running over-square

wrote in
oups.com:

On Sep 17, 5:36 pm, Roy Smith wrote:
wrote:
The old no-more-than-square thing was a rule of thumb for pilots
who flew engines that had little or no operating instructions,

Keep in mind that there's nothing magic about "square" operation.
Square means "the manifold pressure is that same as the prop speed".
That's hogwash; the numbers only work out the same because of an
accident of what units we use.

There's nothing that says we have to measure prop speed in RPM; we
could just as easily measure it in radians per second or Mega-degrees
per fortnight. There's also nothing that says we need to measure
manifold pressure in inches of mercury. It could be in mm/Hg, torr,
atmospheres, PSI, Pascals, etc.

No, there's nothing magic about it. Just that the old guys
often avoided oversquare operation unless they could find
manufacturer's data recommending it.

Some of these old practices get
carried forward into newer engines where they make no sense. Old
engines often had to run on low-octane fuels that suffered detonation
at low RPM and high MP, and the accident of RPM vs. MP was a handy way
to avoid it. Detonation was a sure way to end up on foot miles from
anywhere hospitable, and since fuel was cheap and the boss was paying
for it anyway, it was safer to use more and get home.

It's not an old practice. Many prewar engines were run over square and
100 octane fuels were commonly available immediatly after the war, not
to mention 115/145.
The only time I ever ran an engine like that was in training, and that
was only for ease of operation as a quick rule of thumb. never during
actual revenue operation.


IMO it's a practice that crept in over the years out of ignorance of the
way engines operate and ignorance of the loads and forces at play when a
piston is whizzing up and down.


bertie