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From: doug@terak.UUCP
Newsgroups: net.physics
Subject: Re: odd number of cylinders
Message-ID: <1030@terak.UUCP>
Date: Thu, 30-Jan-86 16:47:26 EST
Article-I.D.: terak.1030
Posted: Thu Jan 30 16:47:26 1986
Date-Received: Sun, 2-Feb-86 01:06:21 EST
References: <1379@garfield.columbia.edu>
Organization: Calcomp Display Products Division, Scottsdale, AZ, USA
Lines: 25

> i saw the discussion on the yaw of airplane engines...and was 
> wondering if anyone knew why the number of cylinders on rotary
> airplane engines seems to be an odd number?

I don't know about rotary engines, but on radial engines the reason
is that these are 4-stroke engines with a single crank throw.

Since each cylinder on a 4-stroker fires once every two revolutions,
the odd number gives a smoother power flow.

By the way, to be more precise there are an odd number of cylinders
*per bank*.  On the more powerful radials, there were multiple banks
on the same crankshaft.  For example, a 36-cylinder radial can be
created by using a 4-throw crankshaft and mounting 4 9-cylinder
radials back-to-back.

The conceptual difference between a radial and a rotary is that in a
radial the cylinders are mounted to the plane and the crankshaft turns
with the propellor -- on a rotary the crankshaft is mounted to the plane
and the cylinders rotate with the propellor.

P.S. If you want still more info, try net.aviation -- lots of folks
over there with lots of good scoop on this sort of stuff.
-- 
Doug Pardee -- CalComp -- {hardy,savax,seismo,decvax,ihnp4}!terak!doug