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Aviation History
1948
1948 - 0086.PDF
74 FLIGHT JANUARY 15TH, 1046 AIRCRAFT PNEUMATICS Some Current Applications DURING the recent war, advances were made in the appli-cations of compressed air to aircraft auxiliary powercircuits. Originally used for powering wheel brakes and guns, compressed air operation was later extended to variousengine controls. These extensions in the use of pneumatic components were largely rendered possible by the developmentand large-scale production of a suitable compressor of high per- formance and low weight. Satisfactory performance of pneu-matic systems in Service aircraft led to their installation in almost all British aircraft built during the war, and the possi-bilities of extending the use of pneumatics still further were recognized by aircraft designers who had an eye to the futureon medium-sized civil aircraft and trainers. By th«- f-nd of tho war, more than one designer had adoptedpneumatics for his main ancillary power supply to operate, in addition to v. heel brakes, the flap andundercarriage rams and local extensions such as engine priming. In each case asaving in weight was found by compari- son with other power forms. Depend-ing on designers' choice, these systems worked on pressures of either .130 or600 p.s.i. In most cases these generating system pressures were reduced to a lower figure for ram operation to take advantageof the expansion factor in compressed air, so that any normal drop in bottle pressure is not reflected in the ram. Because Service Embodiment Loan Equipment was laterbased on a compressor pressure of 450 p.s.i. some trainer pneu- matic systems were designed around this equipment, and areasonable weight-S-aving by comparison with other power sup- plies was found. This advantage, coupled with those of a re-duction in the number of power services, a greatly reduced fire risk and a clean operating medium, has led to the adoption ofcompressed air at higher pressure in more recently developed training aircraft. In civil aircraft the same trend appears, in that with develop-ments in compressor design it has proved economical in some cases to increase the working pressure, since a system weightreduction is achieved. The final choice of pressure, there- fore, rests largely with the designer, who must decide on theeconomics of the aircraft and on the availability of servicing equipment on the routes on which their products will fly.Present trends show, however, that for some time to come the economical generating pressure for most aircraft wouldappear to be 1,000 p.s.i. and this pressure is being accepted By 7 A. H. A. BASTABLE, A.M.I.Mech.E. A.F.R.Ae.S., Grad. I.P.E by the operating^ companies both at home and overseas.When designing an aircraft pneumatic system, the basic aim should be simplicity, since this means, in most cases, theleast weight, lowest cost and reduced maintenance. There are cases where secondary considerations due to undercarriage orflap characteristics necessitate additional equipment for satis- factory operations. In Fig. i is shown a, pneumatic circuitbased on a twin-engined installation. Fig. 2 shows an alterna- tive approach, while Figs. 3 and 4 show yet other possiblesolutions. The merits of each circuit will now be discussed. Tracing the circuit shown in Fig. 1, it will be seen thateach compressor delivers air, not only to the two main ait bottles, but charges the emergency air bottle when necessaryJBtThis storage, system enables either compressor to charge ail ^ three bottles, if required, without loss of air through the othergenerating units, since there is a noB- return valve incorporated in the regu-lator next in line to the bottles. Fre- quently it is possible for the emergencybottle to be of the same size as the main units, due to its limited require-ments, BO that spares are reduced. From these comments on the generating and storage sys-tems it would appear that a likely solution to the location of the components would be for them to be housed in the enginenacelle, since there is often space to spare aft of the firewall for the bottles. The three bottles are interconnected by trans-verse balance pipes which, on 1,000 p.s.i. systems, can be quite small. This positioning of bottles eliminates the prejudice ofsome designers to high-pressure air storage in the fuselage. The control valves can then be conveniently locatpd on some smallpanel. At this point it may be observed that while as short alength and large a bore of pipe as practicable is desirable on the generating side, considerable weight-saving in piping can beobtained by the use on the control and actuating circuit of small-bore tubing This applies particularly to 1,000 p.s.i.circuits, where that pressure is fed to the ram. With the reduction in pipe size the unions are correspondingly reducedin size and weight. Criticism has been levelled at the use of high-pressure air linesin the cockpit on the grounds that if these fractured the line would whip, seriously damaging the airframe, while the noise ofescaping air would alarm the passengers. Tests have shown conclusively that this criticism is quite unfounded, and thsfc, . : :- EXHAUST NRV ANTI-FREEZE RELIEF "^ ' '- -" \/ J UNIT. VALVE COMPRESSOR AIR RESERVOIRS •! OIL & WATERSEPARATOR TO BRAKES 'NOT ALWAYS NEEDED IN O PRESSURE MAINTAINIMG VALVE £^ VALVE SELECTOR OTHER SERVICES SELECTOR F.B.= FLOW RESTRICTOR S.V. = SHUTTLE VALVE NBV= NONRETURN VALVE U'C RAM FLAP RAM Fig. I. For the circuit shown (generating pres- sure 600 p.s.i., actuating pressure 450 p.s.i.), the weight of the compon- ents, excluding rams and piping, is 44.8 Ib. If 1,000 p.s.i. generating and ram pressures were used the comparative circuit weight would be 41 Ib.
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