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Aviation History
1947
1947 - 1425.PDF
AUGUST 28TH, 1947 FLIGHT 217 Fig. S. Air-cooled cylinder with single sleeve valve. Fig. 3. Alternative arrangement to that shown in Fig. 2. Fig. 4. Penthouse or hemispherical head with overhead camshaft. present. It is not absolutely essential, but I would specify a Flat Six for this 250 h.p. engine, instead of the in-line Six, and I would suggest that it should be of about 7 to 7J litres capacity, modern in design, about 35 h.p. per litre. The three-deep Flat Six gives better cooling—it is assumed that it must be air-cooled—and it should have direct fuel injection to each cylinder, on the score of distribution and fire risk'. We now come to the vexed question of how can we get complete reliability from such a size of engine, and adequate cooling that will give us 700 or 800 hours between over- hauls? My answer to this, most definitely, is that it is impossible with the accepted vertical push-rod rocker gear. The alternatives are a penthouse or spherical head, with inclined valves and more complex push-rods; or an over- head camshaft; or a Burt McCollum single-sleeve valve. Here I know I am strongly prejudiced in favour of the sleeve valve, but a little further investigation will show that there is a good argument for this type of valve mechanism for an air-cooled aircraft engine, and if this is not accepted, then a more expensive, spherical-headed poppet-valve type must be selected. The accompanying diagrams illustrate my point. "/ - ~ - Advanced Cylinder-head Technique To have this aircraft functioning efficiently, we want to be able to cruise with about 350 total h.p. continuously from the two engines, and still have 700 or 800 hours' life be- en overhauls, and we must live good fuel consumption. 'This means a good deal more • advanced cylinder-head tech- nique, with maximum cylinder temperatures under 200 deg C, and a cruising consumption at 65-70 per cent take-off power of Fig. 7. Another suggested layout in which both engines are on the e.g., with short shafts which would save weight. 0.44 lb per b.h.p. hour. These are considerably better figures than are common practice to day. They can be achieved, it is believed, with complete reliability with the very latest poppet-valve technique, but wben you have done this you have got an engine whit h is just about as expensive as a sleeve valve to make, is not as silent, and costs more in maintenance. A good example of an air-cooled aircraft engine with a good but expensive air-cooled valve gear, such as I have outlined, is the American Ranger. I would, therefore, lean towards the sleeve valve as being the best all-round solution. From these comments, it will be clear, I hope, thai in order to achieve this ideal aircraft one of the first bedrock points is to provide an engine of better thermal efficiency. This necessitates a new form of valve gear, a shorter and stiffer unit of smaller capacity, and a better consumption than is general practice to-day in this category of engine. Although 250 h.p. is not too much to take from a six- cylinder engine, it is just about enough, and although a six-cylinder, in-line might well do this job with up-to date valve gear and adequate cooling, I would personally lean to the opposed Flat Six, as I think the cooling is easier and the crankshaft system is certainly easier, seeing that gearing will have to be introduced into the system Now if we are to take the engines away from the tradi- tional place on the leading edge of the wing (and I have no doubt in my mind as to the necessity of doing this), where are they to go ? At first glance it would appear that inside the wings is the right place but, after discussions with practical aircraft designers, one does agree that this presents difficulties from their point of view. Accessibility is not good—although I am not so keen about engine acces- sibility per se, and I think it is apt to be an over-estimated quality. You want to be able to get at the magnetos, sparking plugs, fuel injector and engine accessories ; after that I suggest it is better to be able to whip out the engine quickly and simply. If you are going to take the engines away from the traditional place on the leading edge of the wing, you naturally have to be careful not to upset the aircraft designer from the balance point of view. And to introduce complications which would entail carrying bal- last, etc., when the machine was flying light would be a fatal handicap. Some considerable thought has gone into the layout of &iich art aircraft from the power-plant installation and balance point of view, and of eleven possible alternatives that have been studied, diagrams 6 and 7 seem to be the most promising. No 6 provides for the two engines in the nose, with very accessible cowling, which can be made of the "alligator" type, and which can be opened complete and immediately for inspection and servicing. The lay- out entails long shafting and some considerable develop- ment, but most of these problems have been dissipated by the electronic art of strain gauging. The second machine, shown in diagram No. 7, has the engines on the centre of gravity, requires all the pas?en-
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