FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1944
1944 - 2008.PDF
346 FLIGHT SEPTEMBER 28TH, 1944 Fig. 3 (above). Possible increase in power Fig. 4. at constant height with improved fuels. AIR-COOLED ENGINES 4,000 h.p. at present-day b.m.e.p. and speeds may be expected to meet the require ments for a 5,000 to 6,000 h.p. engine in the period we are considering. As may be seen in Table I, all modern high-duty air craft engines at present give 3 about 4.8 h.p. per sq. in. of piston area, regardless of type. A figure of 5 h.p. per sq. in. represents the best attainable with normal design and, on this basis, a 4,000 h.p. engine will require about 800 sq. in. of piston area, and this could be obtained with any of the cylinder arrangements shown in Table II. Engines of these sizes would give about 5,700 h.p. in seven years time. All the cylinder arrange ments shown, except the 32- and 48-cylinder, have been run in recent years and, although for any one purpose there is presumably only one optimum design, any of the above number of cylinders from 18 to 32 is entirely practicable for an air-cooled engine of this size. The H-engines would, of course, require two crankshafts geared together. I think on the 32 X and the 32-cylinder four-crank radial it would be desirable to use two shafts with the reduction gear in the centre. If this system is not adopted the 32-cylinder X engine shaft becomes very long and torsional vibration difficulties are considerable. In the case of the four-crank radial the advantage is that it halves the power to be transmitted through the maneton joints. A split master rod for more than seven cylinders per crank is impracticable, so one must have a build-up shaft. Sleeve-valve Engine Design Of the radial arrangements the 18-cylinder or the 28- cylinder seems the obvious choice. I believe that most engineers would go for the 28-cylinder and, given equal skill in design and development, I believe it would prove the best. I don't think, however, there is anything im practicable in the 18-cylinder, large-bore engine. The 21-cylinder does not offer enough reduction in cylinder bore to justify the extra complication, and I think that the 22-cylinder imposes too many difficulties in master- rod and big-end bearing design for an engine which is to run at the speeds which modern single-sleeve valve design makes possible. I will now consider the air-cooled radial sleeve-valve type in relation to the problems to be faced in regard to the increase in specific power and increase in cylinder size. The prob lems are due to increased cylinder pressures, increased speed and higher rates of^ heat dissipation Better detail design and manufac ture of reduction gears will allow sub stantial increases in stress over accepted British radial engine prac tice, and we may gain a little from better steels. There is an urgent need for reliable nitrided steels of high core strength and the greatest surface ** a * *~ i & Z 130 JU V 1 V 0 K £ »- 8 - a < 1 C&T J 1 tfioo, .../ / HtCH^MlJtTUffE ? > COMPRESSION RATfO 7 / / / t / A tfSP/ * / / / // A / &, . iK y #, / -JQ liO 130 140 150 160 17 PERCENTAGE INCREASE IN FUEL RATING AT CONSTANT INDUCTION T •WOO iSCC J400 1300 I2O0 I10O ICOO 900 eoo 700 600 z EMPERATURE b •J UJ 5 1 i * 50%o COMPRESSION RATIO 1 A i - -i ISO ' •V * / \ 200 CROSS BM.E.fi-lb/O' . 250 . / '> i •Si - • 30 Relationship of maximum to mean cylinder pressures. 10* z y ^ y OJ d •* I 9* $ BO 86 --' X / ..._ 2000 2200 Pig- 5 Volumetric efficiency 10m engine tests at ground level and zero boos', corrected for constant induction temperature of 100 deg. C. (Hercules engine). hardness that can be combined with resistance to flaking or chipping. Work on such a steel was instigated by the Bristol Co. some six years ago, and should bear fruit in the period under consideration. The crankshaft is one of the heaviest components of any aircraft engine, but it is so important that its design is not usually considered in relation to weight so much as many other components. In the future, material of higher core strength, improved quality forgings, and full exploita tion of vibration dampers will have to be used to allow the nominal stresses to be raised. Other possibilities of keeping the weight in bounds are the development of Hirth joints and the use of plain journal bearings as adopted by Pratt and Whitney. Steel crankcases can be made strength for strength of comparable weight with those of light alloy, but oh this basis it is difficult to get sufficient rigidity. To obtain a smooth-running engine with high gas pressures this ques tion of crankcase stiffness may well become critical. The Bristol Company is in the unique and happy posi tion of being able to use ordinary good quality tin base* white-metal for big-end bearings. This material is alrriost completely resistant to bearing corrosion and is very tolerant of dirt. Its fatigue resistance is, however, rela tively low. Fatigue stresses have been avoided by having the bearing-metal on a sleeve fixed to the crankpin, and this is so rigid that the stresses due to flexure are neglig ible. On the most highly loaded point the pressure has only a cyclic variation of about 20 per cent. If the white- metal was on the master-rod bore, the load at any point would vary from zero to maximum every revolution and fatigue would occur. From all points of view cooling of the piston is a major factor. Cooling controls the tendency for ring gum ming and the piston strength. Ameri can practice is to use an open design of piston which is well ventilate! and gives any oil splashing around easy access to the crown. However, a long gudgeon pin is necessary and a long pin must be of large outside diameter 2400 K PM 2600 28O0 3OO0
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
