Talk:Savoia-Marchetti SM.81 propulsion
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This page was created in an attempt to tidy and reduce the size of the SM.81 main article. Its content is simply a copy from the original main article's text. It requires a thorough work-out for spelling, grammar, references, links, etc. etc. --Red Sunset 17:43, 5 September 2007 (UTC)
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About the performances of the three-engine versus two-engine aircraft, it must be said that both the solutions had positive and negative sides. The real problem was the availability of a reliable engine. Italian project were usually either reliable but underpowered or powerful but much less reliable. As reliability, overall, it's obvious that the three-engine solution, with the power obtained from three quite reliable engines, was better than having the same output made by only two more powerful but less reliable engines, for several reasons: if one engine was shut down or damaged (in military aircraft it' prevedible), with two-engine solution there is an asymmetric thrust, that make the handling of the aircraft much difficult. With three engines, if one is shut down the aircraft has still a not dramatically asymmetric thrust or even no asymmetry if the engine is the one on the nose. Obviously, if the two engines remained are also quite reliable and better placed in the aircraft, the possibilities that aircraft return to home is much better. Moreover, the loss in power is only 1/3. If the aircraft with two engines lost one, it lost half power, it had asymmetrical thrust and the only engine that remained is less reliable on itself. Moreover, the speed falls and the struggle for the survived engine, already less reliable, is bigger, and with the risk to overheat (with emergency power and a modest airflow) and break it as well.
This is a good explanation of why three-engine machines were so liked by Italians. The shortcoming is that three engines are more heavy, more costly and more complex than one.
As aerodynamical side, three engines are usually draggier than one. But this is only partially true. In fact, the less powerful engines often had also smaller diameter, and so, less drag. This drag could have been reduced with carenatures on the propellers and tight engine rings, thanks to the lower termic emissions (less need to a bigger airflow) and so, giving a modest drag overall. If the aircraft had one small engine in the nose, drag could be not so dramatically bigger than having a free nose. But the diameter of powerful engines in the wing is bigger than smaller ones, so the overall drag of fuselage+wings is not necessarily lower for twin (big)engined than tree-(small) engined aircraft, while the total power output of 3 engines could be higher and obtained with much more reliability both as single engine and as overall architecture. All this explain quite well why S.M.79 (3x760 hp) was as fast if not faster than BR.20 (2x1000), and much more powerful, agile and reliable. At lower speed, as climbs and manoeuver, an aircraft with the same speed but draggier and so, with more power (to reach these speeds), had an advantage in P/W ratio, so while BR.20 was competitive with S.79 in horizontal speed, but nowhere in agility and climb (climb at 6,000 m was 25' compared to 17' at 5,000). The S.79 had a 15% more power with only a minimal increment of the weight.
The three-engine configuration was superseded essentially when engines both reliable and powerful became available, but still Z.1007ter whipped C.1018 overall, even if slower (when the speeds became higher the difference of design are more important than at lower ones, because mainly drag is attrition and so is related to the square of speed).
