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Aviation History
1948
1948 - 0087.PDF
JANUARY 15TH, 194S FLIGHT 75 Aircraft Pneumatics with the size of pipe normally in use, the fracture of a reason-ably clipped pipe is a harmless local event. The pipe wall thickness of commercially available tubing, coupled with thethickness associated with practicable handling, gives a most reassuring safety factor. While in theory the moisture content in the compressed air,small as it is after the oil and water trap, should indicate a copper-base pipe, because of corrosion on the untreated inter-nal walls of light-alloy pipes, so far practice has shown that light-alloy pipes have a satisfactory life. Too much care cannotbe expended on the piping and pipe joints of a pneumatic sys- tem, since the system's satisfactory performance can dependon the air-tightness of each joint. The function of the pressure-maintaining valve is to isolatethe actuating system if the pressure drops below brake pres- sure usually 220 p.s.i.—so that the air in the main air bottlesis available for brakes. In this case operation of the flaps /nd undercarriage is effected by using the emergency air, whichi*Ls taken to the rams by a separate pipe line. At the ram the emergency line is connected to a shuttle valve which, on opera-tion by the emergency air supply, isolates the ram from the normal circuit. Thus the emergency circuit is quite separatefrom the normal air lines and does not rely on the safe work- ing of any valves and pipe line which are in that circuit. Theweight of the emergency air line is considered worthwhile in view of the. addi- tional safety which isgiven. From Fig. 2 it will beseen that the emergency air is introduced to thenormal system by a single TO BRAKES REDUCING VALVE Fig. 2. This diagram shows an alternative arrangement of the emergency air supply. This, though lighter than the circuit shown in Fig. I, is not so safe since it has no separate emergency pipe line. TCSERVICES shuttle valve. The saving in piping will be appreciated, butthis assumes that all joints will remain leak-proof and that all s of equipment will be in perfect condition. While this^position should be correct, it does not allow for any •i.jage to the pipe line or to any item of equipment; it is sug-sted, therefore, that this arrangement to some extent nega- uves the installation of the emergency system. The use of aspecial bottle, charged at higher-than-system pressure, leads to difficulties in charging and the adoption of non-standard re-ducing valves. This applies equally to a small high-pressure CO, bottle which, at very low temperatures, suffers from apressure reduction. ;.. -, ; .*.-. • Weight Saved by High Pressure By using a 1,000 p.s.i. pressure for the generating system, and for the major load operation, the system can be by the use of a reducing valve only on the side of the ram where low- pressure air is used, as shown in Figs. 3 and 4. It will be appreciated that in this scheme the ram-pressure will fall, de- pending on the storage bottle capacity, but it can be arranged that the temporary minimum pressure on which the ram will work will be approximately 800 p.s.i. This means a great difference in the size of the ram, and the ram weight saving can be substantial. For instance, a flap ram having a com- bined stroke of 6.5m weighs 1.56 lb; the equivalent weight ofa 35° ps.i. ram is approximately 4.5 lb for a load of 1,100 lb. In a recent layout prepared for a designer it was possible to offer the alternatives of a 600 p.s.i. and a 1,000 p.s.i. system, and for this twin-engined layout it was found that a saving of approximately 10 lb was made by using the 1,000 p.s.i. system, the system weights for components only being 38 lb and 48 lb respectively, each exclusive of brake control valves. These weights included the estimates for the rams. Proposals put forward to another designer showed a saving compared with his estimate of the equivalent hydraulic circuit; the suggested pneumatic system weighed 38 lb, the weight given for the proposed hydraulic circuit was 72 lb. In another case the comparison with electrical actuation of flaps and tri- cycle undercarriage showed that pneumatics would weigh approximately 75 1b, electrics 240 lb. In this case, to indi- vidual weights of motors and control gear was added the weights of torque tubes, bearings, bevel gears and steel screw jacks; the local stiffening of the airframe for these components was not assessable. Due to geometrical design or aerodynamic loading, it is fre- quently possible to arrange that the e'ffort exerted by one side of the ram can be achieved by using air at a lower pressure whereby a saving in air consumption is effected. Fig. 4 shows how this can be arranged with existing standard components. Due to the reducing valve being in effect a non-return valve, a b5'-pass has to be introduced which includes a non-return valve set to pass air in the opposite direction only. This added complication unfortunately introduces yet more com- ponents and pipe jsints. To simplify this layout, and to stan- dardize a basic unit, the Hymatic Engineering Company, Limited, recently introduced a new multi-purpose valve. .-"•:.•::• -'.:-;-.d:_ New Multi-purpose Valve : v #-s This new valve is designed to give a free return flow when the selector side pressure is reduced to atmospheric, so that the non-return valve shown in Fig. 3 is unnecessary. In addi- tion, if this property is not required, the return 'flow is can- celled by an alteration to the delivery union. Again, if it is a circuit wh;-re a pressure-maintaining valve is followed by a re- FROM ducing valve, as in Fig. 1, then RESERVOIR by an alteration to the inlet union, the two valves may be combined in one with an increase in weight of only approximately x>.i2 1b on EXHAUST that of the reducing valve, giv- ing a nett weight reduc- tion of 0.32 lb. To this simplification must be added the weight and FROM RESERVOIR Fig. 3. Frequently it is possible to use a reduced pressure to operate one ' side of the ram, so em- ploying the expansive pro- perty of air. cost of pipe and pipejoints to appreciate fully the importance of thisnew valve. So far, attention hasbeen directed to the more commonly operatedunits, but compressed air has been applied success-fully to other services. Air-operated fuel jettisonvalves have been de- veloped, using a single-acting or spring-return ram. In this application the valve is held closed by the spring acting internally on one side of thepiston. To operate, air is applied to the opposite side of the piston. Spring-loaded rams are also used for various enginecontrols such as hot and cold air intake, air filter and idle cut-off. A typical ram for this is shown in Fig. 6 which, witha bore of 1.4m diameter and a stroke of 2in working on 450 p.s.i. has an air-operated thrust of 500 lb, while the spring-return gives an initial load of 30 lb rising to 84 lb on compres- sion, the ram weight being 0.94 lb. Considerable thought has been given to the development ofpneumatic rams since, unlike hydraulic rams, the operating medium does not provide its own lubrication of the piston.Therefore, in developing rams which would operate satisfac- torily through the complete temperature range, much researchhad to be done both in the composition of the rubber used for the sealing glands and in the form of grease applied to theram during assembly, which must last the life of the ram. It will be appreciated that with compressed air the sealingdifficulties are much greater than with hydraulics, since air will, at equivalent pressures, escape where hydraulic fluidapparently will not. Therefore, the rubber used must be capable of retaining its flexibility in sealing throughout the SELECTOR -H EXHAUST PRESSURE REDUCING VALVE. RAM Fig. 4. By improved reducing- valve design it is now possible to delete the non-return valve shown in Fig. 3, and so simplify and lighten the circuit.
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