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Aviation History
1943
1943 - 1634.PDF
FLIGHT The Cheetah Engine Improved Take-off Power of the Motrk X with Moderate Supercharging : Constructional Details SUPERCHARGERDRIVING GEARS SUPERCHARGER CASING REAR GOVER-U NQUESTIONABLY the best- known radial engine used on training aircraft of the R.A.F. 's the Cheetah, built by Armstrong Siddeley Motors, Ltd. This seven-cylinder radial air-cooled engine has a bore and stroke01 I 33-3 mm- by 139.7 mm., giving a total capacity of 13.66 litres; the com- pression ratio is 6.35 to 1. Of simple and sturdy design from the outset, it has been developed over the years, the latest engine bearing the designation Cheetah X. This particular type is now approved with the Cheetah IX for a full-throttle take-off on 87 octane fuel. The maxi- mum power developed at take-off is 420 b.h.p. and the maximum climb, level and cruising figures have been increased pro- portionately. A remarkable fact about this engine is that the official overhaul period since November, 1941, has been 1,200 hours. Quite lately in Canada the Cheetah X has been approved at 1,400 hours be- tween overhauls. Thousands of R.A.F. pilots have received their elementary training in the air aboard machines fitted with this engine* as it is largely used in connection with the Empire Training Scheme apart from other services. Arduous conditions are naturally im- posed upon training aircraft, particu- larly when they are used, as in this case, under extreme variations of climatic con- ditions. The latest forms of the Cheetah engine, as mentioned, are the Mark IX and Mark X, the latter being the modi- fied version incorporating provision for the use of a variable-pitch airscrew. The nett dry weight of "the engine is 694 lb. It is moderately supercharged at plus 2\ Ib./sq. in. boost for take-off and for maximum climbing plus J lb./«q. in. In applying the supercharg- ing apparatus the design has been kept simple by the use of a single gear train only to drive the rotor. Applied to the supercharger is the Claudel Hobson type AV 70 M updraught carburettor, and sundry other auxiliaries, in- cluding the twin magnetos, are also mounted at the rear. The oil pumps and pressure filters are mounted outside the crankcase nose, and in this position provide ready accessibility. The same observation may be made con- cerning the sump filter, which is located between two lower cylinders. The crankshaft is a one-piece steel forging of the single-throw type having a brass balance weight fitted to each crank web and revolves in two roller bearings. A thrust ball race is pressed into the front cover and further secured by the interference fit of the thrust plate. The master rod and the six auxiliary connecting rods are air-hardened steel stampings. Aluminium alloy heads are used, and into them a VALVE ROCKER COVERS An "exploded" drawing of the Armstrong Siddeley Cheetah X which emphasises the has been developed over the past 11 years from 260 h.p. to its present 420 h.p. In its seven cylinders of the 14-cylinder Panther whereas the further secured by a locking ring which acts as a fin. To assist cooling there is close-pitch finning on both the cylinder head and the barrels.. There are two valves per cylinder operated by enclosed push-rods from roller tappets on the front extension of the crankcase, the. exhaust valve#being sodium cooled. A series of cams on the periphery of a drum driven at half engine speed bj' epicyclic gearing operate the tappets. Valve seat- ings are stellited. The valve rockers are completely en- closed, and have their ball-bearing fulcrum spindles mounted in forks which are integral with rods extend- ing to bosses on the side of the cylinder head casting. Dry Sump Lubrication When the engrne has attained normal temperature, the rods expand lengthwise to maintain a constant gap between the valve rocker and valve step. The light-alloy pistons have four rings, one compres- sion ring near the .crown, two above the gudgeon-pin boss and a double scraper ring at the bottom of the skirt. Lubrication is on the dry-sump principle, the pump unit being driven by a bevelled gear bolted on to the The unit incorporates two pressurecam drum carrier, steel barrel is screwed and shrunk,., the two parts being pumps and a scavenge pump; the main pressure pump
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