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Aviation History
1931
1931 - 0534.PDF
FLIGH'J, JUNE 5, 1931 FRONT AND REAR VIEWS OF THE POBJOY " R" ENGINE : Note that the propeller hub is raised in relation to the centre of the engine. THE POBJOY "R" TYPE ENGINE THE Pobjoy " R " engine,which was illustrated in ourissue of April 3, 1931, hasrecently passed the type tests imposed by the British Air Ministry,and as a result the engine is now avail- able for use in aircraft, with fullCertificate of Airworthiness. Remark- able features of the Pobjoy " R "type engine are the very low specific weight (1.73 lb./h.p. on "normal "power), and the fitting of a reduction gear which allows of good propeller efficiency, thus giving very good thrusthorse-power for take-off and climbing. The normal engine rating is 75 b.h.p. at 3,000 r.p.m., and the maximum per-missible speed for short periods is 3,300 r.p.m., at which speed the engine develops 85 b.h.p. The type tests werecarried out without a single hitch, and without replace- ments of any kind. Uponbeing stripped, the engine was found to be in excel-lent condition. During the 50 hours' run neither theK.L.G. plugs nor the con- tact breakers of the B.T.H.magnetos were cleaned or adjusted in any way. Thepetrol consumption was re- markably low, i.e., 0.53pint per b.h.p. per hour, which corresponds to about4J gallons per hour at cruising revs. This veryexcellent fuel consumption is doubtless due in no smallmeasure to the Claudel- Hobson carburettor, whichhas been specially designed for the Pobjoy " R "engine. One somewhat re- markable feature of thetype tests was that they were carried out with thespecial Pobjoy exhaust col- lector in place, so thatfuture purchasers may re- gard the silencer as havingbeen type tested as well. Having passed the British Air Ministry Type Tests recently, this engine is now available for use in light aircraft, with full Certificate of Airworthiness. For an overall diameter of 647 mm. and a weight of only 130 lb., the engine develops a normal power of 75 b.h.p. at a speed of 3,000 r.p.m., while the reduction gearing reduces the corresponding propeller speed to 1,400 r.p-m. THE POBJOY "R" TYPE ENGINE: The exhaust collector shown was in place throughout the type tests of the engine. •<•••• 1 v 496 The fitting of this exhaust collectorreduces the noise very greatly, and prevents oil being thrown back on themachine. In fact, the engine is re- markably clean in this respect.Although the Pobjoy " R " type of engine is of small capacity (2,835 c.c),its normal power is such that it should suffice for light two-seater aeroplanes,the more so as the propeller reduction gear fitted results in good airscrewefficiency, and consequently a useful thrust horse-power during take-off and climbing. From this point of view,the nominally smaller power of this engine is largely illusory, and in practice the engine is equivalent to anengine developing a good deal more power, but having direct airscrew drive. Add to this fact the low weight(130 lb. complete), and it will be realised that in the Pob- joy " R " we have a newengine of quite exceptional promise. Not only is theengine very suitable for single-seater and two-seateraeroplanes, but the very low specific weight makesit a practical proposition to produce twin-engined orthree-engined aircraft of re- latively small size, thussecuring in smaller machines the same advan-tages of multiple power plants as have hithertobeen the prerogative of " giant air liners," as thegeneral Press loves to call our commercial aircraft. The Pobjoy " R "engine is of the seven- cylinder radial air-cooledtype, but differs from other engines of this type in thata plain reduction gear is fitted, which raises thepropeller shaft and hub above the centre line ofthe engine. The argument has sometimes been
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