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Aviation History
1964
1964 - 0955.PDF
546 FLIGHT International, 2 April 19b* "Flight lncern»tional" photograph NO CONCESSIONS... ment programme had fallen into place and it was obvious that some fairly major changes to the design of the droopable wing leading edge would be required on the final production aircraft to improve low-speed handling and performance. As the detailed testing was still in its early stages and was being adequately covered by the first and third aircraft, it was decided not to use the fourth in the main Trident 1 programme but to convert it to be representative of the later Trident IE, with fixed full-span slats in place of the droop. This aircraft flew in January 1963 and was operated in parallel with the main programme for about ten months. The fifth aircraft was deferred until the first set of low-speed modifications was available in production form, flying for the first time in May 1963. This aircraft was eventually used for the route- proving at the end of 1963. It is not possible to describe here in detail all aspects of the deve- lopment testing, and in retrospect the whole of the flight test pro- gramme now appears to have been comparatively routine hard work, which was commendably free from those spectacular crises or incidents which can so easily slow down the progress of a brand- new aircraft. This situation was, of course, materially assisted on the engineering side by the extensive pre-flight rig-testing of, for instance, the flying controls, hydraulic and electrical systems (on which very little flight work has subsequently been required) and by the continuous full-scale support of the engine and component manufacturers. In addition, the aircraft itself rapidly proved to be comfortable and quiet, with a good standard of serviceability at home, and a still better standard when operating abroad. Many of the features of the Trident which represent advances over aircraft like the Comet have not proved unusually difficult to develop, because of the high standard of controllability. As an example, the use of airborne reverse thrust has been cleared for operation at all altitudes up to the maximum operating speed and during the landing flare, because the trim-changes with one or two reversers operative are small. The large all-moving tailplane with geared elevator has enabled a large e.g. range (30 per cent of the mean chord length) to be cleared without difficulty and ensures a healthy recovery from any slightly delicate predicament near the stall. The engine-out minimum control speeds are never limiting and cannot be measured in practice, even for academic interest. The available rates of roll are generally high but the behaviour in turbulence in roll was initially compared by some pilots to a light aircraft. A roll damper operating on one aileron cured this prob- lem and it was subsequently found that this system could also serve as a yaw damper if required. Two areas of the testing stand out in their individual ways, how- ever, and merit fuller description. The first, the clearance of the aircraft to its high-speed design boundary, proved to be less of a problem than on any other aircraft we have tested in the last decade, although a relatively larger and very much more complex organiza- tion was involved. The initial speed and Mach number clearances for the first period of flying, which was based on the results of calculations and tests on flutter models, was high enough to allow the cruising perform- ance to be measured at the scheduled maximum operating speeds and to ascertain that the stability at high Mach numbers was satis- factory without a Mach trimmer. This represented a great advan- tage, and offset the effects of subsequent delays to the start of the final clearance. The final clearance was started in the autumn of 1962 on the first aircraft, after a grounding period to install the test equipment. Two electrically controlled linear hydraulic inertia exciters with a sepa- rate power supply were installed, one in the nose of the aircraft in place of the radar and the Other above the centre engine intake in Landing at Hatfield after the first flight, January 9, 1962 the position normally occupied by the HF notch aerial. Each exciter was capable of being rotated in flight to act in either the horizontal or vertical plane and each was controlled by a control unit mounted in the cabin to sweep through a frequency range of 2£c/s to 35c/s in six to eight minutes. The continuous excitation was stopped automatically at 35c/s, but the sweep could be started and stopped as required. The airframe response was measured by a large number of com- pensated accelerometers mounted on the main structure and by transducers for main control angles. Some of these outputs were recorded in the aircraft on magnetic tape and a selected number were telemetered to the ground with exciter information, where they were also recorded on tape. Selected accelerometer outputs were visually displayed in the cockpit together with a frequency signal, and the instinctive cut-out button normally used to disengage the autopilot was wired to stop the exciter in emergency. On a typical flight, the aircraft flew at the required conditions of speed and height over a course in East Anglia which had been specially surveyed to ensure good telemetry reception at Hatfield. The test results were presented on two plotting tables via resolvers in the form of Nyquist diagrams, and also on an ultra-violet re- corder. These diagrams were analysed on the spot by a separate team of four people, each plot taking about five minutes, so that a maximum number of cases could safely be completed on each flight. Completing the Programme Thirty-one flights were required to complete the programme, in- cluding the setting-up and checking out of the equipment, which proved reliable and not unduly difficult to service. A further four flights were used to make re-checks with the autopilot engaged. The general handling at high Mach numbers was assessed in parallel with the flutter programme, together with measurements of struc- tural loads. The aircraft was eventually cleared to a true Mach number slightly above the design value of MT = 0.95, the sole problem encountered being an airspeed static system "flick" at just above MT = 0.92 caused by shock-waves passing over the static plates. The second area, at the other end of the speed scale, namely, the development of good low-speed performance and reasonably forgiving handling characteristics, was probably the largest and certainly the most expensive problem in the flight programme, because the importance of these qualities to a modern civil jet cannot be over-emphasized. The low-speed handling, which had been fairly thoroughly assessed on the first flight, was initially unsatisfactory because of early flow separation over the outer wings, as shown by surface tufts and a lack of definition of the stall. The separation was aggra- vated by poor sealing between the droop and the wing at the closing plate (the "knuckle") and was temporarily improved by locking the droop down and doping a fabric strip along the undersurface of the droop joint. The discontinuity at the inboard end of the droop had been de- sensitized at a late stage in the design by fitting a large solid wooden fillet across it to remove any possibility of a handling problem on the pod engines during initial flying. This was known to have a high drag, but the discontinuity presented a serious problem when it was removed and it was therefore hastily replaced at a very early stage of development by an equally temporary simple wooden fairing. The present Kruger-type flap at the wing root serves as a simple means of putting an efficient, retractable fairing at this junction. The optimum lift characteristics were obtained by successive small alterations to the droop angle and it was found that the best results were obtained when the main flow separation took place at
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