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Aviation History
1953
1953 - 0484.PDF
48o FLIGHT Engine Procurement and Development . . . It was usual to estimate spare engines on the basis of $t per cent of the number of engines fitted in an aircraft. It was unlikely that a manu facturer would machine more than two or three complete sets of engine parts at the outset, leaving the remainder in the form of rough forgings and castings, since a radical change and redesign might become necessary as a result of failure in the early development phase. The contract price and conditions for the first engines made provision for an acceptance-test at powers to be decided later. In practice the conditions of the acceptance-test were very flexible and were dictated by past experience and the knowledge of the engine submitted. In practice the test was regarded as practical proof of completion of the initial prototype engine and that it was capable of running. Each and every prototype engine had to pass an acceptance-test, the severity of which was increased progressively according to the state of development and the behaviour of the engine previously built, until a firm rating had been established for the type. Engines accepted subsequently had then to pass a two-hour test based on this rating. The object was to achieve a better standard for each subsequent engine. After the acceptance-test, and stripping and inspection by the Ministry and the firm, bench running began on another contract calling for x hours of experimental running at a cost of £y per hr. If a radical redesign for, say, an increased performance requirement was called for, still another contract would be issued to cover this revision. Flight- and Type-Tests.—A. Cdre. Banks then turned to flight- clearance and type-tests. For special-category or flight-clearance to permit experimental flying to begin, a test of 25 hours' duration was normally made, but a longer period might be specified, depending upon the state of development of the engine. There was at present in draft form—awaiting approval—a flight-test schedule the object of which was to regularize and standardize flight- testing procedure. To date, this procedure had been left rather in the hands of the individual engine builder. Finally there came the type- or model-test of 150 hours' duration; the engine manufacturer tried to complete this test as soon as he was satisfied that the performance and mechanical standard of the engine justified an attempt. The official type-test was usually preceded by a number of 150-hour tests run to type-test conditions. British official engine tests were done at the plants of the manufacturers, not at Government establishments, but were under Ministry supervision. It was considered that our type-test conditions permitted very close matching with actual engine conditions in flight. A. Cdre. Banks then turned to the next phase of development, the completion of the type-test having normally permitted the sealing of the Drawing Introduction Sheet (D.I.S.), which was a list of approved drawings for the engine type or model. Production could then proceed. If urgently required, permission could be given to release material and tooling in advance of the type-test, and separate sections of the D.I.S. might be approved and sealed in advance to enable machining of com ponents to commence. An I.T.P. could also be issued for speeding production preparations. Until the D.I.S. was sealed, design changes were largely the firm's concern, but afterwards any further design changes required the prior agreement of the Engine Modifications Committee (E.M.C.) at the M.o.S. At the firm there was also a Local Modifications Committee (L.M.C.) operating under the chairmanship of the Resident Technical Officer. The members of this committee were drawn from the firm, the R.T.O. staff and A.I.D. The R.A.F. and the Navy were represented on the main E.M.C. Committee. All these proceedings were extremely flexible, and in cases of special urgency could be expedited—for example, by telephone agreement between local R.T.O. and chief R.T.O., the paper work being put in order at the next meeting. Performance Acceptance.—British procedure for the performance acceptance of production engines was then described by the lecturer as follows: First, the brochure performance was vetted by the Ministry; if it was accepted as the performance likely to be achieved, the type-test conditions for the engine were then based upon a combination of r.p.m. and jet-pipe temperature calculated to give as nearly as possible the brochure power. Production engines were expected to meet this per formance but were allowed.a low limit of 4 per cent below type-test power to pass, provided the jet-pipe temperature was within the permissible limits. As the Air Commodore understood it, U.S. practice was to declare the nominal maximum engine rating as the minimum acceptable for clearing production engines. There was little to choose between the two procedures, but at present we favoured our own method. An attempt was always made to obtain from type-test results