X-Mozilla-Keys:                                                                                 
Russell Kent wrote:

Don Sherman wrote:

This is neither a trick question nor a Wankel question:  why do virtually all
rotaries [radials] (piston type) have an odd number of cylinders in each bank?
(Yes, I know about the master/slave connecting rod designs within.)  Air & Space
asked me this poser a couple of years ago and I have not yet found a credible
answer.  DS

Go search Deja.com in rec.aviation.homebuilt; there was a discussion of this very
topic a few months ago.  Basically it boils dow to this:

   * in order to minimize vibration, and because irregular ignition sequences were
     darned hard to make, one wants the cylinders to fire in a regular, uniform
     pattern
   * for a four-stroke radial engine, in order to satisfy the above one has to have
     an odd number of cylinders, firing half (+/-1) with each crankshaft revolution

An example might clarify.  Assume one has an 8 cylinder radial engine, with the
cylinders at 45 degree radii.  Here are your ignition options:

  1. 1-2-3-4-5-6-7-8 (spark every 45 degrees).  This fires each cylinder in turn on
     the first (combustion-exhaust) crankshaft revolution, and the crankshaft coasts
     on the second (intake-compression) revolution.  Obviously not a good design.
  2. 1-3-5-7-2-4-6-8 (spark about every 90 degrees, except for the 8-1 transition).
     The obvious problem is how to have the ignition *reliably* generate that spark
     train.

The ignition is much easier for the 9 cylinder engine: the ignition is
1-3-5-7-9-2-4-6-8, with the cylinders at 40 degree radii, and the spark every 80
degrees.

Russell Kent

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