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Aviation History
1967
1967 - 1988.PDF
584 FLIGHT International, S October 1967 Test Flying the Nimrod By J. G. HARRISON* The prototype Nimrod retracting its undercarriage during climb-out "Flight" photograph BASED ON THE COMET COMMERCIAL AIRLINER, yet with adifferent shape, different engines and systems and atotally different role to perform, the Nimrod is the world's first pure-jet maritime reconnaissance aircraft, with performance far surpassing that of the contemporary propeller- driven aircraft used in the role. As such, its development pro- gramme is likely to provide an unusually interesting task for the flight test team. The programme started at the end of May with the maiden flight of the first prototype. A converted Comet, it has the correct Nimrod shape, Spey engines of considerably greater thrust than the Comet's Avons and the completely new a.c. electrical system of the Nimrod. It is otherwise largely un- representative of the production aircraft, still retaining some Comet systems and being fitted with the comprehensive flight- test instrumentation normally associated with prototypes. During the first few flights a number of checks were made to demonstrate that the main programme could proceed with safety. For example, before engine re-lights could be attempted, it was necessary to show that engine bay drainage was satisfac- tory. This was done by shutting down an engine, opening the high-pressure cock to pour unburnt fuel through the jet pipe, then landing so that the whitewashed engine bay could be inspected for fuel stains. This test, incidentally, provided early evidence of the com- pletely innocuous nature of a landing with asymmetric power in the Nimrod. Before stalling, the anti-spin parachute was streamed—first on landing, then at 10,000ft. To provide photo- graphic evidence of a satisfactory stream, a Shackleton flew in formation with the Nimrod. This close proximity of the old and the new emphasised the great difference in noise level of the two aircraft—the Shackleton's Griffons being clearly heard by all aboard the Nimrod. It obviously makes good sense to carry parachutes on a first prototype if a means of escape can be found, so the rear passenger door was opened in flight on several occasions. Running in parallel with these safety checks were some routine longitudinal stability and control tests on items not expected to be significantly different from the Comet. These were mainly concerned with establishing elevator angles to trim through the speed range, the changes of trim with power, flaps and airbrakes and the stick forces required for manoeuvr- ing—each test being made at three different positions of the aircraft centre of gravity. At the same time, the first exploratory tests were being made of the lateral and directional characteris- tics which, due to the completely changed side-elevation shape of the Nimrod, might be expected to be different from the Comet. In this way, after seven flights had been completed, a broad-brush picture had emerged which confirmed that things were as expected longitudinally, but that lateral and directional behaviour warranted more detailed investigation. The next 20 hours or so were spent mainly on the flight "Chief Test Pilot, Hawker Siddeley Aviation (Manchester). flutter clearance in order to remove the speed and Mach number limitations imposed for the early flying. On each flight some 250-300 sharp jerks were applied to the controls. Before clearance could be given to increase speed on the next flight, a careful scrutiny of many trace records was required to ensure that no part of the structure exhibited signs of un- damped oscillations following a disturbance. At the same time it was necessary to ensure that structural speed limitations were not exceeded inadvertently, so, dovetailing with the flutter tests, measurement of errors in the Nimrod's pitot and static systems was made. Real height, speed and Mach number were established and compared with the indicated values by photo- graphing low runs over the airfield and by formation runs at high altitude with HS.125 and Buccaneer aircraft which had already been calibrated. These tests showed that the static error was rather larger than desirable so six other static vent posi- tions were investigated and from these a new low error position has been selected. The bomb-doors were first opened on the twelfth flight at low speed with negligible change of trim or buffeting and this good behaviour has subsequently been con- firmed at the highest speed permitted for bomb-door operation. Now that the limitations had been lifted and speeds could be determined accurately, it was time to measure a few spot points in the Nimrod's performance envelope. So far, these have been confined to the important cruise and search condi- tions. Although time-consuming, the high-altitude cruise tests only require accurate levels to be flown and these are not usually upset by the weather. For the low-level search case, however, the typical English summer provided far too much cloud and turbulence for accurate performance measurements so the Nimrod was taken to its true environment out to s^ where smooth air could be obtained. With the two outboard engines shut down it sailed quietly along until the time came to re-light. A unique feature permits this to be done very rapidly by using the engine starter—a comforting thought for Coastal crews! In practice, of course, the engines would not be shut down at aircraft weights which would prevent the attainment of an adequate rate of climb in the event of losing one of the remaining engines. In its first two months the Nimrod flew nearly 60 hours. To the test pilot this is the most interesting period in the life of a new aircraft because everything is being done for the first time. It is also a period which generates a big workload for the whole flight test team including the many engineers of different specialist qualifications as well as the flight crews. Each flight requires careful planning, instrumentation results have to be analysed, many written reports are needed and, above all, discussions and meetings are practically non-stop- Many months of systematic flight testing are still required. The second prototype with its search, navigation and attack sysienis joined the programme on luly 31 and all the evidence at this stage indicates that the Nimrod will finally prove to be an outstanding aircraft in the hands of Coastal Command crews. To date a total of over 90 flying hours has been logged by both prototypes.
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