# Why do aircraft radial engines always use an odd number of cylinders?

**URL:** <https://boards.straightdope.com/t/why-do-aircraft-radial-engines-always-use-an-odd-number-of-cylinders/747124>\
**Category:** Factual Questions\
**Created:** [February 25, 2016, 7:08pm UTC](https://boards.straightdope.com/t/why-do-aircraft-radial-engines-always-use-an-odd-number-of-cylinders/747124 "2016-02-25T19:08:11Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![system](https://global.discourse-cdn.com/straightdope/original/2X/e/e489c3b7d8fce19c4b355dd4fc3f88cc39c34b87.png) [@system](https://boards.straightdope.com/u/system)\
**Post date:** [February 25, 2016, 7:08pm UTC](https://boards.straightdope.com/t/why-do-aircraft-radial-engines-always-use-an-odd-number-of-cylinders/747124/1 "2016-02-25T19:08:11Z")

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Why do aircraft radial engines always use an odd number of cylinders?

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**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\
**Post date:** [February 25, 2016, 7:25pm UTC](https://boards.straightdope.com/t/why-do-aircraft-radial-engines-always-use-an-odd-number-of-cylinders/747124/2 "2016-02-25T19:25:22Z")

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A. Radials are built with an odd number of cylinders to give a better firing order and corresponding smoothness with lack of vibration.

[www.radialengines.com/frequently-asked-questions/](http://www.radialengines.com/frequently-asked-questions/)

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**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\
**Post date:** [February 25, 2016, 7:32pm UTC](https://boards.straightdope.com/t/why-do-aircraft-radial-engines-always-use-an-odd-number-of-cylinders/747124/3 "2016-02-25T19:32:58Z")

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Wiki’s visuals nail it:

> **[Radial engine](https://en.wikipedia.org/wiki/Radial_engine)**
>
> The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. It resembles a stylized star when viewed from the front, and is called a "star engine" in some other languages.
> The radial configuration was commonly used for aircraft engines before gas turbine engines became predominant.
> Since the axes of the cylinders are coplanar, the connecting rods cannot all be directly attached t...

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**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\
**Post date:** [February 25, 2016, 7:34pm UTC](https://boards.straightdope.com/t/why-do-aircraft-radial-engines-always-use-an-odd-number-of-cylinders/747124/4 "2016-02-25T19:34:15Z")

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Reported. This is the 2nd time today (that I’ve noticed) that a newbie has come in and posted the same question as another poster did just a short while earlier.