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Aviation History
1953
1953 - 1572.PDF
726 FLIGHT, 4 December 1953 HIGH-ALTITUDE COCKPIT COMFORT Design-problems Discussed in an S.A.E. Lecture PROBLEMS arising in the pressurizing and air-con ditioning of fighter cockpits formed the subject of one of the lectures delivered at the recent National Aero nautic Meeting, held at Los Angeles, of the Society of Automotive Engineers. The paper was by Mr. George A. Lemke, a design specialist of Consolidate Vultee Aircraft Corporation. The lecturer's aim was to clarify and co-ordinate certain factors in this sphere of design-activity. Basically, he pointed out, the air- conditioning system had to provide cooling for an electronic installation which produced 60 times more heat than the pilot's own body, as well as anti-icing, windscreen demisting and rain clearance. These functions were carried out by a system utilizing air bled from the engine compressor, ram-air-driven turbines and heat-exchangers (Fig 1). Since the electronic equipment was designed for air cooling and required four times the cooling needed for the cockpit, it was essential to use air-cycle-type refrigeration. The requirement for constant cooling-air flow at varying engine speeds, altitudes and outside air temperatures necessitated a turbine with a wide operating range and therefore with variable-area nozzles. Further more, the electronics required four separate types of alternators and generators which, for convenience, should be remotely driven with a single speed control for all four. It was found that both for weight and power-consumption a turbine driven by compressor air bleed was preferable to a direct engine accessory drive. The generator driving turbine was given the name A.T.M. (air turbine motor). The A.T.M. was located after the heat-exchanger, since it had been found that the flow of air bled from the compressor was still sufficient, after passing through the heat-exchanger, for the turbine's power requirements. With such an auxiliary power source all the aircraft's electronics and other services could be ground-run with a normal ground compressor for test and repair purposes. Both pressurization and cooling were provided by the air bled from the engine compressor and led through the ram-air heat- exchanger and then the A.T.M. expansion turbine. These two together reduced the air temperature sufficiently for cockpit- cooling purposes, while still providing a sufficient mass flow for pressurization. The majority of cooling air passing through the A.T.M. was used in the cockpit, but a proportion was led through electronic compartments for cooling [as illustrated below, Fig. 1]. Primary flow, however, passed into the cockpit, cooled and pressurized it, and was then led to electronic comoartments, to the A.T.M. compart ment and finally to the engine-accessory compart ments. When the air returned to the accessory com partment it had a temperature of 160 deg F, which was still cool in comparison with the 800 deg F mi the engine itself. A cockpit pressure regulator (shown in Fig. 1) maintained sufficient back pressure to pressurize the cockpit while yet allowing an adequate cooling flow to reach the various compartments to be cooled. Mr. Lemke then gave an explanation of the characteristics of pneumatic power drives, follow ing which he turned his attention to the actual operating conditions through which adequate cooling, pressurization and services would have to be maintained. He dealt first with the power require ment. A critical condition, he said, was during idling at extreme altitude. This condition—met mainly during let-down, throttled back, from altitude—was only transient in that power delivered improved as denser air regions were reached. The following com promises could be allowed: A.T.M. power-loss could be treated as a calculated risk; automatic monitoring of non-essential electrical loads could be provided; non-essential electrical loads could be cut out by switches on the throttle control; and cabin pressure could be reduced with throttle closed. Conditions arising during approach to land could be dealt with similarly. For cooling, Mr. Lemke denned four conditions : die ratio of turbine inlet pressure to discharge pres sure, the effectiveness of the heat-exchanger, the efficiency of the turbine and the ram cooling-air temperature. The critical condition would be during high-speed flight at sea level on a hot day. Ram air would be hot and mass flow high enough to close the turbine nozzles to the minimum position, causing considerable reduction in cooling efficiency. The follow ing compromises should be considered: to allow cockpit tempera ture to rise, taking advantage of the temperature lag in both equipment and pilot; to inject water into the heat-exchanger ram airflow; to inject water into the A.T.M. discharge, reducing temperature but increasing humidity; or to use a second heat-exchanger. So far as cooling of electronic equipment was concerned, Mr. Lemke thought that the best progress could be made with equip ment which would operate at higher temperatures rather than with increased cooling. While such equipment was being developed he suggested the following compromises: that materials sensitive to high temperatures should be eliminated; that com ponents which create a high temperature should be insulated by using shielding and reflective or absorbent surface finishes; that evaporative cooling systems should be used as an alternative (such devices were already under development) and, finally, that reduced cooling during transient conditions of short duration, taking advantage of temperature-lag in the equipment, should be allowed. Concerning the heat-exchanger itself, Mr. Lemke defined the optimum ratio of ram flow to compressor-bleed flow as 3 to 1 This ratio, however, could not be maintained, since compressor- bleed mass flow increased with altitude whereas ram airflow decreased. The varying ratio of flows with altitude led to a reduction in temperature-drop and a decrease in power per lb of air in the A.T.M. The following compromises couM he rmde in selecting the heat-exchanger: to increase the size of the heat exchanger (within space and weight limitations), or to provide variable efflux area for the ram cooling air duct. Mr. Lemke next dealt with cockpit "inside fog" caused by water entrainment. This occurred when the A.T.M. discharge tem perature dropped below the wet-bulb temperature corresponding to the pressure in the cockpit. The water in the air reached its dewpoint and condensed, forming water vapour. Reduction of cockpit visibility through inside fog was, for obvious reasons, Fig. 1. The cockpit conditioning and ancillary power system described by jthe lecturer. This system is typical of modern American practice. RAM AIR" ENGINE COMPRESSOR 40-70 1/ MIN ENGINE TURBINE - THRUST "(REDUCED APPRO* ' IX PER IX BLEED] I 30-60 »/MIN 180-90 l/MIN HEAT EXCHANGE BY PASS -160-500 -*7MIN -RAM AIR WINDSHIELD RAIN | CLEARING AIR TURBINE MOTOR 5 PAD GEAR BOX ACCESSORY DRIVE ANTI-ICING 1 TAIL WING ENGINE DUCT FLOW CONTROL SPILL VALVE DEMIST EXCESS AIRFLOW 25 */MIN SPLITTER VALVE COCKPIT PRESS. REGULATOR 15J/MIN AFT ELECTRONICS FWD ELECTRONICS 1-2 l/MI N MISC. 3000 osi COMP. CANOPY SEAL FUEL TANK PRESSURIZATION MISC. TAN!' PRESS. ATM COMPARTMENT ENGINE ACCESSORIES COMPARTMENT -OVERBOARD
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