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Why are there planes with 1, 2, 3, 4, 6, and 8 engines, but (as far as I know) none with 5 or 7?

By KevinW42 Asked Jul 7 2008 12:37PM
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Top Answer out of 2

by Its time for QueenR to kick ass on Jul 7, 2008 at 12:42 pm Permalink

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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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Avatar KevinW42 Jul, 07 2008 at 12:58 PM
Well the 727 has three.
Avatar Its time for QueenR to kick ass Jul, 07 2008 at 01:07 PM
Yes, an engine in each wing and one in the tail.

Answer 2 out of 2

by Mr. Meaulnes on Jul 7, 2008 at 12:43 pm Permalink

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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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Avatar KevinW42 Jul, 07 2008 at 12:57 PM
For five engines you could put two on each wing, like on a 747, and one in the tail, like on a 727.
Avatar Mr. Meaulnes Jul, 07 2008 at 01:02 PM
True enough...
I don't claim to know, but that have been inefficient in terms of weight at the time when five engines would have been state-of-the-art. Or maybe it was just more convienient to move up to six...
There are so many factors at work in technological development:
How durable and in-demand the pre-existing models are, for one.
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Just look at tehcnology like the AK-47. It's still in wide use, because it's a good product. The same may well hold true for some four-engine aircraft out there, making the five-engine design obsolete before it was even made.


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Why are there planes with 1, 2, 3, 4, 6, and 8 engines, but (as far as I know) none with 5 or 7?

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