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Aviation History
1962
1962 - 0701.PDF
Most who mocked Bristol Aircraft on the lateness of the I88's first flight did not have to work around the clock in the rain ahead of further flameholder gutters. Mixture strength is thus kept approximately constant, and the effect of selecting increased aug mentation is to spread the reheat combustion radially until at full reheat it fills the entire pipe. Peak reheat temperature is in excess of 2,000 nK, and de Havilland believe that the thrust augmentation achieved is nearer to the theoretical maximum than that attained in any other powerplant without wasteful reheat fuel consumption. Notwithstanding the very high reheat temperature, all metal parts are held well below 1,000°C by a film-cooled heat shield inside the pipe. This shield is flexibly suspended, and is of "wiggle-strip" construction in Nimonic 80. Design and development of an intake capable of achieving opti mum performance over a wide range of Mach numbers is a laborious undertaking; the nozzle, although apparently simpler, poses problems at least as great. It is impossible to make a nozzle perfect for all anticipated flight conditions, but with clever mechanical and aerodynamic design a practicable nozzle can be made which approaches the theoretical optimum very closely. After passing through numerous development stages the Bristol 188 nozzle has at present assumed the form shown in the illustra tions. The basic nozzle is of the single hinged-petal type, supple mented by a fixed divergent section aft of the petals. Air is bled from the main intake and, after cooling and ventilating the nacelle on its way aft, is passed out between the petals and the fixed nozzle. This secondary air stabilizes the main jet flow and reduces the base drag in the cases when the petals are not fully open. Like most of the nozzle assembly, the ring of petal flaps is fabricated in Nimonic 90. Eight of the petals are positioned by pneumatic actuators fed with engine bleed-air; the ram pistons do not move, but the actuator bodies, which are cast in pairs, are linked to a petal at either end. Each of the eight powered petals is thus positioned by two double actuators, to guard against drive failure, and interleaved between these eight petals are eight idler flaps which move with them. Similar engines installed in the D.H. Engines Javelin test-bed have petals actuated by ICI Silco- dyne H, a silicone-based fluid stable from —65 to 600°F. In all non-reheat conditions the flaps are closed to form a conver gent nozzle. As reheat is brought in the resulting change in turbine pressure-ratio is sensed by the control system faired beneath the nacelle, which progressively opens the nozzle until with maximum reheat at Mach numbers up to 1.4 the petal flaps are parallel. At higher Mach numbers the petals open outwards until at the maxi mum design Mach number they are aligned with the divergent (fixed) outer nozzle. In some respects, the development of a supersonic intake is an easier task, although great effort is needed to perfect a control system which not only varies the geometry according to the flight regime but also safeguards the installation against engine failure. Below about Ml.4 it is possible to employ a direct pitot intake. At greater speeds the loss in pressure-recovery due to the normal shock becomes unacceptable, but up to about M2.2 it is possible to achieve adequate efficiency by making an inclined shock with a fixed centrebody spike (or, as in the F-104, half-body) and control ling the swallowing capacity by a variable bleed system. Above the M2.2/M2.5 level such an intake becomes inadequate, and re course must be had to a variable centrebody and diffusion through at least three Shockwaves. Fully optimized intakes may well be developed with the 188 in the course of time, but the present pattern has a fixed geometry. On the other hand, carefully designed bleeds and auxiliary intakes not only match the swallowing capacity of the intake to the de mands of the engine, but also preserve a satisfactory Shockwave system under all supersonic conditions. At low forward speeds it is impossible for a supersonic intake to take in the maximum airflow required by the engine unless a system of auxiliary intakes is provided. In the existing 188 intake additional air is admitted through ten rectangular "e.g.r." (engine ground running) intakes surrounding the diffuser upstream of the engine. Each of the ten ducts is provided with a door which closes flush with the cowling at high forward speeds. Below about M0.7 the pressure in the diffuser is lower than ambient, causing the ten doors to spring inwards past a dead-centre and thus increase the effective intake area by some 60 per cent. These auxiliary intakes overcome the problem of insufficient swallowing capacity at low forward speeds. At the upper end of the speed range the high indicated airspeed can ram in air greatly in excess of engine requirements, and, although some of this surplus can be put to good account as a secondary ventilating flow (as presently described), by far the greater part must be discharged overboard. To do this efficiently without incurring a severe drag penalty a second ring of apertures is provided in the diffuser section, leading to ten aft-facing exits controlled by spill-valves The latter are normally shut, but when intake swallowing exceeds engine demand they are opened mechanically. Control of the spill-valves depends upon a system which senses the position of the normal shock inside the intake. An aft-facing pitot is stationed at the correct shock location, and its pressure is compared with that sensed by a static orifice on the centrebody spike. Should mis-matching tend to move the shock forwards, the sudden increase in dP across the pitot/static system triggers a servo which opens the spill-valve flaps by means of a bleed-air motor and a chain-drive to individual screwjacks. Within the nacelle there are no firewalls, and the entire area is fully ventilated by a large airflow bled from the diffuser. Below about M0.5 the diffuser ram pressure is lower than the static pressure inside the nacelle, and in order to maintain the proper nacelle airflow (and prevent hot gases from being ingested through the nozzle) engine bleed air is discharged from aft-facing perfora tions in an inducer ring surrounding the sprung fire flaps where the flow leaves the diffuser. Although already heated by ram com pression, the nacelle airflow. carries away high heat energy from the jetpipe and afterburner, which is partially converted into useful thrust in the nozzle. Immediately aft of the combustion chamber the flow is divided radially by a titanium heat shield, which substan tially reduces nacelle metal temperatures. The large bulges above each cowling are fairings over temporary fire-suppression bottles. Development of the complete powerplant has been assisted by
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