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Aviation History
1931
1931 - 0535.PDF
FLIGHT, JUNE 5, 1931 anced POWER CURVE: against thisthat, with the 0 r , ','. c 11 e r eccentricallyolaccd even cooling of the cylinders is difficult. Prac-tical experience with the poL,joy does not appear tobear this out, as no cooling troubles have been encoun-tered, or, if they were, they have now been overcome,the engine passing its type tests without trouble.Po b j oy Airmotors, Ltd., have, in conjunction withthe Comper Aircraft Com- pany, carried out experi-ments with a special type of cowling which promisesto give extremely good re- sults both in the matter ofreducing drag and in keep- ing the engine cool. More-over, the excentric placing of the airscrew does notappear to interfere with the symmetry of the cowl-ing, as the photographs of the Comper '' Swift '' show. The cylinders of the Pob-joy " R " engine have steel barrels, with aluminium heads screwed on and lockedin place by a special patented method. The pistons are of the slipper type, and are made of special hard-wearingaluminium alloy. The combustion chamber of the cylin- ders is of very clean design, a fact which contributes tothe large power output at high speeds, while the effective arrangement of the cooling fins gives freedom from over-heating and detonation. There are two valves per cylinder. In the design of the overhead valve gear, special carehas been taken to avoid wear in places where this usually occurs. The tappets retain their adjustment for prolongedperiods, and a patented compensating arrangement ensures that tappet clearances remain nearly constant at all enginetemperatures. The crankshaft is built in two parts, secured togetherby a special form of patented joint. The crank pin is hardened, and the floating case-hardened wrist pins areheld in the hardened big-end ring by a patented, quick- release, positively-locked joint. All bearings in the engine,valve rockers included, are of either the ball or roller type, with the exception of those of the big end, the cam ring,and the front end of the crankshaft, which are carried on floating bushes and supplied with an ample quantity ofoil at a pressure of 35 lb./sq. in. To assist in steadying the airscrew drive, a small flywheel is secured to the crank-shaft alongside its gear wheel. This flywheel also acts as a centrifugal oil filter, the oil passing through this filterbefore going to the various bearings. This centrifugal filter separates out the very finest particles of foreign M 6! fr ^77 K iw 72 71 a / / • / / / — / i / / / FUEL CO / / / —— ~~~—^- __ • ——^~-^i o I I M«O »TOO *»O0 «»OO 3«0o 3>»© z»oo i-Jwv i*- iMRSCRCW R.P. M SO r* 91 U 22 y J tto 65 j fin6 m J. POBJOW "R* TYPt CNGINC matter, and contributeslargely to the durability of the bearings and othermoving parts. The induction system ofthe Pobjoy "R" is somewhat unusual, and consists, aswill be seen from the illus- trations, of a " spider," inwhich all the induction pipes are of the same lengthand symmetrically/ arranged. In the centre they meet ina common exhaust-heated " hot spot," which is fedby a single-barrel car- burettor. The inlet mani-fold forms the back cover of the crankcase, and iswarmed by the hot oil. Full dual ignition is pro-vided by two B.T.H. mag- netos firing two plugs percylinder via seven-point distributors. A handstarter is fitted, operated by cable from the cockpit,and to start the engine it is only necessary to turnit half-a-dozen times with the petrol turned on andthe throttle and air strangler closed, when an easy pull on the hand starterlever starts the engine with one pull. It is never necessary to touch the propeller when starting up. Pressure feed lubrication is used, with two gear-type oilpumps, one being a scavenge pump, which returns the oil to the service tank, and the other a pressume pump whichdelivers oil to the bearings via the centrifugal oil filter. Two gauze niters protect the inlets to the two pumps, andcan be removed and cleaned without disturbing anything except the two screwed caps which hold them in place.The centrifugal filter flywheel is exposed for cleaning by removing the front cover of the crankcase. The oil pumpsand gauze filters form a complete unit. The airscrew reduction gear consists of a robust pairof double helical gears, which are remarkably silent, so that, in conjunction with the slow-running airscrew andvery silent overhead valve mechanism and the effective exhaust collector, the Pobjoy " R " type engine is quiteunusually silent when fitted in an aircraft. A special type of exhaust collector has been designedfor the Pobjoy engine, in which one branch collects the exhaust from three cylinders and the other from fourcylinders, and discharges through two pipes underneath the fuselage of the aircraft. This exhaust collector, as well ascowlings, oil tank and cooler, petrol pump, etc., is supplied with the engine, and the makers have preferred to shoulderresponsibility by supplying the engine complete in every way, even, if desired, to the engine mounting, so that, ifhe wishes, the aircraft designer need not concern himself THE POBJOY ENGINE IN A COMPER " SWIFT" : Actually these photographs show an earliermodel, but the "R" type will be fitted with a similar cowling. 497
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