a nominal rating which should represent the average production engine; and it had also been found, in practice, that the average negative tolerance on production test was nearer 2 than 4 per cent. There was little advantage, and sometimes considerable disadvantage, in erring on the high side of jet-engine performance in production, because of the effect it might have upon fuel consumption and range. In the case of multi-engine aircraft, it was important that the average hourly fuel consumption should not be substantially different from the nominal figures supplied to the aircraft designer. For the single-engine fighter, a wide variation in thrust or fuel consumption between production engines could also be embarrassing. A reduction in the negative allowance, from 4 to 2 per cent while still retaining the present nominal average rating, might soon be agreed. In this connection, we had successfully tried, in certain cases, the U.S. method of adjusting propelling-nozzle areas by means of trimmers or "mice." Another allowance was also made in the type-test, to meet engine behaviour at altitude. At present, British engines employed a method of control in which the fuel supply was regulated by throttle setting and ram pressure, with a top limit controlled by a maximum-speed (r.p.m.) governor but without any link-up with jet-pipe temperature. Since j.p.t. often tended to increase with altitude, it was necessary to set the limiting temperature, for acceptance of the engine, at 20 to 30 deg C (36-54 deg F) below the operating limit fixed by the type-test, in order to meet this temperature increase; but the minimum acceptable thrust must remain within the 4 per cent drop allowed for production engines. Automatic j.p.t. control was now being developed and would be introduced on all British engines in the near future. This would relieve the pilot of continually having to adjust the throttle to avoid exceeding engine-temperature limitations. The reason for accepting production engines at a lower j.p.t. than that cleared on type-test was not only to cover the altitude case, but also to meet the sea-level tropical condition, of high inlet temperature. Often, type-tests were run with smaller-than-standard final nozzle area to clear for high temperature—which procedure, incidentally, gave higher-than- brochure thrust. In so far as reliability was concerned, the test-bed thrust was secondary in importance to r.p.m. and turbine-inlet temperature. But, taking the case of the propeller-turbine, we in Britain were interested in the actual shaft horsepower developed on account of reduction-gear and airscrew stressing. Details of the actual tests of the strip and examination following pro duction test, of regular technical meetings to discuss progress and of continued development and design refinement were then discussed by A. Cdre. Banks, who then turned to the organization of his directorate in the M.o.S., illustrating by means of a chart (reproduced here). Directorate Organization.—The lecturer drew particular attention to the total number of personnel employed—scientific, technical, clerical, at headquarters and at the engine firms—namely, 144. He said that since the man and not the machine was the controlling factor in the creation of Fig. 2. Organization of the Directorate of Engine Research and Development. Its total personnel strength is 144. PRINCIPAL DIRECTOR OF ENGINE RESEARCH AND DEVELOPMENT (P.D. ENG. R.D.) I Service Officers Director of Engine Research and Development (D. ENG. R.D.) Deputy Director of Engine Research and Development (D/D. ENG. R.D.) Staff Officer (Technical) Staff Officer (Naval) I U.S.A.F Representative Assistant Director Eng. R. (Engine research; fuels and lubricants research and development) Assistant Director Eng. R.D.1 (Engine projects; tender designs and specs.; type-test requirements; performance data) Assistant Director Eng. R.D.3 (Development of turbo prop and piston engines; helicopter propulsion) Assistant Director Eng. R.D.2 (Development of gas turbines; fuel systems and accessories for turbine and piston engines) Chief Resident Technical Officer (Engines) (Chairman, Engine Modifications Committee Resident Technical Officers (Engines) Chief Technical Adviser Eng. R.D. Plans Staff Officer (Administration) Eng. R.D. Admin. Eng. R.D. Contracts I Eng. Eng. Eng. Eng. R.D.4 (Development of airscrews, auxiliary gearboxes and auxiliary airborne power plants) R.D. Defects R.D. Mods R.D. Fits and clearances Assistant Director Eng. R.D.6 (Research and development of ramjets and rocket motors) Alvis A. Siddeley (Visiting tech. off. to Metro- Vickers and Brockworth Engineering) Bristol (Visiting tech. off. to B.O.A.C, Treforest) B.T.H. (Visiting technical off. to Joseph Lucas, H. M. Hobson, S.U. Carbs., Lodge, Amal, Automotive Products, Electro- Hydraulics) deHavilland Engines de Havilland Napier Propellers Rolls-R oyce (Visiting tech. off. to Standard Motors) Rotax (Visiting tech. off. to Ultra Electric and Simms Motor Unit) Rotol (Visiting tech. off. to Dowty Fuel Systems)
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