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Aviation History
1951
1951 - 1257.PDF
778 FLIGHT, 29 June 1951 JET AIRLINERS IN SERVICE . . . Turning to the special features of the Jet-liner, the lecturer said the aircraft had a com- pletely automatic pressurization system capableof operating under chosen conditions up to a maximum differential of 8.3 lb/sqin. Thisrather high pressure had been chosen for two reasons: (a) it was desirable for the passengersto have the cabin altitude as low as was prac- ticable, and 8.3 lb/sqin provided a 4,000ftcabin at 30,000ft; (b) the importance of climb- ing and descending as rapidly as possible to andfrom the cruising height in the interests of fuel economy, whilst submitting passengers to aslow an apparent rate of climb as was feasible. The structure had to be tested to at least16.6 lb/sq in, and this meant that the fuselage structure had to be as reliable as the wing itself.Another factor influencing the structure was the speed at which jet aircraft flew. This inevitablygave rise to special structural problems, and methods of design and construction, if theweight penalty associated with greater speed was to be minimized. In the case of the Jetliner,a structure-weight percentage (in terms of the take-off gross weight) of the order of 25 per centspoke well for the job the engineers had done. Referring to the power installation, Mr. Atkinsaid that the Jetliner employed water-methanol thrust augmentation, which provided up to12 per cent thrust increase under hot-day conditions. Aerodynamic features of the Jetliner whichcould be considered as peculiar to jet aircraft design were the unbalanced control surfacesand, whilst these could not be claimed as an advantage by comparison with other types ofaircraft, they were becoming increasingly a feature of high-speed flight. In advancing thecase for the jet transport, the cleanness of air- flow over the airframe, unspoiled by an airscrew,was probably the main point. It gave rise to more effective control, absence of extraneousasymmetric effects, and a general smoothness over the cabin, resulting in absence of vibrationand a pleasanter and generally lower noise level throughout the greater part of the cabin length.By contrast to airscrew-driven types, there was a rise in noise level toward the after-part of the cabin. However, by adequate soundproofing,this need not be higher than that which obtained in piston-engined and turboproptypes. Throughout the remainder of the cabin —which was usually three-quarters of itslength—the superiority of the turbojet was outstanding. In conclusion, Mr. Atkin said that, as aresult of much investigation, Avro Canada had demonstrated that the Jetliner, or any other jettransport, could be operated economically v+U adequate paylr>^d, and at the same time carryfuel reserves £?hich met all normal airline requirements, iftie factors involved in achiev-ing maximum fuP-economy were brought out best by evaluating operation of the aircraft inpassenger-miles per year as the product of block speed and passenger capacity. If theapproximation was accepted that the present- day turbojet consumed 70 per cent more fuelthan the turboprop, and 160 per cent more fuel than a piston-engine, whilst the jet fuel wasroughly two-thirds the cost of piston-engine fuel, then it could be seen that the cost per milewas approximately the same. In other words, the jet aircraft was competitive with the othertypes, while having the added advantage of irtereased speed and comfort. FROM THE ENGINE MANUFACTURER'S ANGLE •pOLLOWING the papers, above, presenting* the view of the operator and the aircraft manufacturer, those of the engine builders•were given by Mr. B. H. Slatter, in The Mamba Engines in the Apollo Aircraft. Mr. Slatter gave a brief description of theMamba and stated that, in building it into a power unit for the Apollo, the main aims hadbeen to keep the frontal area of the nacelle to a minimum, to group all the accessories on theengine unit, and, at the same time, to provide good accessibility. The engine was fullycowled, the maximum diameter of the cowling being 3iin, and the nose cowling formed a3j-gal oil tank. The power plant was divided into three zones by fireproof bulkheads. Theengine, fuel and oil systems, and the airscrew governor and feathering pump were all locatedround the compressor casing, and ready access to the accessories for servicing was afforded byhinged and detachable cowlings with toggle fasteners. Drive was taken from the engineaccessories casing to a bevel box in the leading edge of the wing, where the drive was bifur-cated, and taken outwards to subsidiary gear- boxes carrying generators, cabin blowers andhydraulic pumps. The power plant was easily detachable from the airframe by disconnectionof the auxiliary drive-shaft, fuel and electrical services and removal of four mounting bolts. Engine-control Problems After referring briefly to the ventilation ofthe nacelle and touching on the fact that turbine units required higher-capacity ground-startingfacilities than did piston engines—in which con- nection it would probably prove advantageousto employ higher voltage starting systems in order to minimize the weight of cables and con-nections—the lecturer dilated upon controls. It had been necessary to decide first of allwhat proportion of the total power output should be given to the airscrew, and whatshould be left in the exhaust jet. Calculations and tests had shown that for air speeds up to350 m.p.h. the maximum thrust-horsepower would be obtained by converting' about 85 percent of the available power into shaft horse- power for the airscrew. Unlike a jet unit withfixed propelling nozzle, the rate of fuel feed to a turboprop and the speed at which it ran werenot fixed automatically by the characteristics of the engine. There was a variety of possibleways in which the engine could be controlled, but it was decided to govern speed by means ofthe airscrew (as in piston-engine practice), and to govern power by scheduling the fuel supply. As the speed and power of the engine wereindependent, it was next necessary to choose the relationship