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Workshop and Production Section AIRCRAFT ENGINEERING May, 1944 Heywood pneumatic components are a familiar The Automatic Regulator Valve feature of British aircraft, but the details of the The Hcywood automatic regulator valve operates variou s units are not as well known as they might primaril y through th e medium of a metallic bellows be . The following notes illustrate some of the assembly, th e bellows being subject to the air pres principle features:— sure in the reservoir. When the reservoir reaches th e maximum operating pressure the bellows lift a plunge r against a counter spring to open a valve The Compressor which by-passes the compressor output, t o atmo sphere, thus relieving the load on the compressor. Compressed air systems (Fig. 1) on British air Conversely, when th e air controls are operated and craft normally operate at reservoir pressures of th e reservoir pressure falls, th e counter spring over 300 lb./sq. in. or 450 lb./sq. in., through reducing valves t o th e pressures required a t th e service points . comes the pressure exerted through the bellows, Th e Heywood compressor is actuall y able t o deliver closes th e bypass valve and th e compressor outpu t is air a t pressures u p t o 850 lb./sq. in . The compressor again delivered into th e reservoir. (Fig. 2) is a single-cylinder, two-stage recip A simple safety device embodied in th e regulator rocating machine, air cooled, designed for flange valve safeguards the system against the possibility mountin g direct to the engine or gearbox drive. of bellows failure and provides for th e maintenance Th e weight of the compressor is rather less than of reservoir pressure for so long a s ma y be necessary 5 lb., with a cylinder capacity of 45 c.c. On the unti l attention can be given t o th e regulator. downward stroke of the piston c air is drawn into Oil and Water Trap th e cylinder through the suction valve d in the cylinder head. The suction valve closes as th e pis Th e air supplied from the compressed air system to n reaches the limit of th e downward stroke. On t o th e air services should be free from oil an d a s far as th e upward stroke of the piston, the air in the first possible, free from moisture and for this reason the stage , above the piston, is compressed and during standar d system includes an oil and water trap th e stroke opens the transfer valve in the piston (Fig. 3). head . The air is thereb y transferred into th e second Th e tra p (Fig. 3) is simple, and consists of a light- stag e volume formed by the annular space between alloy die casting fitted with inlet and outlet con th e large cylinder diameter and the small piston nexions and a needle-type draining valve operated diamete r below the top land. On the following b y a handwhccl. downward stroke, the air in.the second stage is Th e compressed air, cooled in its passage from the further compressed and is discharged through the compressor, is conducted to the inlet connexion B delivery valve c on the side of th e cylinder. At the which projects inside th e trap. Here, by reducing sam e time, th e first stage volume is refilled through th e air passage, a high velocity air stream is directed th e suction valve, and the cycle is complete. agains t th e inner wall of a chute A, causing th e oil Th e compressor is designed for normal constant an d water contents t o bo precipitated. Meanwhile, driving speeds of 1,200 r.p.m., with higher speeds for partia l expansion take s place and the moisture and oil-free air passes filter D to leave the tra p at the shor t periods. At ground level th e compressor has outle t connexion C. The liquid contents collect at a n output of approximately 14; cubi c feet of air per th e bottom of th e trap. Around the outlet filter is minut e against 450 lb./sq. in. pressure when driven fitted a baffle which prevent s th e spillage of th e liquid a t 1,200 r.p.m. The time take n t o raise th e pressure content s under all conditions of flight. The tra p is i n a 400 cubic in. reservoir a t ground level from 250 drained by opening the spring-loaded needle valve t o 450 lb./sq. in. is approximatel y two minutes . The standar d compressor is capable of re-charging th e air E, which, by releasing th e handwheel F, returns to secured into a split crankcase b y a flange and locking reservoir a t altitudes u p t o 45,000 ft. or more . It is it s seat automatically. The effectiveness of the ring. The flange is formed around the lower end i n thi s connexion tha t th e specialized features of the moistur e separation is dependen t upon th e air being of th e cylinder and locates in a continuous groove Heywood two-stage single cylinder compressors reasonably cool as i t enters th e tra p and it is there machined in both halves of the crankcase. The show to advantage and are directly applicable to fore necessary for th e trap to be installed in a cool locking ring is screwed on a threaded shoulder on fighter aircraft. The first stage pressure is so place. th e cylinder outer wall and bears against the top of arranged that atmospheric pressures at high alti Engine Starting th e crankcase in such a way tha t th e top face of the tude s are sufficient to refill the first stage volume. flange on the cylinder is held firmly against th e top Th e modern pneumatic system leads t o the con Th e compressor is lubricate d under pressure direct side of the groove in the crankcase. sideration of engine starting by compressed air, from th e engine or gearbox lubrication system. The since th e latest design mark s a considerable advance Th e cylinder is machined from chrome-nickel supply t o th e compressor is metere d a t the oil intake on earlier efforts in this field. The Heywood Com air hardening steel, and the piston, crankcase and an d by collector grooves on the crankshaft journal pan y has been working on the development of a cylinder head arc aluminium alloy die castings. from which th e oil is diverte d to th e big end bearing high-pressure injection starter and have designed Th e outer half of th e crankcase houses a ball bearing an d th e cylinder bore (Fig. 2). Crankcase evacua thi s system to suit aircraft needs. _ It s possibilities which carries the crankshaft and a plain phosphor tion and the oil return to the engine lubrication a s a acro-enginc starter are considerable in two bronze bearing is used on the drive end. The drive syste m is cither direct through the crankcase flange mai n respects. Firstly, in the certainty of starting end of th e crankshaft is hollow, with a central boss or by external pipe connexions. an d secondly in th e reduction of weight. splined internally to engage with th e driving shaft. Considerable attention has been given to design Thi s end of th e driving shaft is also hollow and car Th e air equipment described in these notes is features having a bearing upon.th e weight and com rie s a coiled spring which is held in compression by mad e under British patents, b y th e Heywood Com pactnes s of the compressor unit. In the cylinder/ a circlip. The driving shaft is floating and allows, pressor Co. Ltd., Redditch, Worcestershire, from crankcase assembly, for example, the cylinder is if necessary, for a slight misalignment. whom full particulars can be ha d on request.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: May 1, 1944
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