ANSWERS: 2
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Well really you need an even number of engines (having more weight on one wing is avery bad thing), however, one engine can be put in the fuselage, or two engines in the wings and one in the tail etc.
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Probably just because technology made small, coincidental leaps forward at the proper times for five and seven to get skipped. But it might also have something to do with the fact that five and seven are both uneven prime numbers higher than the number of basic frame components on a typical aircraft (3: body, and two wings), and as far as I can tell, that makes them rather awkward numbers of engines to try to position aerodynamically on a conventional aircraft frame. In order to make five engines aerodynamically functional, you'd practically have to build a plane on the model of a Thai Fighter. And I can't even imagine how you'd engineer an aerodynamic balance for a large, prime number like seven...
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