between these variables. Thepower/speed characteristics of a turbine were considerably less flexible than were those ofa piston engine. In the lower part of the speed range, the power capabilities were limitedeither by the maximum temperature which the turbine materials could withstand, or by the possibility of overloading the compressor, whichresulted in breakdown of the aerodynamic flow. A "working line" was thus chosen whichenabled the engine to develop any given power with the minimum fuel consumption. Sincethe speed of the engine was governed by a v.p. airscrew, it was quite simple to arrange for theengine speed to approximate to the "working line" in night, even when the engine power wasreduced to zero. On the ground, however, it was desirable that the engine idling speed shouldbe lower to avoid excessive idling thrust, and to keep fuel consumption low. The ground idlingspeed (approximately half max r.p.m.) was, in fact, the speed corresponding to the fuel flowwhich gave satisfactory engine starting. Be- tween the ground and flight idling speeds, itwas possible to over-fuel the engine if the throttle was opened too rapidly, and this hadcaused some trouble but, by the introduction of a temperature control, a satisfactory remedywas found. There was no need to provide the pilot withseparate control of speed and power; this would, in fact, be unacceptable, since therewould be nothing to prevent selection of an impossible power at low speed, with consequentdamage to the engine by overheating. Thus, to achieve the desired relationship, the controllever of the fuel metering unit and the airscrvw governor control lever were both connected tothe pilot's throttle. The connection to the airscrew governor was made through a servosystem which, in effect, enabled the governor to anticipate the change in power which wasbeing made. The high-pressure cock in the fuel system was connected to a separate leverin the cockpit which was also linked to the feathering over-ride on.the airscrew governor.Thus, if the lever was operated in flight, the fuel supply was cut off and the airscrew controlsimultaneously over-ridden to select coarse pitch. As the engine fuel requirement varied withthrottle setting, air-intake pressure and tem- perature, it was obvious that a sensitive andaccurate control was required to achieve the accuracy of temperature control desired at, ornear, full power when th,s engine was operating close to the maximum allowable temperature.Furthermore, the Mamba jet-pipe temperature changed about 3 deg C for every one deg Cdeviation in ambient air temperature from standard. To meet these difficulties, a fuel meteringcontrol sensitive to jet-pipe temperatures was developed. It comprised an electrical amplifier,fed with signals from thermo-couples in the jet- pipe, and from a temperature selector linkedwith the throttle. The output of the amplifier controlled a fuel valve. Although a magneticamplifier had been chosen in the interests of reliability, it was not desirable to rely entirelyon a somewhat complicated electrical system. Consequently, the temperature-control func-tion was limited to about 30 per cent of the total fuel flow, so that it acted as an accurate trimmer of the operating temperature, but any failur. ofthe temperature control could not have a catastrophic effect on the power of the engine. Mr. Slatter then went on to give a briefdescription of the airscrews used on the Apollo and next discussed re-starting in flight. Insome ways the turboprop was easier to start quickly in flight than was a turbojet, owing tothe fact that the airscrew could be used to accelerate the engine rapidly. In effect, it wasonly necessary to unfeather the airscrew with fuel and ignition turned on. However, becauseof the relatively high inertia of a turbine engine, rapid unfeathering could result in a momentarybut considerable amount of drag while the aii screw fed power into the engine to accelerateit. Furthermore, should the combustion chambers fail to light, the drag would remainas the airscrew continued to windmill the engine at high speed. However, the reverse torque switch dealtadmirably with re-starting. In effect, it con- trolled the rate of unfeathering the airscrew,prevented more than 50 h.p. being fed into the engine, and automatically arrested unfeatheringwhile the blade pitch was still quite coarse, until such time as *'.ie combustion chamberslighted and th--,^ .ie began to develop power. Modifications for extra power Mr. Slatter'then told how, by increasing themaximum r.p.m. to 15,000, and altering the aspect ratio of the blading, the output of theMamba had been lifted to 1,250 h.p., and went on to say that when the modified engines wereinstalled in the Apollo, the jet-pipe diameters would be expanded up to the maximum sizepermitted by the wing spars in order to revert as far as possible to the intended distribution ofpower output between the airscrew and the residual jet. Various methods of ensuring reliable opera-tion under icing conditions had been considered, and the use of a methanol spray in the engineintake had been chosen for the Apollo. A single-spray jet was fixed to the periphery of theairscrew spinner, so that the methanol was dis- tributed around the annular intake, the rate offlow being from 10 to 40 lb/hr, depending upon the severity of the icing. Ice formation couldnot occur on the leading edge of the engine cowling, as the nose cowl was the oil tank andwas therefore kept warm. It was interesting to note that, although it had originally been fearedthat the small and relatively fragile blades of the Mamba compressor might be particularly sus-ceptible to damage by ice, the engine had, in fact, suffered no damage under natural orartificial icing conditions in flight, even when no anti-icing protection had been provided. In conclusion, Mr. Slatter observed that,although an engine might reach a satisfactory state of development on the test-bed, thereremained problems associated with its inclu- sion in a suitable power plant for associationwith an aircraft, and with its operation in flight.
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