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180 AIRCRAFT ENGINEERING July, 1932 Some Particulars of the 80 h.p. Seven Cylinder Pobjo y Air-Cooled Radial TH E 80 h.p. seven-cylinder Pobjoy "R " Th e exhaust collector system is specially designed engine, fitted as standard to the twin- to avoid expansion troubles, and is heavily engined Monospar monoplane, is of the chromiu m plated all over. radial air-cooled, geared, dry sump type. The A petrol pum p and drive can be fitted if required. cylinder bore is 77 mm. (3 1/32 in. ) with a stroke of Th e low weight of the engine makes it easy to 87 mm. (3 27/64 in.) , the total volume being 2,835 handl e by two men. In the absence of a suitable cu. cm. The power at normal revolutions, 3,000 wooden stand provided with seven recesses, the r.p.m., is 75 rated and 77-79 actual b.h.p., while engine is most conveniently laid airscrew shaft at the maximum of 3,300 r.p.m. it develops 85 downward s with th e cylinders resting on two blocks b.h.p. The weight, complete with airscrew hub, of wood, bu t not on th e sparking plugs or the y may is 135-140 lb. At normal horse-power th e power- be broken. weight ratio is, therefore, 1∙87 lb./h.p . At full Th e airscrew hub is usually packed separately in throttle the fuel consumption is 0∙62 pints/b.h.p., th e case, and around the airscrew shaft is fitted or 6 gallons per hour; the oil consumption being a dummy tube to prevent the shaft from being 1¼ pint s per hour. accidentl y driven back into the gearcase. This The light alloy crankcase is in four parts, and tub e must be removed before the hub is fitted, and carries the patented single-throw two-piece crank th e engine must never be rested upon the airscrew shaft on four substantial bearings. At the front shaft. Th e two independent ignition systems each end is a plain bearing through which the pressure Th e lid of th e makers' standard packing case has consists of a B.T.H. magneto firing its respective oil reaches the crankshaft. On either side of the a steel framework provided, to which the engine is sparkin g plug in each cylinder through a separate crank-throw are two large roller bearings, and the bolted when in transit. The box itself is merely a H.T . distributor. Shielded ignition can be provided shaft is located endwise by th e rear ball bearing. cover. This lid and stand form a very convenient for use wit h radio. The single crankpin is hardened, and on it runs suppor t for th e engine until ready for installation. Th e Claudel-Hobson carburettor is set for a floating bronze bush running inside th e hardened Prolonged air and bench tests of this engine economy when cruising, but on fully opening the eye of the master connecting-rod, to which are extendin g over thousands of hours have proved throttl e a special power jet is brought into opera linked the six articulated connecting-rods by a tha t sump outlet temperatures up to 110 deg. C. tion . A mixture control is provided for use at patented arrangement giving perfect security and ar e perfectly safe with th e oils recommended. The altitudes . lubrication. temperatur e of the oil upon return from the tank Th e hand-starter mechanism, of th e ratchet type, The aluminium slipper-type pistons run in steel t o the engine should, however, never exceed is completely disengaged when in the normal cylinders to which aluminium cylinder heads are 75 deg. C. nor fall below 50 deg. C., and in some runnin g position. Spring stops limiting its travel screwed and locked by a patented method. Each installations and climates it may be dilficult to in bot h directions are provided inside th e crankcase. head carries one inlet and one exhaust valve, kee p below th e former figure without some form of actuated by ball-bearing overhead rockers carried oil cooler. The engine makers manufacture a small on patented brackets to avoid expansion troubles, simple and neat cooler in two sizes for temperate and operated by push rods. The rockers are dust- LEADING PARTICULARS an d tropical use respectively, and unless specified proof and self-lubricating, but grease-gun attach Bore. 77 num. (3 1/32 in.). one of these coolers will be supplied with each ments are provided for replenishing the grease at Stroke. 87 mm. (3 27/64 in.). engine. infrequent intervals. Instantly detachable alu Capacity. 2,835 c.c. An AC-Sphinx petrol pump type "B " can be minium covers are provided over th e rockers. Power at Normal r.p.m. (3,000r.p.m) 75 b.h.p. (rated); 77-79 fitted on a special driving gear a t the rear of the The reduction gear to the airscrew shaft consists (actual). engine, and being of the self-regulating diaphragm of a pair of substantial double-helical gears. To Maximum Power at Maximum r.p.m. (3,300 r.p.m.). 85 b.h.p. type , may be coupled direct to the carburettor. serve th e double purpose of steadying th e drive and Weight Complete (with Airscrew Hub). 135 to 140 lb. I n th e latter case, however, should the level of the of centrifugally filtering the lubricating oil, a Weight per horse-power. 1∙87 lb./h.p. petro l in the tank be lower than the pump, some patented centrifugal or hollow filter-flywheel is Weight of Standard Exhaust Collectors. 8 lb. provision must be mad e for flooding th e carburettor, secured to the crankshaft alongside its gear. This Fuel Consumption. At 0∙9 throttle and less, 0∙53 pints/b.h.p. suc h as a hand priming pump supplying petrol filter need only be cleaned a t top overhaul. hour. At full throttle, 0∙62 pints/b.h.p. hour, unde r pressure to a T piece in the pipe from pump The duplex oil-pump mounted accessibly at the Fuel Consumption per hour. At 0∙9 lull throttle, 4½ gallons. At t o carburettor. front of the engines scavenges the crankcase and full throttle, 6 gallons. returns the oil to th e tank, whence it is drawn by I n later models a large flange is provided at the Oil Consumption. 1¼ pints per hour. the pressure pum p and delivered at 35 lb. pe r sq. in. rea r of the petrol-pump drive casing, so that if Valve Timing (clearances set to 0∙012 in.) :— into the hollow crankshaf t through the front cover. desired a sub-bulkhead can be attached isolating Inlet Opens 4 (leg. early. The big-end bearing and the cam drum bearing th e petrol pum p from the engine bay. ,, Closes 50 deg. late. are supplied under pressure, the remaining bearings Exhaust Opens 60 deg. early Th e engine crank lever should be operated by a ,, Closes 11 deg. late. being lubricated by splash. The inlets to the two steel cable. The normal force required is of the pumps are protected by readily accessible gauze orde r of 50 lb., so tha t a leverage of 2 to 1 is desir filters. able in th e cockpit.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jul 1, 1932
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