ence of a ground electrical supply was also considered an opera-tional advantage. The engine company and B.T.H. accordingly developed a single-breech cartridge starter, employing a cart-ridge of what was at that time exceptional size. Such starters also became standard on the Avon RA.3. In order to accommodate die considerable torque applied verysuddenly by such a starter, a special form of shock-damping drive was evolved. The starter turns a spring-drive shaft which extendsinside the compressor to beyond the fifth stage. The rear end of this shaft is splined to an outer, final-drive shaft, which engageswith die compressor rotor via (on the RAJ) a pawl-and-ratchet mechanism. The hot exhaust from the starter escapes throughfaired pipes in the intake and thence through ports in die cowling. The RA.7 introduced a new triple-breech starter developedby Rolls-Royce but based upon B.T.H. patents. This unit is used on most of the fighter Avons at present in service. In opera-tion it resembles die single-breech unit, but is larger, and the
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
