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Aviation History
1964
1964 - 0954.PDF
fLIGHT International, 2 April 1964 545 No Concessions THE STORY OF THE TRIDENT FLIGHT-TEST PROGRAMME By A. J. Fairbrother* THE original planning for the Trident 1 flight test programmewas done at the end of 1958, just after the completion of thedevelopment programmes on the Comet 4 and Comet 4B. The flight testing of the basic Comet 4 series via the Comet 3 proto- type, running in almost exact parallel with the Boeing 707-80/120 programme, had by this time extended a little over four years, whereas the time-scales which were considered for the Trident 1 in the early planning stages were all approximately half this figure from the outset. The estimates of the flying hours required to obtain the C of A and the corresponding aircraft development time required were based on statistics obtained from analysis of the various Comet programmes extending back to the Comet 1 in 1949. It was then realized that although there was considerable scatter in the flying hours taken to clear any particular development problem, which left the production of new estimates more in the category of an art than a science, the overall utilization of the various Comet develop- ment aircraft in terms of hours flown per year had averaged out at a surprisingly uniform figure. The methods used for the estima- tion of time-scales required for the Trident were thereafter consider- ably simplified by making the assumption that each aircraft used in the programme would fly at a constant average number of hours per year on home-based development work irrespective of the precise task. This assumption also implied a constant allowance for the periods spent in the shops per month or per year to incor- porate development modificatons, and it has subsequently proved in practice to have been reasonably correct. Sharing the Load It was finally decided to share the main workload of the develop- ment programme among three aircraft, which would be fully instrumented, and to complete the mandatory 200 hours' route- proving on a fourth fully representative airframe. The three development aircraft were intended for handling tests, systems and engineering tests and performance testing respectively, but all were equipped with a common standard of basic instrument wiring and piping to enable programmes to be rapidly interchanged if necessary. As finally flown, however, each aircraft was fully equipped to measure performance, including all main engine parameters, and a standard limited number of channels for handling at all times, irrespective of the basic role. The instrumentation used was conventional in the sense that combinations of auto-observers and galvanometer cameras were used for recording the bulk of the development data, but all the component parts of the system had been considerably refined in detail design. Extensive use was made, for instance, of printed circuit cards to enable between-flight changes to be made as rapidly as possible. Each aircraft was fitted with an auto-observer covered by two cameras recording at two frames per second, used primarily for the recording of quasi-static quantities such as engine and systems pressures and temperatures, and a master flight recorder as standard basic equipment. The master flight recorder, which was specially designed for use in the Trident, is a large galvanometer camera using 12in paper and capable of a wide range of paper speeds and a variety of uses. In its basic role it records two sets of data continuously throughout the flight, using a slow paper speed of about one inch per minute, to enable the whole flight to be inspected easily. The first set of chan- nels consists of the basic performance parameters—airspeed, alti- tude, air temperature, fuel used in the form of gallons gone, etc, for which there is a consistent requirement in every flight test pro- gramme. The second set of channels record the sequence of opera- tions of the whole of the recording equipment in the aircraft. This enables the subsequent data-processing to be reduced to the mini- mum by economizing on film reading requirements. Magnetic tape was used for only a limited number of general development problems requiring rapid analysis of frequency res- * Flight test manager, de Havilland division of Hawker Siddeley Aviation. ponse but formed a fundamental part of the recording system used for the flight flutter programme, described in more detail later. The first Trident 1 emerged in the early autumn of 1961 fitted with two very early Spey engines for preliminary engine runs and some rather sedate taxying trials. This mild exercise, which was in- valuable because it showed the need for some minor changes to the unddxarriage damping at an early stage, was followed by the re- mainder of the pre-flight checks on the aircraft systems, which were completed with the arrival of the flight engines (and the snow) in mid-December 1961. The aircraft was finally cleared for flight on December 21, ten days before the original promise. Weather conditions did not permit movement until January 8 (1962) when two or three hops were made at light weight to assess the control gearings, to fix the take-off setting of the extremely powerful all-moving tail. The Hatfield runway is 6,000ft, which is not over-generous for piototype flights, and about 300ft of PSP strip had been laid on one grass undershoot as an insurance to im- prove the bearing strength in case the weather was bad. The hops were, in fact, carried out using only the main runway surface for the whole of the acceleration, flight and stopping distance without any difficulty, the ten seconds or so of airborne time proving adequate for the qualitative response checks. These checks showed that the control gearings of the tailplane and rudder were of the right order, but the lateral control gearing (by ailerons and linked spoilers) was on the high side. The first flight, made on January 9, 1962, lasted lhr 20min and this aircraft then maintained a steady average of 22£ flying hours per month for the next eight months, mainly on low-speed handling work. It was then grounded for a period to install the equipment required for the flight flutter programme. This performance com- pared favourably with the basic target for utilization mentioned earlier, which was 23hr per month. The second aircraft, for systems development, flew in May 1962, again to the original schedule, and the third, primarily intended for performance measurement and development, flew in mid-July. Both these aircraft were exactly similar in their external condition to the first aircraft as first flown. By this time, however, the pattern of the priorities in the develop- Corps d'elite of the DH jet airliner flight-test team, continuously since the Comet I of 15 years ago, has comprised: at foot of steps, John Cunningham (right) and Peter Bugge (left) and in ascending order J. A. Marshall, A. J. Fairbrother (author of this article) and £. Brackstone- Brown. This picture was taken ten years ago, after the Comet 3's first flight
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